A number of genes have been identified that are induced when plants or plant parts are subjected to
stresses resulting in cellular water deficit. Yet the function or role that most of these gene products play
is still elusive. Some of the changes in gene expression may be adaptive, having functions that promote
plant survival during water deficit, but this cannot be assumed. The function of many genes can be pre-
dicted based on the amino acid sequence of the gene product deduced. Clues to function may also be ob-
tained from expression characteristics such as timing of expression and organ, tissue, cellular, and sub-
cellular location. However, the significance of gene expression with respect to stress tolerance cannot
always be predicted.
At this time, most researchers have concentrated on identifying genes that respond to stress, begin-
ning to determine the function, and studying their regulation. In this chapter, genes that are regulated by
water deficit will be categorized based on functional predictions. Utilization of the techniques of plant
molecular biology is providing new insights into plant responses to the environment with respect to the
function of these changes and the regulation of these responses and the commonalty of the response
among different stresses and among different species.
II. PREDICTED FUNCTIONS OF STRESS- AND ABA-INDUCED GENE
PRODUCTSMany different genes, encompassing many classes of gene products, are induced by abiotic stresses. To
fully understand the significance of these changes in gene expression, the function of the stress-induced
gene products and the mechanism by which these genes are regulated must be understood. Many of these
stress-induced genes are responsive to ABA application. In terms of function, these genes can be divided
into different classes based on their DNA sequence, expression characteristics, and/or predicted func-
tions. Unfortunately, at this writing, there are few examples in which the in vivo function of the gene prod-
uct has been demonstrated. In many cases, functions have been predicted based on deduced amino acid
sequence, but no biochemical or physiological data have been obtained to prove the function in vivo.
Therefore, several of the gene classifications are based on predicted functions, and as the true functions
are determined these categories may need to be corrected. The classifications used for abiotic stress–in-
duced genes are hydrophilic gene products (Figure 1), enzymes, those with other predicted functions, and
those for which a function has not been predicted.
A. Hydrophilic Proteins Predicted to Have a Protective Function
A number of genes induced during periods of water deficit have been identified that encode proteins
which are overwhelmingly hydrophilic, are soluble upon boiling, and are therefore expected to be located
in the cytosol. These characteristics have led to the prediction that these gene products are involved in
protecting cellular structures and components from dehydration associated with water deficit, salinity,
and low-temperature stress. Many of these genes were first shown to be expressed during seed desicca-
tion, the period of seed development following maturation, and are referred to as late embryogenesis
abundant (lea) genes [3,4]. Dure et al. [4] established three groups of leagenes based on the publication
of homologous genes found in seeds of other species. This classification is useful because of the prepon-
derance of different names for genes within these classes. However, it must be acknowledged that the
nameleafor genes expressed during stress in vegetative organs may not be appropriate because a num-
ber of these stress-induced genes are not expressed during seed development. It was suggested by Dure
et al. [4] that the name WSP (water stress protein) be applied to proteins of this class for that reason. How-
ever, this naming system has not been adopted, possibly because the name WSP also has problems; this
name does not acknowledge that these proteins may be induced by other stresses, such as low tempera-
ture or salt stress, and it does nothing to add to our understanding of the function of these genes. The genes
discussed in this section on hydrophilic gene products include genes that have been identified after peri-
ods of stress in vegetative organs and share significant DNA sequence homology with leagenes from cot-
ton. In addition, several new genes have been identified that are stress induced, overwhelmingly hy-
drophilic, yet have not been found in desiccating cotton seeds.
- Em Family (Group 1)
This family of proteins is hydrophilic with no notable structural domains predicted by the amino acid se-
quence [5]. Cysteine and tryptophan are not found in the proteins of this group. The entire deduced pro-
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