Plant Science 159 2000 135 – 148
Characterization of salt-induced changes in gene expression in tomato Lycopersicon esculentum roots and the role played by
abscisic acid
Jun-Zhi Wei
1
, Ananchanok Tirajoh, Jollanda Effendy, Aine L. Plant
Department of Biological Sciences, Simon Fraser Uni6ersity,
8888
Uni6ersity Dri6e, Burnaby, BC, Canada V
5
A
1
S
6
Received 12 April 2000; received in revised form 10 July 2000; accepted 12 July 2000
Abstract
Examination of tomato Lycopersicon esculentum Mill root mRNA profiles by differential display-polymerase chain reaction DD-PCR revealed that a salt treatment induced, promoted or repressed the expression of a number of genes. The majority of
the observed changes were indicative of a rapid and transient salt-induced alteration in gene expression. Twenty partial cDNAs corresponding primarily to salt-induced or up-regulated mRNAs were subsequently cloned and sequenced. The role of abscisic
acid ABA in regulating salt-responsive gene expression in roots was explored. The DD-PCR data indicate that the majority of the salt-induced changes in the root mRNA profile occurred in an ABA-independent manner. The expression of genes
corresponding to six cDNAs was shown unequivocally to be responsive to a salt treatment by RNA blot hybridization. Just two of these were responsive to exogenous ABA and, in salt-treated roots of the ABA-deficient mutant flacca, all were expressed to
a level comparable to that in the wild-type. The identity of two of the salt-responsive partial cDNAs is known. The deduced amino acid sequence of one was similar to that of laccases that polymerize a variety of substrates to form resilient structures within the
cell wall. One other shared amino acid sequence similarity with the C-terminus of a tobacco pathogen-induced oxygenase PIOX. It is possible that the PIOX is involved in generating signaling molecules that mediate a general stress response. © 2000 Elsevier
Science Ireland Ltd. All rights reserved.
Keywords
:
Abscisic acid; Differential display; Gene expression; Lycopersicon esculentum; Salt stress www.elsevier.comlocateplantsci
1. Introduction
Roots play a number of important roles during plant growth and development and typically are
the first and critical part of the plant to encounter soil salinity, a major stress that drastically affects
crop productivity. Approximately 10 of the world’s arable land and 23 of the cultivated land
is covered with various types of salt-affected soils and this situation will worsen due to the continu-
ous build-up of salt in cultivated soils as a result of irrigation [1]. When growing in saline soils,
roots have to cope with two types of stresses. The first of these is an osmotic stress resulting from
high salt concentration in the soil that results in lowered water potential and a consequent loss of
cell turgor in the roots. The second is an ionic stress induced by changes in the concentrations of
specific ions in the root-growing medium and within root tissues. These stresses in turn cause a
reduction in the water uptake and the inhibition of root elongation [2].
Physiological and metabolic changes in salt-af- fected roots are accompanied by changes in gene
expression. Salt alters the accumulation of several polypeptides in roots [3 – 8] and subsequently a
Abbre6iations
:
ABA, abscisic acid; AC, Ailsa Craig; DD-PCR, differential display polymerase chain reaction; flc, flacca; MS, Mu-
rashige and Skoog; PIOX, pathogen-induced oxygenase; ROS, reac- tive oxygen species; RT, reverse transcription.
Corresponding author. Tel.: + 1-604-2914461; fax: + 1-604- 2913496.
E-mail address
:
aplantsfu.ca A.L. Plant.
1
Present address: USDA-ARS, Forage and Range Research Labo- ratory, Utah State University, Logan, UT 84322-6300, USA.
0168-945200 - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 8 - 9 4 5 2 0 0 0 0 3 4 4 - 7
number of cDNA clones corresponding to salt-re- sponsive genes have been identified and isolated
from cDNA libraries constructed using RNA from salt-treated plant tissues [9 – 13]. Those that have
been isolated from salt-treated roots include ger- min [14], the early salt-stress responsive cDNAs
from Lophopyrum elongatum [9] and several cD- NAs associated with salt-tolerance in rice [15,16].
The expression of many of these genes in roots is responsive to abscisic acid ABA and it has subse-
quently been proposed that ABA is the primary regulator of salt-induced changes in gene expres-
sion [17]. ABA is known to regulate the expression of many genes in response to other environmental
stresses, particularly water-deficit-stress [18]. How- ever, in salt-challenged plants, a role for ABA has
largely been based on enhanced gene expression in response to the application of exogenous ABA to
unstressed plant tissues. Several studies have ad- dressed the role of endogenous ABA in regulating
salt-responsive genes, however they, have exam- ined expression in either leaves or whole seedlings
[19 – 22]. Very few studies have examined directly the role of endogenous ABA in eliciting salt-in-
duced changes in gene expression in roots.
The recently developed differential display or DD-PCR technique [23] provides a sensitive and
flexible approach to the identification of differen- tially expressed genes. This method has been used
successfully to identify several cDNAs correspond- ing to genes regulated by gibberellic acid [24,25],
ozone [26], salt [27], heat [28], senescence [29], sucrose [30] and those differentially expressed dur-
ing development [31 – 34]. Differential display was used in this study to analyze changes in mRNA
populations that occur in salt- and ABA-treated tomato roots and to identify novel salt-responsive
genes. DD-PCR and RNA blot hybridization data are presented that indicate that a substantial pro-
portion of the gene expression that occurs in salt- treated
roots appears
to be
regulated independently of ABA.
2. Methods
