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182
Phosphate transport and signaling
KG Raghothama
The discovery of phosphate (Pi) transporter genes has
provided a basis for the molecular study of the complex pattern
of Pi transport in plants. Over the past two years, a significant
amount of information has been generated on the molecular
regulation of phosphate transport in plants. Recent
developments in plant genomics will soon allow the complete
dissection of the signal transduction pathway(s) associated
with plant responses to Pi limitation in the rhizosphere.
root hairs called proteoid roots [4,7]. Proteoid roots efficiently produce and secrete organic acids and phosphatases
[8], and absorb Pi more rapidly than non-proteoid roots [7].
In many dicots, secretion of organic acids and chelating
agents that enhance Pi availability from the roots is also
documented (see for review [2••]).
Abbreviations
Michalis–Menten constant
Km
Pi
phosphate
Psr1
phosphorus starvation response 1
It is becoming evident that many of the biochemical, physiological and morphological changes that occur in response
to Pi deficiency are associated with altered gene expression
[1••,2••,9]. These changes in gene expression are initiated
as a direct and specific response to Pi starvation. Some of
the induced genes are directly involved in enhancing the
availability of Pi and promote its uptake. The phosphate
transporters, phosphatases, and enzymes involved in organic-acid synthesis and anion channels facilitating organic acid
release are examples of the proteins encoded by genes
whose expression is induced by Pi deficiency. This review
will provide an overview of the recent developments in our
understanding of phosphate transport and signal transduction in plants experiencing Pi starvation.
Introduction
Phosphate acquisition and transport in plants
Phosphate (Pi) is one of the essential nutrients required by
plants. It is a key player in all metabolic processes including energy transfer, signal transduction, biosynthesis of
macromolecules, photosynthesis and respiration [1••,2••].
Nevertheless, the interactions of Pi with soil constituents
such as Al, Fe and Ca, its existence in organic forms, and
its low rates of diffusion make Pi the least readily available
nutrient in the rhizosphere. The low availability of Pi in
the acid soils of the tropical and sub-tropical regions of the
world is of serious concern to agriculture in these regions.
In response to persistent Pi deficiency, plants have developed multifaceted adaptive mechanisms to acquire Pi from
the soil [2••]. Enhanced uptake and efficient utilization of
Pi are among the primary biochemical and molecular
changes associated with Pi deficiency.
Plants acquire Pi despite a steep concentration gradient
across the plasma membrane: Pi concentrations within
plant cells are typically 1000-times those outside. Under
experimental conditions, both high and low affinity Piuptake mechanisms have been observed in plants
[2••,10••]. Nevertheless, it is generally accepted that at Pi
concentrations that are within the micromolar range
(1–10 µm), which corresponds to Pi concentrations in cultivated soils, the high-affinity transporters mediate Pi
uptake. An energy mediated co-transport process, driven
by protons generated by a plasma membrane H+-ATPase,
has been proposed as the mechanism of Pi uptake in plants
([2••,10••,11•] and references therein). Additional evidence of the involvement of protons in Pi uptake comes
from the use of inhibitors that dissipate the proton gradient across membranes and that suppress Pi uptake
[12•,13]. The Km (i.e. Michalis–Menten constant, the substrate concentration that allows the reaction to proceeed at
one-half its maximum rate) for high-affinity transporters
varies from 1.8 to 9.9 µM [10••]. The high-affinity uptake
process is induced when Pi is deficient, whereas the lowaffinity transport system appears to be expressed
constitutively in plants.
Addresses
Center for Plant Environmental Stress Physiology, Department of
Horticulture and Landscape Architecture, Purdue University,
West Lafayette, Indiana 47907-1165, USA;
e-mail: Ragu@hort.purdue.edu
Current Opinion in Plant Biology 2000, 3:182–187
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Many of the morphological and biochemical changes that
are induced in roots growing in Pi-deficient conditions are
geared towards enhancing Pi uptake. Root modifications
include enhanced root growth, altered root architecture,
and increased production and elongation of root hairs [3,4].
The root hairs are responsible for nearly 63% of Pi
absorbed by Pi-deficient plants [5]. The association of
mycorrhizal fungi with roots is an important symbiotic
interaction that enhances Pi acquisition [6]. Mycorrhizal
fungi function as an extended and highly efficient root system in that their hyphae acquire, concentrate and transport
Pi from soil that would otherwise be beyond the reach of
the roots. Non-mycorrhizal plants, such as white lupin,
respond to Pi deficiency by producing highly branched
bottlebrush-like clusters of rootlets that are covered with
Plant phosphate transporters
High-affinity Pi transporters are membrane-associated proteins that translocate Pi from an external media containing
low concentrations (i.e. 1–10 µM) into the cytoplasm
where Pi concentrations are much greater (i.e. mM). The
availability of Arabidopsis expressed sequence tags has led
to the isolation of several genes that encode high-affinity
Phosphate transport and signaling Raghothama
183
Table 1
Amino acid identity (%) among Pi transporters isolated from Arabidopsis shows that the high-affinity phosphate transporters are
highly conserved in plants.
AtPT1 AtPT2
AtPT3
AtPT4
AtPT5
AtPT6
CrPT1
LePT1
LePT2
MtPT1
MtPT2
NtPT1
StPT1
StPT2
TaPT1
AtPT1 100
AtPT2
AtPT3
AtPT4
AtPT5
AtPT6
CrPT1
LePT1
LePT2
MtPT1
MtPT2
NtPT1
StPT1
StPT2
TaPT1
78.4
100
98.5
78.6
100
93.6
79
94
100
75.8
77.5
76.7
77.3
100
67.4
67.1
67.2
67.2
64.6
100
78.8
82.3
78.6
79
77.7
68.1
100
77.1
79.6
77.1
78.7
76.3
68.6
83.6
100
78.1
76.8
78.7
78.8
75.6
66.5
78.5
79
100
79.1
77.1
79.5
80
77
66.2
77.2
75.9
81
100
79.2
77.7
80.2
80.8
77.7
66.3
77.9
76.3
81.6
98.3
100
80.4
82.9
80.4
80.2
78.4
67.9
87.6
89
81.1
76.9
77.6
100
80.2
83.2
80.6
80.8
78.5
66.2
85.7
76.7
81.3
78.2
78.2
93.5
100
75.2
75
75.6
76.4
73.3
64.5
75.8
76.7
93.8
79
79.6
79
78.9
100
74.2
75.3
74.2
73
72.7
69.2
78.5
78.3
71.2
71.9
72.1
80.3
80.1
68.8
100
AtPT1 to AtPT6 [14,15,17,18]; tomato LePT1 and 2 [12•,19•]; Medicago, MtPT1 and 2 [20•]; Catharanthus, CrPT1 [21]; potato, StPT1 and 2
[13]; Triticum, TaPT1 (Accession No. AF110180); tobacco, NtPT1 (Accession No. AB020061) was calculated by using the FASTA algorithm
software of the Genetics Computer Group (GCG, Madison WI). Three more Arabidopsis Pi transporter homologs, AtPT7 (BAC F9H16.16), AtPT8
(Accession No. AC015450) and AtPT9 (Accession No. AC007551) showing 40–55% amino-acid identity are also reported. In addition, several Pi
transporters have also been cloned from barley roots [31].
Pi transporters [12•,13–18,19•,20•,21]. The functions of
these isolated genes have been determined by their complementation of yeast mutants that are deficient in
high-affinity Pi uptake [12•,13,14,20•,21], and by their
expression in cultured tobacco cells grown in Pi-limiting
media [16]. The salient features of the expression of genes
encoding high-affinity phosphate transporters include:
preferential expression in roots, rapid induction when Pi
becomes deficient, reversibility upon resupply of Pi and a
specific response to Pi deprivation [13,19•].
