Plant Science 150 2000 1 – 19
Review
myo-Inositol metabolism in plants
Frank A. Loewus
a,
, Pushpalatha P.N. Murthy
b
a
Institute of Biological Chemistry, Washington State Uni6ersity, Pullman, WA
99164
-
6340
, USA
b
Department of Chemistry, Michigan Technological Uni6ersity, Houghton, MI
49931
, USA Received 3 June 1999; received in revised form 19 July 1999; accepted 19 July 1999
Abstract
The multifunctional position supplied by myo-inositol is emerging as a central feature in plant biochemistry and physiology. In this critique, attention is drawn to metabolic aspects and current assessment is made of manifold ways in which myo-inositol and
its metabolic products impact growth and development. The fact that a unique enzyme, common to all eukaryotic organisms where such assessment has been undertaken, controls conversion of
D
-glucose-6-P to 1
L
-myo-inositol-1-P provides a useful point of departure for this brief metabolic survey. Some aspects such as biosynthesis, phosphate and polyphosphate ester hydrolysis, and
O-methylation of myo-inositol have captured the consideration of molecular biologists, yet other aspects including oxidation, conjugation, and transfer to phospholipids remain virtually untouched from this viewpoint. Here, an attempt is made to enlist new
interest in all facets of myo-inositol metabolism and its place in plant biology. © 2000 Elsevier Science Ireland Ltd. All rights reserved.
Keywords
:
Galactinol; Galactopinitol; Glycosylphosphatidylinositol; Glycosylinositolphosphorylceramide; IAA-MI conjugates; myo-Inositol; 1L-myo-Inositol-1-P; Ins3P
1
synthase; MI kinase; MI monophosphatase; MI oxidation pathway; O-Methyl Inositols; Ononitol; Phosphatidyli- nositols; Phytic acid; Pinitol; Raffinose
www.elsevier.comlocateplantsci
1. Introduction
Isolation of myo-inositol MI from muscle ex- tract by Scherer in 1850 led to 80 years of intense
interest on the natural occurrence, properties, derivatives, and stereoisomers of the cyclitols.
Then, discovery that MI functioned as a growth factor for certain microorganisms and as a re-
quirement for growth of certain mutant forms of yeast prompted fresh interest in its biochemical
and biological features. Soon it became apparent that MI played a central role in growth and devel-
opment [1]. This was especially true in plant biol- ogy
where molecular
entities containing
or utilizing MI were involved in structure and func-
tion [2]. Fig. 1 summarizes this information by categorizing specific products of MI metabolism
with particular interest to plant biologists and by identifying avenues of inquiry which may lead to a
better appreciation of this unique molecule and its position in plant science. Colored backgrounds in
Fig. 1 attempt to provide a sense of related func- tions while avoiding the confusion of ‘metabolic
mapping’.
Conversion of
D
-glucose-6-P to
1L
-MI-1-P con- stitutes the first committed step in MI biosynthesis
[1 – 3]. Metabolic processing of MI beyond biosyn- thesis produces other stereo-forms of inositol and
leads to a host of functional roles, all of which require this unique cyclitol. These include:
Abbre6iations
:
ABA, abscisic acid; IAA, indole-3-acetic acid; MI, myo-inositol; Ins3P
1
, 1L-myo-inositol-1-P; InsP
6
or MI-P
6
, phytic acid; MIOP, myo-inositol oxidation pathway; SNOP, sugar nucle-
otide oxidation pathway. Corresponding author. Tel.: + 1-509-335-3413; fax: + 1-509-335-
7643. E-mail address
:
loewusmail.wsu.edu F.A. Loewus 0168-945200 - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 1 6 8 - 9 4 5 2 9 9 0 0 1 5 0 - 8
Cycling of
1L
-MI-1-P and free MI by MI phos- phatase and MI kinase [4,5].
Oxidation of free MI to
D
-glucuronic acid with its subsequent role in biogenesis of uronosyl
and pentosyl units of pectin, hemicelluloses, and related structures in plant cell walls [1,2,6].
Esterification of MI to form auxin IAA esters and their glycosides [7,8].
Conjugation of free MI with UDP-
D
-galactose to form galactinol, the galactosyl donor for
biosynthesis in the raffinose and galactopinitol series of oligosaccharides [9,10].
Isomerization and methylation of MI and other isomeric scyllo-, chiro-, muco-, and neo- inosi-
tols to form O-methyl inositols sequoyitol, bornesitol, quebrachitol, pinitol, ononitol, etc.
which participate in stress-related responses, storage of seed products, and production of
inositol-glycosides such as pinitol-galactosides [9 – 13].
Fig. 1. Functional roles of myo-inositol in plant metabolism.
Fig. 2. myo-Inositol.
If one takes biosynthesis as a basis of assign- ment and traces the origin from
D
-glucose 6-phos- phate,
then clockwise
assignment preserves
biosynthetic relatedness. Unhappily, recent ad- vances involving key roles for MI polyphosphates
involved in signal transduction where addition or loss of a single phosphate may alter assignment
from
D
- to
L
- or vice-versa have created confusion for those unfamiliar with rules of cyclitol nomen-
clature. The upshot is a tentative agreement by the International Union of Biochemistry to relax rules
of nomenclature so that
1L
-MI-1-P, the product of myo-inositol-1-phosphate synthase, may be desig-
nated
1L
-MI1P
1
,
1D
-MI3P
1
, or simply, Ins3P
1
where the symbol Ins signifies MI with counter- clockwise
numbering from
1D
. Thus,
1D
- MI1,4,5P
3
, an important physiological signal generated during phosphatidylinositol-4,5-bisphos-
phate metabolism, becomes simply Ins1,4,5P
3
. More detailed discussion of the stereochemistry of
MI and its phosphate esters is found in [14] and on the Internet at http:www.chem.qmw.ac.uk
iubmbnomenclature.
3. MI biosynthesis