Recently, a gene encoding a low-affinity (high Km) Pi transporter (Pht2;1) that is expressed preferentially in the leaves
of Arabidopsis was described for the first time [22••].
Functional analysis of this gene in mutant yeast cells indicated that it is a Pi:H symporter, which has an apparent Km
of 0.4 mM for Pi [22••]. Homologs of this gene are also
found in other plant species including rice [22••]. The lowaffinity transporters belonging to the Pht2;1 family are
probably involved in loading Pi within shoots.
Structure of plant Pi transporters
All of the high-affinity Pi transporters whose encoding
genes have been cloned are integral membrane proteins.
Each protein consists of 12 membrane-spanning regions
that are separated into two groups of six membrane-spanning regions by a large hydrophilic charged region [23•].
Plant Pi transporters appear to have evolved by tandem
intragenic duplication of the original structural unit of a six
membrane-spanning protein [23•]. Interestingly, Pi:H
symporters have been isolated and characterized only from
fungi and plants. The remarkable similarities in the structure and function among the Pi transporters indicates that
these are among the oldest and highly conserved proteins
in plants. The low-affinity Pi transporter Pht2;1 is also a
member of the 12-membrane-spanning-region Pi:H symporter family. Its primary structure, however, is different
from that of the high-affinity phosphate transporters, suggesting that it belongs to a novel subfamily of Pi
transporters [22••].
Spatial distribution of Pi transporters
Genes encoding the high-affinity Pi transporters are preferentially expressed in Pi-starved roots [13,14,19•]. The
expression of these genes in the root hairs and root epidermis indicates that these transporters are specifically
targeted in those cell layers that are exposed to relatively
high concentrations of Pi, thereby facilitating its uptake
[12•,19•]. Recent studies show that Pi transporters are distributed along the entire length of the roots of Pi-starved
plants [24••]. Moreover, a relatively uniform rate of Pi
uptake was observed along the proteoid root axis [7].
These data support the hypothesis that the entire root system retains the potential to transport Pi at an increased rate
in response to Pi starvation [7, 24••]. The high-affinity Pi
transporters of tomato (LePT1) and potato (StPT1) are
also present in organs other than roots, such as leaves,
stem, tubers and flowers [13,19•]. These transporters may
be involved in the transport of Pi within plants in addition
to Pi acquisition by the roots. The gene encoding the lowaffinity Pi transporter (Pht2;1) is constitutively expressed
in leaves of Arabidopsis [22••].
Complexity of Pi transport in plants
The number of Pi-transporter-encoding genes that have
been cloned from plants is rapidly increasing. In
Arabidopsis, there are at least nine genes showing similarity to those encoding high affinity Pi transporters (Table 1).
184
Physiology and metabolism
Figure 1
P movement in plants
(a)
Phloem loading
Unloading
Xylem loading
P retranslocation
Unloading
P
P
Uptake
Xylem loading
Efflux
(b)
(a) Phosphate acquisition and movement in
plants: phosphate acquired by the high-affinity
Pi transporters is loaded into the xylem in
roots. Pi can move freely in both the xylem and
phloem. The loading and unloading of Pi into
the xylem or phloem, and recycling of the
nutrient during Pi deficiency and senescence
may involve different sets of phosphate
transporters. Pi efflux via anion channels helps
to maintain Pi homeostasis under conditions
in which Pi is not limiting. (b) Phosphate
transport across the plasma membrane and
tonoplast. Several high affinity Pi transporters
are involved in Pi acquisition from the external
media. The low affinity transporters (Pht2;1type) are also involved in the uptake and intercellular movement of Pi. The plasma
membrane proton ATPase provides the
energy to drive these transport processes.
The efflux mechanism helps to maintain Pi
homeostasis in cells. At the tonoplast, the
proton ATPases and pyrophosphatases may
provide the required energy for Pi transport. Pi
transport at the tonoplast is bi-directional.
Additional low affinity transporters may also
be involved in Pi transport.
e
an
br
?
Pi
Pi
Pla
s
ma
me
m
Pi efflux
Pi
ATP
H+
To
n
op
la st
ADP
H+
Pi
H+
Pi
H+
PPi
Pi
Low affinity
(Pht2;1-type)
High affinity
Current Opinion in Plant Biology
The existence of multiple high-affinity Pi transporters is
certainly a reflection of the complexity of the Pi transport
process within plants (Figure 1). Evidence of the existence
of Pi transporters that are involved in xylem loading within roots comes from a well-characterized Arabidopsis
mutant (pho1) which lacks the ability to load Pi into the
xylem [25]. The phosphate is unloaded from the xylem
into actively photosynthesizing leaf tissues. Phloem loading and unloading are important components of the
internal movement of Pi within plants. In addition, the
pho2 mutants of Arabidopsis provides genetic evidence of
the involvement of Pi transporters in phloem loading of Pi.
The uncontrolled uptake of Pi in pho2 phenotype may be
caused by alterations in the function of Pi transporter(s) or
Phosphate transport and signaling Raghothama
by the involvement of a regulatory gene that controls Pi
loading in the shoot [26•]. Even though nine phosphate
transporter homologs have been identified in Arabidopsis,
none of them is allelic with pho1 and pho2 mutants.
Interestingly, AtPT1 and AtPT2 are the only major transcripts that have been detected by Northern analysis
[14,15,17]. This could be explained either by the low
expression levels of other transporters or by the existence
of high sequence similarity among some of the nine homologues. Detailed analysis of gene expression at various
developmental stages using gene-specific probes (i.e. 3′ or
5′ untranslated regions of cDNA), in situ hybridization and
immunolocalization studies should help to describe complex processes involved in the expression of genes
encoding Pi transporter.
Intricately regulated movement of Pi across the tonoplast,
in addition to Pi uptake and efflux, regulates cellular Pi
homeostasis. The cytoplasmic Pi concentration (which is in
the mM range) is generally maintained at a constant level
under varying levels of Pi supply, whereas the vacuolar Pi
levels change significantly when Pi is deficient [10••]. The
transport of Pi across the tonoplast requires ATP and cytoplasmic alkalization [10••,11•]. The bi-directional flux of Pi
across the tonoplast takes place whether Pi concentrations
are high (i.e. mM) in the vacuole, the cytoplasm or both,
indicating that transport systems that have fast uptake rates
(i.e. high Vmax) are operating at the tonoplast. The tonoplast H-ATPase or pyrophosphatase could provide the
energy required to maintain an electrochemical gradient
across the tonoplast thereby facilitating Pi transport. The
tonoplast-associated Pi transporters have, however, yet to
be isolated and analyzed in plants. Pi efflux is another
important mechanism for regulating Pi concentrations in
the cytoplasm. When Pi is present in sufficient concentrations, high rates of Pi efflux almost compensate for Pi
influx, supporting the hypothesis that when Pi availability
is non-limiting the Pi homeostasis is primarily controlled by
Pi efflux [10••]. As further evidence of the complexity of Pi
transport, significant amounts of Pi are recycled in plants
during Pi deficiency or senescence.
Transcriptional regulation of phosphate transporters
Transcriptional activation of Pi transporters in response to
Pi starvation seems to be a major regulatory mechanism for
Pi uptake. It is becoming apparent that Pi deficiency rapidly induces the expression of genes that encode Pi
transporters leading to increased transcription and protein
synthesis, the assembly of the proteins in the plasma membrane of the outer cell layers of roots, and enhanced Pi
uptake [24••]. Recent studies in our laboratory have shown
that expression of the luciferase reporter gene driven by
the AtPT2 promoter is strongly induced in roots of Pistarved seedlings (KG Raghothama, unpublished data).
Our studies also show that the disappearance of negative
DNA-binding protein factors may be associated with
expression of genes in Pi-starved plants (U Mukatira,
D Varadarajan, KG Raghothama, Amer Soc Plant Physiol
185
Abstr 1999:189). These results further substantiate the
hypothesis that Pi transporters are transcriptionally regulated in response to Pi deficiency.
Signal transduction during Pi starvation
Plants probably have at least two different signaling mechanisms that maintain Pi homeostasis, one operating at the
cellular level and another involving multiple organs and
most probably arising from the shoots [2••]. At the cellular
level, movement of Pi from and to the vacuole, and regulated efflux and influx are the primary mechanisms that
maintain Pi homeostasis. Changes in cytosolic or vacuolar
Pi concentrations could trigger a signal transduction pathway leading to the activation of Pi-starvation rescue
systems similar to those found in microorganisms [2••]. The
whole plant response is much more complex. The transport
of Pi from old to young tissues, or from root to shoot and
back to roots, is likely to affect Pi-stress signaling.
Phosphate transporters are induced rapidly in response to
Pi starvation. The expression of Pi-transporter mRNA in
cell cultures was evident 3–6 h after transfer to medium
that contained no Pi (DH Kim, U Muchhal,
KG Raghothama, Amer Soc Plant Physiol Abstr 1998:136).
Both the transcripts of Pi transporter mRNA and concentrations of the transporter proteins themselves increased
within 12–24 h of the onset of Pi starvation in tomato
[19•,24••]. A rapid increase in the number of Pi transporters
occurs before the appearance of any visible Pi-deficiency
symptoms, suggesting that signals are initiated as a consequence of subtle changes in some cellular Pi pools.
Divided-root studies support the existence of internal signaling mechanisms that lead to Pi-starvation-induced gene
expression [19•]. Uptake studies in the pho2 Arabidopsis
mutant, a hyperaccumulator of Pi in shoots, also support
this mechanism [26•]. In addition, the formation of proteoid
roots in white lupin is also linked to changes in internal Pi
concentrations [7]. This evidence together suggests that
signals that result in the induction of gene expression are
initiated in response to changes in internal concentration of
Pi in higher plants.
In Pi-deficient plants, changes in cellular Pi concentrations
are accompanied by the translocation of considerable
amounts of carbohydrate to the roots. It is possible that information about the nutrient status of the shoot is transmitted
to the root via certain carbon assimilates or changes in Pi fluxes. A close relationship between altered root growth and Pi
deficiency suggests that phytohormones may be involved in
the response to low Pi availability [27]. Phytohormones, such
as auxin and ethylene, must be involved in altering root
growth, the elongation of root hairs, and the formation of proteoid roots. Nevertheless, at present there is no direct
evidence of a role for phytohormones as primary signals in
the response to Pi starvation. The expression of genes
encoding Pi transporters in tomato cells not experiencing Pistarvation did not change significantly in response to the
auxin NAA (alpha-naphthylacetic acid) or the ethylene
186
Physiology and metabolism
precursor ACC (1-aminocyclopropane-1-carboxylic acid),
indicating that these hormones are not directly involved in
regulating Pi uptake (DH Kim, U Muchhal, KG
Raghothama, Amer Soc Plant Physiol Abstr 1998:136). In addition, abscisic acid (ABA) may not play a major role in the
Pi-stress-induced response [28]. Although no direct evidence
is available at present, it is likely that ethylene and auxin
have roles in altering root architecture and promoting root
hair elongation in response to Pi starvation [27]. One can also
envision a role for calcium in Pi-starvation-induced signal
transduction. There is evidence of increased Ca2+-ATPase
transcript accumulation in the roots of Pi-starved tomato [29].
Cytosolic calcium levels and their modulation through the
activity of specific Ca2+-ATPases are thought to play a role
both in the response and the adaptation of plants to Pi starvation. Direct evidence for the involvement of calcium in
Pi-starvation-induced signaling is, however, still lacking.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
••
Plaxton WC, Carswell MC: Metabolic aspects of the phosphate
starvation response in plants. In Plant Responses to Environmental
Stresses: From Phytohormones to Genome Reorganization. Edited by
Lerner HR. New York: Dekker; 1999:349-372.
An excellent review of biochemical changes occurring during phosphate starvation. This article addresses the effect of Pi starvation on photosynthesis, energy transfers, glycolysis and respiration among the other biochemical changes.
2. Raghothama KG: Phosphate acquisition. Annu Rev Plant Physiol
•• Plant Mol Biol 1999, 50:665-693.
This informative review provides a comprehensive overview of the physiological and biochemical adaptations of plants that enable them to acquire
and utilize Pi in Pi-deficient conditions. A special emphasis is placed on the
recent developments in the molecular biology of Pi acquisition.
3.
Bates TR, Lynch JP: Stimulation of root hair elongation in
Arabidopsis thaliana by low phosphorous availability. Plant Cell
Environ 1996, 19:529-538.
4.
Marschner H: Mineral Nutrition in Plants, edn 2. San Diego:
Academic Press; 1995.
At present, the information about signal transduction in
Pi-starved plants is rather sketchy at best. The discovery
of Psr1 (phosphorus starvation response 1), a regulatory
protein in Chlamydomonas may shed some light on the signal transduction pathway involved in the Pi-starvation
response [30••]. The Psr1, a putative transcriptional activator, is crucial for acclimation of the unicellular green
alga, Chlamydomonas, to Pi starvation. Homologs of this
gene are present in vascular plants, and they may be
involved in a similar type of regulation as has been
observed in algal cells.
5.
Gahoonia TS, Nielsen NE: Direct evidence on participation of root
hairs in phosphorus (32P) uptake from soil. Plant Soil 1998,
198:147-152.
6.
Harrison MJ: Molecular and cellular aspects of the arbuscular
mycorrhizal symbiosis. Annu Rev Plant Physiol Plant Mol Biol
1999, 50:361-389.
7.
Keerthisinghe G, Hocking PJ, Ryan PR, Delhaize E: Effect of
phosphorus supply on the formation and function of proteoid roots
of white lupin (Lupinus albus L.). Plant Cell Environ 1998, 21:467-478.
8.
Gilbert GA, Allan DL, Vance CP: Phosphorus deficiency in white
lupin alters root development and metabolism. In Radical Biology:
Advances and Perspectives on the Function of Plant Roots. Edited by
Flores HE, Lynch JP, Eissenstat D, Rockville MD: Amer Soc Plant
Physiol; 1997:92-103.
Conclusions
9.
Raghothama KG: Molecular regulation of phosphate acquisition in
plants. In Plant Nutrition — Molecular Biology and Genetics. Edited
by Nielsen GG, Jensen A. Boston: Kluwer Academic Publishers;
1999:95-103.
In recent years, significant progress has been made in isolating and characterizing molecular determinants of Pi
acquisition and transport. We now know that the ‘simple
process’ of Pi transport is regulated by multiple transporters that are expressed and controlled in a spatial and
temporal manner. It is becoming clear that changes in the
intracellular concentrations of Pi could initiate signal
transduction pathway(s) leading to the adaptation of
plants to Pi deficiency. Nevertheless, the task of understanding how plants sense and respond to the amount of
available Pi in the rhizosphere is still a challenging and
ongoing area of research. New genomics tools such as
expressed sequence tags, sequence-tagged mutants,
gene-chips, microarrays and the nearly completed
Arabidopsis genome sequence will allow researchers to
dissect the molecular complexities of Pi transport and
signaling. This will certainly lead to a better understanding of the complex physiological and biochemical
responses observed in Pi-starved plants, and ultimately to
the development of molecular strategies to improve Pi
efficiency and crop productivity.
10. Mimura T: Regulation of phosphate transport and homeostasis in
•• plant cells. Int Rev Cytol 1999, 191:149-200.
This extensively illustrated review provides in-depth information about phosphate transport in plants. The article contains extensive citation of previous
work and detailed information about Pi-transport kinetics and cellular distribution of Pi.
11. Schachtman DP, Reid RJ, Ayling SM: Phosphorus uptake by plants:
•
from soil to cell. Plant Physiol 1998, 116:447-453.
This paper provides a brief but comprehensive overview of Pi uptake and distribution in plants, and of the role of mycorrhizae in Pi acquisition.
12. Daram P, Brunner S, Amrhein N, Bucher M: Functional analysis and
•
cell-specific expression of a phosphate transporter from tomato.
Planta 1998, 206:225-233.
In an elegant piece of work, the tomato phosphate transporter was functionally characterized in the yeast mutant that lacks the high-affinity Pi transporter. This study also provides evidence of the expression of Pi transporters
in the epidermal and root hair cells of tomato.
13. Leggewie G, Willmitzer L, Riesmeier JW: Two cDNAs from potato
are able to complement a phosphate uptake-deficient yeast
mutant: Identification of phosphate transporters from higher
plants. Plant Cell 1997, 9:381-392.
14. Muchhal US, Pardo JM, Raghothama KG: Phosphate transporters
from the higher plant Arabidopsis thaliana. Proc Natl Acad Sci
USA 1996, 93:10519-10523.
Acknowledgements
15. Smith FW, Ealing PM, Dong B, Delhaize E: The cloning of two
Arabidopsis genes belonging to a phosphate transporter family.
Plant J 1997, 11:83-92.
Research in my laboratory is supported by a grant from the US Department
of Agriculture. I thank U Mukatira, a graduate student, for compiling the
data presented in Table 1. My sincere thanks are due to my colleague
M Jenks for critical reading of the manuscript. I apologize to those
colleagues whose work is not directly cited because of space limitations.
16. Mitsukawa N, Okumura S, Shirano Y, Sato S, Kato T, Harashima S,
Shibata D: Overexpression of an Arabidopsis thaliana high-affinity
phosphate transporter gene in tobacco cultured cells enhances
cell growth under phosphate-limited conditions. Proc Natl Acad
Sci USA 1997, 94:7098-7102.
Phosphate transport and signaling Raghothama
17.
Okumura S, Mitsukawa N, Shirano Y, Shibata D: Phosphate transporter
gene family of Arabidopsis thaliana. DNA Res 1998, 5:1-9.
18. Lu YP, Zhen RG, Rea PA: AtPT4: a fourth member of the
Arabidopsis phosphate transporter gene family (accession No
U97546). Plant Physiol 1997, 114:747.
19. Liu C, Muchhal US, Mukatira U, Kononowicz AK, Raghothama KG:
•
Tomato phosphate transporter genes are differentially regulated
in plant tissues by phosphorus. Plant Physiol 1998, 116:91-99.
This paper describes the cloning and characterization of phosphate transporters from tomato. Studies show that the epidermal layers of Pi-starved
roots are enriched in Pi transporters. The authors also provide evidence of the
regulation of gene expression by internal signals in response to Pi starvation.
20. Liu H, Trieu AT, Blaylock LA, Harrison MJ: Cloning and
•
characterization of two phosphate transporters from Medicago
truncatula roots. Regulation in response to phosphate and to
colonization by arbuscular mycorrhizal (AM) fungi. Mol Plant
Microbe Interact 1998, 11:14-22.
An interaction between mycorrhizal colonization and the expression of the
genes encoding two plant phosphate transporters is examined in this interesting paper. The fungal colonization results in down-regulation of plant Pi
transporters. The transcripts of both the transporter genes are present in
roots, and transcript levels increase in response to Pi starvation.
21. Kai M, Masuda Y, Kikuchi Y, Osaki M, Tadano T: Isolation and
characterization of a cDNA from Catharanthus roseus which is
highly homologous with phosphate transporter. Soil Sci Plant Nutr
1997, 43:227-235.
22. Daram P, Brunner S, Rausch C, Steiner C, Amrhein N, Bucher M:
•• Pht2;1 encodes a low affinity phosphate transporter from
Arabidopsis. Plant Cell 1999, 11:2153-2166.
This paper describes the cloning and characterization of the first low-affinity
phosphate transporter from plants. The gene that encodes this transporter is
constitutively expressed in leaves and, interestingly, the transport is driven by
a proton:phosphate symport process.
23. Pao SS, Paulsen IT, Saier MH: Major facilitator superfamily.
•
Microbiol Mol Biol Rev 1998, 62:1-34.
The authors have done an excellent job of compiling extensive data on the
major facilitator superfamily (MFS) in an easy to understand review. An informative discussion on the origin and functional significance of the MFS is presented: these H:Pi symporters are presumed to be of ancient origin and
unique to plants and fungi.
24. Muchhal US, Raghothama KG: Transcriptional regulation of plant
•• phosphate transporters. Proc Natl Acad Sci USA 1999,
96:5868-5872.
This is the first report to describe the transcriptional regulation of any
major nutrient transporter in plants. In addition, this study also shows
187
that the plasma membranes of Pi deficient roots are enriched in Pi transporters, a physiologically relevant location for the high-affinity
Pi transporters.
25. Poirier Y, Thoma S, Somerville C, Schiefelbein J: A mutant of
Arabidopsis deficient in xylem loading of phosphate. Plant Physiol
1991, 97:1087-1093.
26. Dong B, Rengel Z, Delhaize E: Uptake and translocation of
•
phosphate by pho2 mutant and wild-type seedlings of
Arabidopsis thaliana. Planta 1998, 205:251-256.
In this study, the Pi acquisition and transport by the pho2 mutant, which
accumulates excessive Pi in its shoots, and wild-type Arabidopsis plants
were compared. The Pi uptake rate of pho2 is nearly twice that of wild-type
plants. Interestingly, Pi uptake rates of isolated roots of the mutant and wildtype plants are similar. These data support the hypothesis that the shoot
influences Pi uptake by the roots. It is presumed that the pho2 mutation may
lead to a defect in the phloem transport of Pi or lack of regulation of internal
Pi levels in the shoot.
27.
Lynch J, Brown KM: Ethylene and plant responses to nutritional
stress. Physiol Plant 1997, 100:613-619.
27.
Lynch J, Brown KM: Ethylene and plant responses to nutritional
stress. Physiol Plant 1997, 100:613-619.
28. Trull MC, Guiltinan MJ, Lynch JP, Deikman J: The responses of wildtype and ABA mutant Arabidopsis thaliana plants to phosphorous
starvation. Plant Cell Environ 1997, 20:85-92.
29. Muchhal US, Liu C, Raghothama KG: Calcium-ATPase is
differentially expressed in phosphate starved roots of tomato.
Physiol Planta 1997, 101:540-544.
30. Wykoff DD, Grossman AR, Weeks DP, Usuda H, Shimogawara K:
•• Psr1, a nuclear localized protein that regulates phosphorus
metabolism in Chlamydomonas. Proc Natl Acad Sci USA 1999,
96:15336-15341.
This paper is of special interest because it describes the characterization of
a regulator of phosphorus metabolism in a photosynthetic organism. Psr1 is
a Pi-starvation-induced regulatory gene isolated from Chlamydomonas,
which plays an important role in acclimation of the organism to Pi starvation.
This putative transcriptional activator is localized in the nucleus under Pi-deficient and sufficient conditions. The existence of homologs of Psr1 in vascular plants suggests a role for similar proteins in the adaptation of plants to
Pi-limiting conditions.
31. Smith FW, Cybinski DH, Rae AL: Regulation of expression of genes
encoding phosphate transporters in barley roots. In Plant
Nutrition — Molecular Biology and Genetics. Edited by Nielsen GG,
Jensen A. Boston: Kluwer Academic Publishers; 1999:145-150.
Phosphate transport and signaling
KG Raghothama
The discovery of phosphate (Pi) transporter genes has
provided a basis for the molecular study of the complex pattern
of Pi transport in plants. Over the past two years, a significant
amount of information has been generated on the molecular
regulation of phosphate transport in plants. Recent
developments in plant genomics will soon allow the complete
dissection of the signal transduction pathway(s) associated
with plant responses to Pi limitation in the rhizosphere.
root hairs called proteoid roots [4,7]. Proteoid roots efficiently produce and secrete organic acids and phosphatases
[8], and absorb Pi more rapidly than non-proteoid roots [7].
In many dicots, secretion of organic acids and chelating
agents that enhance Pi availability from the roots is also
documented (see for review [2••]).
Abbreviations
Michalis–Menten constant
Km
Pi
phosphate
Psr1
phosphorus starvation response 1
It is becoming evident that many of the biochemical, physiological and morphological changes that occur in response
to Pi deficiency are associated with altered gene expression
[1••,2••,9]. These changes in gene expression are initiated
as a direct and specific response to Pi starvation. Some of
the induced genes are directly involved in enhancing the
availability of Pi and promote its uptake. The phosphate
transporters, phosphatases, and enzymes involved in organic-acid synthesis and anion channels facilitating organic acid
release are examples of the proteins encoded by genes
whose expression is induced by Pi deficiency. This review
will provide an overview of the recent developments in our
understanding of phosphate transport and signal transduction in plants experiencing Pi starvation.
Introduction
Phosphate acquisition and transport in plants
Phosphate (Pi) is one of the essential nutrients required by
plants. It is a key player in all metabolic processes including energy transfer, signal transduction, biosynthesis of
macromolecules, photosynthesis and respiration [1••,2••].
Nevertheless, the interactions of Pi with soil constituents
such as Al, Fe and Ca, its existence in organic forms, and
its low rates of diffusion make Pi the least readily available
nutrient in the rhizosphere. The low availability of Pi in
the acid soils of the tropical and sub-tropical regions of the
world is of serious concern to agriculture in these regions.
In response to persistent Pi deficiency, plants have developed multifaceted adaptive mechanisms to acquire Pi from
the soil [2••]. Enhanced uptake and efficient utilization of
Pi are among the primary biochemical and molecular
changes associated with Pi deficiency.
Plants acquire Pi despite a steep concentration gradient
across the plasma membrane: Pi concentrations within
plant cells are typically 1000-times those outside. Under
experimental conditions, both high and low affinity Piuptake mechanisms have been observed in plants
[2••,10••]. Nevertheless, it is generally accepted that at Pi
concentrations that are within the micromolar range
(1–10 µm), which corresponds to Pi concentrations in cultivated soils, the high-affinity transporters mediate Pi
uptake. An energy mediated co-transport process, driven
by protons generated by a plasma membrane H+-ATPase,
has been proposed as the mechanism of Pi uptake in plants
([2••,10••,11•] and references therein). Additional evidence of the involvement of protons in Pi uptake comes
from the use of inhibitors that dissipate the proton gradient across membranes and that suppress Pi uptake
[12•,13]. The Km (i.e. Michalis–Menten constant, the substrate concentration that allows the reaction to proceeed at
one-half its maximum rate) for high-affinity transporters
varies from 1.8 to 9.9 µM [10••]. The high-affinity uptake
process is induced when Pi is deficient, whereas the lowaffinity transport system appears to be expressed
constitutively in plants.
Addresses
Center for Plant Environmental Stress Physiology, Department of
Horticulture and Landscape Architecture, Purdue University,
West Lafayette, Indiana 47907-1165, USA;
e-mail: Ragu@hort.purdue.edu
Current Opinion in Plant Biology 2000, 3:182–187
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Many of the morphological and biochemical changes that
are induced in roots growing in Pi-deficient conditions are
geared towards enhancing Pi uptake. Root modifications
include enhanced root growth, altered root architecture,
and increased production and elongation of root hairs [3,4].
The root hairs are responsible for nearly 63% of Pi
absorbed by Pi-deficient plants [5]. The association of
mycorrhizal fungi with roots is an important symbiotic
interaction that enhances Pi acquisition [6]. Mycorrhizal
fungi function as an extended and highly efficient root system in that their hyphae acquire, concentrate and transport
Pi from soil that would otherwise be beyond the reach of
the roots. Non-mycorrhizal plants, such as white lupin,
respond to Pi deficiency by producing highly branched
bottlebrush-like clusters of rootlets that are covered with
Plant phosphate transporters
High-affinity Pi transporters are membrane-associated proteins that translocate Pi from an external media containing
low concentrations (i.e. 1–10 µM) into the cytoplasm
where Pi concentrations are much greater (i.e. mM). The
availability of Arabidopsis expressed sequence tags has led
to the isolation of several genes that encode high-affinity
Phosphate transport and signaling Raghothama
183
Table 1
Amino acid identity (%) among Pi transporters isolated from Arabidopsis shows that the high-affinity phosphate transporters are
highly conserved in plants.
AtPT1 AtPT2
AtPT3
AtPT4
AtPT5
AtPT6
CrPT1
LePT1
LePT2
MtPT1
MtPT2
NtPT1
StPT1
StPT2
TaPT1
AtPT1 100
AtPT2
AtPT3
AtPT4
AtPT5
AtPT6
CrPT1
LePT1
LePT2
MtPT1
MtPT2
NtPT1
StPT1
StPT2
TaPT1
78.4
100
98.5
78.6
100
93.6
79
94
100
75.8
77.5
76.7
77.3
100
67.4
67.1
67.2
67.2
64.6
100
78.8
82.3
78.6
79
77.7
68.1
100
77.1
79.6
77.1
78.7
76.3
68.6
83.6
100
78.1
76.8
78.7
78.8
75.6
66.5
78.5
79
100
79.1
77.1
79.5
80
77
66.2
77.2
75.9
81
100
79.2
77.7
80.2
80.8
77.7
66.3
77.9
76.3
81.6
98.3
100
80.4
82.9
80.4
80.2
78.4
67.9
87.6
89
81.1
76.9
77.6
100
80.2
83.2
80.6
80.8
78.5
66.2
85.7
76.7
81.3
78.2
78.2
93.5
100
75.2
75
75.6
76.4
73.3
64.5
75.8
76.7
93.8
79
79.6
79
78.9
100
74.2
75.3
74.2
73
72.7
69.2
78.5
78.3
71.2
71.9
72.1
80.3
80.1
68.8
100
AtPT1 to AtPT6 [14,15,17,18]; tomato LePT1 and 2 [12•,19•]; Medicago, MtPT1 and 2 [20•]; Catharanthus, CrPT1 [21]; potato, StPT1 and 2
[13]; Triticum, TaPT1 (Accession No. AF110180); tobacco, NtPT1 (Accession No. AB020061) was calculated by using the FASTA algorithm
software of the Genetics Computer Group (GCG, Madison WI). Three more Arabidopsis Pi transporter homologs, AtPT7 (BAC F9H16.16), AtPT8
(Accession No. AC015450) and AtPT9 (Accession No. AC007551) showing 40–55% amino-acid identity are also reported. In addition, several Pi
transporters have also been cloned from barley roots [31].
Pi transporters [12•,13–18,19•,20•,21]. The functions of
these isolated genes have been determined by their complementation of yeast mutants that are deficient in
high-affinity Pi uptake [12•,13,14,20•,21], and by their
expression in cultured tobacco cells grown in Pi-limiting
media [16]. The salient features of the expression of genes
encoding high-affinity phosphate transporters include:
preferential expression in roots, rapid induction when Pi
becomes deficient, reversibility upon resupply of Pi and a
specific response to Pi deprivation [13,19•].
Recently, a gene encoding a low-affinity (high Km) Pi transporter (Pht2;1) that is expressed preferentially in the leaves
of Arabidopsis was described for the first time [22••].
Functional analysis of this gene in mutant yeast cells indicated that it is a Pi:H symporter, which has an apparent Km
of 0.4 mM for Pi [22••]. Homologs of this gene are also
found in other plant species including rice [22••]. The lowaffinity transporters belonging to the Pht2;1 family are
probably involved in loading Pi within shoots.
Structure of plant Pi transporters
All of the high-affinity Pi transporters whose encoding
genes have been cloned are integral membrane proteins.
Each protein consists of 12 membrane-spanning regions
that are separated into two groups of six membrane-spanning regions by a large hydrophilic charged region [23•].
Plant Pi transporters appear to have evolved by tandem
intragenic duplication of the original structural unit of a six
membrane-spanning protein [23•]. Interestingly, Pi:H
symporters have been isolated and characterized only from
fungi and plants. The remarkable similarities in the structure and function among the Pi transporters indicates that
these are among the oldest and highly conserved proteins
in plants. The low-affinity Pi transporter Pht2;1 is also a
member of the 12-membrane-spanning-region Pi:H symporter family. Its primary structure, however, is different
from that of the high-affinity phosphate transporters, suggesting that it belongs to a novel subfamily of Pi
transporters [22••].
Spatial distribution of Pi transporters
Genes encoding the high-affinity Pi transporters are preferentially expressed in Pi-starved roots [13,14,19•]. The
expression of these genes in the root hairs and root epidermis indicates that these transporters are specifically
targeted in those cell layers that are exposed to relatively
high concentrations of Pi, thereby facilitating its uptake
[12•,19•]. Recent studies show that Pi transporters are distributed along the entire length of the roots of Pi-starved
plants [24••]. Moreover, a relatively uniform rate of Pi
uptake was observed along the proteoid root axis [7].
These data support the hypothesis that the entire root system retains the potential to transport Pi at an increased rate
in response to Pi starvation [7, 24••]. The high-affinity Pi
transporters of tomato (LePT1) and potato (StPT1) are
also present in organs other than roots, such as leaves,
stem, tubers and flowers [13,19•]. These transporters may
be involved in the transport of Pi within plants in addition
to Pi acquisition by the roots. The gene encoding the lowaffinity Pi transporter (Pht2;1) is constitutively expressed
in leaves of Arabidopsis [22••].
Complexity of Pi transport in plants
The number of Pi-transporter-encoding genes that have
been cloned from plants is rapidly increasing. In
Arabidopsis, there are at least nine genes showing similarity to those encoding high affinity Pi transporters (Table 1).
184
Physiology and metabolism
Figure 1
P movement in plants
(a)
Phloem loading
Unloading
Xylem loading
P retranslocation
Unloading
P
P
Uptake
Xylem loading
Efflux
(b)
(a) Phosphate acquisition and movement in
plants: phosphate acquired by the high-affinity
Pi transporters is loaded into the xylem in
roots. Pi can move freely in both the xylem and
phloem. The loading and unloading of Pi into
the xylem or phloem, and recycling of the
nutrient during Pi deficiency and senescence
may involve different sets of phosphate
transporters. Pi efflux via anion channels helps
to maintain Pi homeostasis under conditions
in which Pi is not limiting. (b) Phosphate
transport across the plasma membrane and
tonoplast. Several high affinity Pi transporters
are involved in Pi acquisition from the external
media. The low affinity transporters (Pht2;1type) are also involved in the uptake and intercellular movement of Pi. The plasma
membrane proton ATPase provides the
energy to drive these transport processes.
The efflux mechanism helps to maintain Pi
homeostasis in cells. At the tonoplast, the
proton ATPases and pyrophosphatases may
provide the required energy for Pi transport. Pi
transport at the tonoplast is bi-directional.
Additional low affinity transporters may also
be involved in Pi transport.
e
an
br
?
Pi
Pi
Pla
s
ma
me
m
Pi efflux
Pi
ATP
H+
To
n
op
la st
ADP
H+
Pi
H+
Pi
H+
PPi
Pi
Low affinity
(Pht2;1-type)
High affinity
Current Opinion in Plant Biology
The existence of multiple high-affinity Pi transporters is
certainly a reflection of the complexity of the Pi transport
process within plants (Figure 1). Evidence of the existence
of Pi transporters that are involved in xylem loading within roots comes from a well-characterized Arabidopsis
mutant (pho1) which lacks the ability to load Pi into the
xylem [25]. The phosphate is unloaded from the xylem
into actively photosynthesizing leaf tissues. Phloem loading and unloading are important components of the
internal movement of Pi within plants. In addition, the
pho2 mutants of Arabidopsis provides genetic evidence of
the involvement of Pi transporters in phloem loading of Pi.
The uncontrolled uptake of Pi in pho2 phenotype may be
caused by alterations in the function of Pi transporter(s) or
Phosphate transport and signaling Raghothama
by the involvement of a regulatory gene that controls Pi
loading in the shoot [26•]. Even though nine phosphate
transporter homologs have been identified in Arabidopsis,
none of them is allelic with pho1 and pho2 mutants.
Interestingly, AtPT1 and AtPT2 are the only major transcripts that have been detected by Northern analysis
[14,15,17]. This could be explained either by the low
expression levels of other transporters or by the existence
of high sequence similarity among some of the nine homologues. Detailed analysis of gene expression at various
developmental stages using gene-specific probes (i.e. 3′ or
5′ untranslated regions of cDNA), in situ hybridization and
immunolocalization studies should help to describe complex processes involved in the expression of genes
encoding Pi transporter.
Intricately regulated movement of Pi across the tonoplast,
in addition to Pi uptake and efflux, regulates cellular Pi
homeostasis. The cytoplasmic Pi concentration (which is in
the mM range) is generally maintained at a constant level
under varying levels of Pi supply, whereas the vacuolar Pi
levels change significantly when Pi is deficient [10••]. The
transport of Pi across the tonoplast requires ATP and cytoplasmic alkalization [10••,11•]. The bi-directional flux of Pi
across the tonoplast takes place whether Pi concentrations
are high (i.e. mM) in the vacuole, the cytoplasm or both,
indicating that transport systems that have fast uptake rates
(i.e. high Vmax) are operating at the tonoplast. The tonoplast H-ATPase or pyrophosphatase could provide the
energy required to maintain an electrochemical gradient
across the tonoplast thereby facilitating Pi transport. The
tonoplast-associated Pi transporters have, however, yet to
be isolated and analyzed in plants. Pi efflux is another
important mechanism for regulating Pi concentrations in
the cytoplasm. When Pi is present in sufficient concentrations, high rates of Pi efflux almost compensate for Pi
influx, supporting the hypothesis that when Pi availability
is non-limiting the Pi homeostasis is primarily controlled by
Pi efflux [10••]. As further evidence of the complexity of Pi
transport, significant amounts of Pi are recycled in plants
during Pi deficiency or senescence.
Transcriptional regulation of phosphate transporters
Transcriptional activation of Pi transporters in response to
Pi starvation seems to be a major regulatory mechanism for
Pi uptake. It is becoming apparent that Pi deficiency rapidly induces the expression of genes that encode Pi
transporters leading to increased transcription and protein
synthesis, the assembly of the proteins in the plasma membrane of the outer cell layers of roots, and enhanced Pi
uptake [24••]. Recent studies in our laboratory have shown
that expression of the luciferase reporter gene driven by
the AtPT2 promoter is strongly induced in roots of Pistarved seedlings (KG Raghothama, unpublished data).
Our studies also show that the disappearance of negative
DNA-binding protein factors may be associated with
expression of genes in Pi-starved plants (U Mukatira,
D Varadarajan, KG Raghothama, Amer Soc Plant Physiol
185
Abstr 1999:189). These results further substantiate the
hypothesis that Pi transporters are transcriptionally regulated in response to Pi deficiency.
Signal transduction during Pi starvation
Plants probably have at least two different signaling mechanisms that maintain Pi homeostasis, one operating at the
cellular level and another involving multiple organs and
most probably arising from the shoots [2••]. At the cellular
level, movement of Pi from and to the vacuole, and regulated efflux and influx are the primary mechanisms that
maintain Pi homeostasis. Changes in cytosolic or vacuolar
Pi concentrations could trigger a signal transduction pathway leading to the activation of Pi-starvation rescue
systems similar to those found in microorganisms [2••]. The
whole plant response is much more complex. The transport
of Pi from old to young tissues, or from root to shoot and
back to roots, is likely to affect Pi-stress signaling.
Phosphate transporters are induced rapidly in response to
Pi starvation. The expression of Pi-transporter mRNA in
cell cultures was evident 3–6 h after transfer to medium
that contained no Pi (DH Kim, U Muchhal,
KG Raghothama, Amer Soc Plant Physiol Abstr 1998:136).
Both the transcripts of Pi transporter mRNA and concentrations of the transporter proteins themselves increased
within 12–24 h of the onset of Pi starvation in tomato
[19•,24••]. A rapid increase in the number of Pi transporters
occurs before the appearance of any visible Pi-deficiency
symptoms, suggesting that signals are initiated as a consequence of subtle changes in some cellular Pi pools.
Divided-root studies support the existence of internal signaling mechanisms that lead to Pi-starvation-induced gene
expression [19•]. Uptake studies in the pho2 Arabidopsis
mutant, a hyperaccumulator of Pi in shoots, also support
this mechanism [26•]. In addition, the formation of proteoid
roots in white lupin is also linked to changes in internal Pi
concentrations [7]. This evidence together suggests that
signals that result in the induction of gene expression are
initiated in response to changes in internal concentration of
Pi in higher plants.
In Pi-deficient plants, changes in cellular Pi concentrations
are accompanied by the translocation of considerable
amounts of carbohydrate to the roots. It is possible that information about the nutrient status of the shoot is transmitted
to the root via certain carbon assimilates or changes in Pi fluxes. A close relationship between altered root growth and Pi
deficiency suggests that phytohormones may be involved in
the response to low Pi availability [27]. Phytohormones, such
as auxin and ethylene, must be involved in altering root
growth, the elongation of root hairs, and the formation of proteoid roots. Nevertheless, at present there is no direct
evidence of a role for phytohormones as primary signals in
the response to Pi starvation. The expression of genes
encoding Pi transporters in tomato cells not experiencing Pistarvation did not change significantly in response to the
auxin NAA (alpha-naphthylacetic acid) or the ethylene
186
Physiology and metabolism
precursor ACC (1-aminocyclopropane-1-carboxylic acid),
indicating that these hormones are not directly involved in
regulating Pi uptake (DH Kim, U Muchhal, KG
Raghothama, Amer Soc Plant Physiol Abstr 1998:136). In addition, abscisic acid (ABA) may not play a major role in the
Pi-stress-induced response [28]. Although no direct evidence
is available at present, it is likely that ethylene and auxin
have roles in altering root architecture and promoting root
hair elongation in response to Pi starvation [27]. One can also
envision a role for calcium in Pi-starvation-induced signal
transduction. There is evidence of increased Ca2+-ATPase
transcript accumulation in the roots of Pi-starved tomato [29].
Cytosolic calcium levels and their modulation through the
activity of specific Ca2+-ATPases are thought to play a role
both in the response and the adaptation of plants to Pi starvation. Direct evidence for the involvement of calcium in
Pi-starvation-induced signaling is, however, still lacking.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
••
Plaxton WC, Carswell MC: Metabolic aspects of the phosphate
starvation response in plants. In Plant Responses to Environmental
Stresses: From Phytohormones to Genome Reorganization. Edited by
Lerner HR. New York: Dekker; 1999:349-372.
An excellent review of biochemical changes occurring during phosphate starvation. This article addresses the effect of Pi starvation on photosynthesis, energy transfers, glycolysis and respiration among the other biochemical changes.
2. Raghothama KG: Phosphate acquisition. Annu Rev Plant Physiol
•• Plant Mol Biol 1999, 50:665-693.
This informative review provides a comprehensive overview of the physiological and biochemical adaptations of plants that enable them to acquire
and utilize Pi in Pi-deficient conditions. A special emphasis is placed on the
recent developments in the molecular biology of Pi acquisition.
3.
Bates TR, Lynch JP: Stimulation of root hair elongation in
Arabidopsis thaliana by low phosphorous availability. Plant Cell
Environ 1996, 19:529-538.
4.
Marschner H: Mineral Nutrition in Plants, edn 2. San Diego:
Academic Press; 1995.
At present, the information about signal transduction in
Pi-starved plants is rather sketchy at best. The discovery
of Psr1 (phosphorus starvation response 1), a regulatory
protein in Chlamydomonas may shed some light on the signal transduction pathway involved in the Pi-starvation
response [30••]. The Psr1, a putative transcriptional activator, is crucial for acclimation of the unicellular green
alga, Chlamydomonas, to Pi starvation. Homologs of this
gene are present in vascular plants, and they may be
involved in a similar type of regulation as has been
observed in algal cells.
5.
Gahoonia TS, Nielsen NE: Direct evidence on participation of root
hairs in phosphorus (32P) uptake from soil. Plant Soil 1998,
198:147-152.
6.
Harrison MJ: Molecular and cellular aspects of the arbuscular
mycorrhizal symbiosis. Annu Rev Plant Physiol Plant Mol Biol
1999, 50:361-389.
7.
Keerthisinghe G, Hocking PJ, Ryan PR, Delhaize E: Effect of
phosphorus supply on the formation and function of proteoid roots
of white lupin (Lupinus albus L.). Plant Cell Environ 1998, 21:467-478.
8.
Gilbert GA, Allan DL, Vance CP: Phosphorus deficiency in white
lupin alters root development and metabolism. In Radical Biology:
Advances and Perspectives on the Function of Plant Roots. Edited by
Flores HE, Lynch JP, Eissenstat D, Rockville MD: Amer Soc Plant
Physiol; 1997:92-103.
Conclusions
9.
Raghothama KG: Molecular regulation of phosphate acquisition in
plants. In Plant Nutrition — Molecular Biology and Genetics. Edited
by Nielsen GG, Jensen A. Boston: Kluwer Academic Publishers;
1999:95-103.
In recent years, significant progress has been made in isolating and characterizing molecular determinants of Pi
acquisition and transport. We now know that the ‘simple
process’ of Pi transport is regulated by multiple transporters that are expressed and controlled in a spatial and
temporal manner. It is becoming clear that changes in the
intracellular concentrations of Pi could initiate signal
transduction pathway(s) leading to the adaptation of
plants to Pi deficiency. Nevertheless, the task of understanding how plants sense and respond to the amount of
available Pi in the rhizosphere is still a challenging and
ongoing area of research. New genomics tools such as
expressed sequence tags, sequence-tagged mutants,
gene-chips, microarrays and the nearly completed
Arabidopsis genome sequence will allow researchers to
dissect the molecular complexities of Pi transport and
signaling. This will certainly lead to a better understanding of the complex physiological and biochemical
responses observed in Pi-starved plants, and ultimately to
the development of molecular strategies to improve Pi
efficiency and crop productivity.
10. Mimura T: Regulation of phosphate transport and homeostasis in
•• plant cells. Int Rev Cytol 1999, 191:149-200.
This extensively illustrated review provides in-depth information about phosphate transport in plants. The article contains extensive citation of previous
work and detailed information about Pi-transport kinetics and cellular distribution of Pi.
11. Schachtman DP, Reid RJ, Ayling SM: Phosphorus uptake by plants:
•
from soil to cell. Plant Physiol 1998, 116:447-453.
This paper provides a brief but comprehensive overview of Pi uptake and distribution in plants, and of the role of mycorrhizae in Pi acquisition.
12. Daram P, Brunner S, Amrhein N, Bucher M: Functional analysis and
•
cell-specific expression of a phosphate transporter from tomato.
Planta 1998, 206:225-233.
In an elegant piece of work, the tomato phosphate transporter was functionally characterized in the yeast mutant that lacks the high-affinity Pi transporter. This study also provides evidence of the expression of Pi transporters
in the epidermal and root hair cells of tomato.
13. Leggewie G, Willmitzer L, Riesmeier JW: Two cDNAs from potato
are able to complement a phosphate uptake-deficient yeast
mutant: Identification of phosphate transporters from higher
plants. Plant Cell 1997, 9:381-392.
14. Muchhal US, Pardo JM, Raghothama KG: Phosphate transporters
from the higher plant Arabidopsis thaliana. Proc Natl Acad Sci
USA 1996, 93:10519-10523.
Acknowledgements
15. Smith FW, Ealing PM, Dong B, Delhaize E: The cloning of two
Arabidopsis genes belonging to a phosphate transporter family.
Plant J 1997, 11:83-92.
Research in my laboratory is supported by a grant from the US Department
of Agriculture. I thank U Mukatira, a graduate student, for compiling the
data presented in Table 1. My sincere thanks are due to my colleague
M Jenks for critical reading of the manuscript. I apologize to those
colleagues whose work is not directly cited because of space limitations.
16. Mitsukawa N, Okumura S, Shirano Y, Sato S, Kato T, Harashima S,
Shibata D: Overexpression of an Arabidopsis thaliana high-affinity
phosphate transporter gene in tobacco cultured cells enhances
cell growth under phosphate-limited conditions. Proc Natl Acad
Sci USA 1997, 94:7098-7102.
Phosphate transport and signaling Raghothama
17.
Okumura S, Mitsukawa N, Shirano Y, Shibata D: Phosphate transporter
gene family of Arabidopsis thaliana. DNA Res 1998, 5:1-9.
18. Lu YP, Zhen RG, Rea PA: AtPT4: a fourth member of the
Arabidopsis phosphate transporter gene family (accession No
U97546). Plant Physiol 1997, 114:747.
19. Liu C, Muchhal US, Mukatira U, Kononowicz AK, Raghothama KG:
•
Tomato phosphate transporter genes are differentially regulated
in plant tissues by phosphorus. Plant Physiol 1998, 116:91-99.
This paper describes the cloning and characterization of phosphate transporters from tomato. Studies show that the epidermal layers of Pi-starved
roots are enriched in Pi transporters. The authors also provide evidence of the
regulation of gene expression by internal signals in response to Pi starvation.
20. Liu H, Trieu AT, Blaylock LA, Harrison MJ: Cloning and
•
characterization of two phosphate transporters from Medicago
truncatula roots. Regulation in response to phosphate and to
colonization by arbuscular mycorrhizal (AM) fungi. Mol Plant
Microbe Interact 1998, 11:14-22.
An interaction between mycorrhizal colonization and the expression of the
genes encoding two plant phosphate transporters is examined in this interesting paper. The fungal colonization results in down-regulation of plant Pi
transporters. The transcripts of both the transporter genes are present in
roots, and transcript levels increase in response to Pi starvation.
21. Kai M, Masuda Y, Kikuchi Y, Osaki M, Tadano T: Isolation and
characterization of a cDNA from Catharanthus roseus which is
highly homologous with phosphate transporter. Soil Sci Plant Nutr
1997, 43:227-235.
22. Daram P, Brunner S, Rausch C, Steiner C, Amrhein N, Bucher M:
•• Pht2;1 encodes a low affinity phosphate transporter from
Arabidopsis. Plant Cell 1999, 11:2153-2166.
This paper describes the cloning and characterization of the first low-affinity
phosphate transporter from plants. The gene that encodes this transporter is
constitutively expressed in leaves and, interestingly, the transport is driven by
a proton:phosphate symport process.
23. Pao SS, Paulsen IT, Saier MH: Major facilitator superfamily.
•
Microbiol Mol Biol Rev 1998, 62:1-34.
The authors have done an excellent job of compiling extensive data on the
major facilitator superfamily (MFS) in an easy to understand review. An informative discussion on the origin and functional significance of the MFS is presented: these H:Pi symporters are presumed to be of ancient origin and
unique to plants and fungi.
24. Muchhal US, Raghothama KG: Transcriptional regulation of plant
•• phosphate transporters. Proc Natl Acad Sci USA 1999,
96:5868-5872.
This is the first report to describe the transcriptional regulation of any
major nutrient transporter in plants. In addition, this study also shows
187
that the plasma membranes of Pi deficient roots are enriched in Pi transporters, a physiologically relevant location for the high-affinity
Pi transporters.
25. Poirier Y, Thoma S, Somerville C, Schiefelbein J: A mutant of
Arabidopsis deficient in xylem loading of phosphate. Plant Physiol
1991, 97:1087-1093.
26. Dong B, Rengel Z, Delhaize E: Uptake and translocation of
•
phosphate by pho2 mutant and wild-type seedlings of
Arabidopsis thaliana. Planta 1998, 205:251-256.
In this study, the Pi acquisition and transport by the pho2 mutant, which
accumulates excessive Pi in its shoots, and wild-type Arabidopsis plants
were compared. The Pi uptake rate of pho2 is nearly twice that of wild-type
plants. Interestingly, Pi uptake rates of isolated roots of the mutant and wildtype plants are similar. These data support the hypothesis that the shoot
influences Pi uptake by the roots. It is presumed that the pho2 mutation may
lead to a defect in the phloem transport of Pi or lack of regulation of internal
Pi levels in the shoot.
27.
Lynch J, Brown KM: Ethylene and plant responses to nutritional
stress. Physiol Plant 1997, 100:613-619.
27.
Lynch J, Brown KM: Ethylene and plant responses to nutritional
stress. Physiol Plant 1997, 100:613-619.
28. Trull MC, Guiltinan MJ, Lynch JP, Deikman J: The responses of wildtype and ABA mutant Arabidopsis thaliana plants to phosphorous
starvation. Plant Cell Environ 1997, 20:85-92.
29. Muchhal US, Liu C, Raghothama KG: Calcium-ATPase is
differentially expressed in phosphate starved roots of tomato.
Physiol Planta 1997, 101:540-544.
30. Wykoff DD, Grossman AR, Weeks DP, Usuda H, Shimogawara K:
•• Psr1, a nuclear localized protein that regulates phosphorus
metabolism in Chlamydomonas. Proc Natl Acad Sci USA 1999,
96:15336-15341.
This paper is of special interest because it describes the characterization of
a regulator of phosphorus metabolism in a photosynthetic organism. Psr1 is
a Pi-starvation-induced regulatory gene isolated from Chlamydomonas,
which plays an important role in acclimation of the organism to Pi starvation.
This putative transcriptional activator is localized in the nucleus under Pi-deficient and sufficient conditions. The existence of homologs of Psr1 in vascular plants suggests a role for similar proteins in the adaptation of plants to
Pi-limiting conditions.
31. Smith FW, Cybinski DH, Rae AL: Regulation of expression of genes
encoding phosphate transporters in barley roots. In Plant
Nutrition — Molecular Biology and Genetics. Edited by Nielsen GG,
Jensen A. Boston: Kluwer Academic Publishers; 1999:145-150.