tion in wheat [91]. Germinlike proteins of barley accumulated only in roots after salt stress. The proteins
are soluble upon boiling and are found mainly in the soluble fraction using cellular fractionation studies
[90]. A similar protein was found in Mesembryanthemum crystallinumroots; however, it was found to de-
crease in response to salt stress. It is proposed that germinlike proteins are sensors of water status, and the
expression of these genes may be involved in the control of growth, depending on plant water status [92].
- Early Light-Induced Proteins
In desiccated Craterostigma plantagineumleaves, a gene, dsp-22, that is targeted to the chloroplast is in-
duced [93]. The deduced amino acid sequence is similar to a protein called early light-induced protein
(ELIP) from pea and barley [94,95]. The protein consists of alternating regions of hydrophobic and hy-
drophilic domains. There is a putative transit peptide at the amino terminus. Light is essential for the ac-
cumulation of this protein. ABA induction does not occur in the dark. DSP-22 may be involved in pho-
toprotection of the photosystems or in maintaining assembled photosynthetic structures essential for
resuming active photosynthesis following rehydration [93].
This description of genes that are induced by water deficit elicited by different types of abiotic
stresses serves to indicate that there are indeed many changes in genes expression in response to changes
in the environment. There is a complex of molecular processes that are altered by the environment. The
functions of the majority of the genes that have been characterized fall in the broad category of protection
of cellular function. Protection is predicted to come from hydrophilic proteins in the cytoplasm, osmolyte
accumulation, degradation of denatured proteins, and protection from pathogens. Genes involved in the
regulation of other genes that are induced by stress have also been identified. These responses occur in
many different plants, species that are tolerant and those that are not, and in response to different stresses
that cause water deficit.
III. METHODS TO EVALUATE THE ADAPTIVE ROLES OF
STRESS-INDUCED GENESThe expression of specific genes during stress implies that the genes are involved in stress tolerance.
However, this may not be a valid assumption. It is possible that gene induction, in addition to promoting
stress tolerance, is a result of an injury or a coincidence because of similar signal transduction pathways
initiated by different stresses. Therefore, techniques must be developed to evaluate the adaptive signifi-
cance of the expression of these genes. In many cases, to begin this evaluation it must first be determined
if the gene products in fact accumulate during stress. In many of the studies that have been completed thus
far, only the accumulation of transcripts has been studied. This does not always ensure that the protein
will accumulate. Using osmotin as an example, it was shown that osmotin mRNA is not always translated
even though it accumulates [96]. Once it has been established that the protein accumulates, further stud-
ies can be completed at the biochemical, genetic, and molecular levels. Several molecular techniques may
be exploited to study the role of specific genes.
A. Under- and Overexpression of Specific Genes
One method to investigate the function of drought-induced gene is to over- and underexpress the genes in
transgenic plants. The rationale behind this strategy is that the altered expression of these genes will alter
stress tolerance if these genes play an essential role in tolerance. Three lea-like genes from Craterostigma
plantagineumwere overexpressed in tobacco driven by the 35S promoter from cauliflower mosaic virus
(CaMV) [97]. The plants were tested for physiological traits that might indicate a difference in stress tol-
erance. An ion leakage test after PEG stress of leaflets was used to test for drought tolerance. However,
no differences were observed between the transgenic plants and the wild type. It is possible that the pa-
rameters measured were not useful for detecting differences in drought tolerance, that these proteins alone
are not sufficient by themselves and need other proteins and/or osmolytes [97], or that these proteins do
not function in drought tolerance.
The antisense strategy, in which antisense RNAs accumulate in transgenic plants in order to elimi-
nate a specific protein, might also be used to evaluate gene function. This technique can be used to con-
struct specific single gene mutants. At this time, antisense plants for stress-induced genes have not been
evaluated and reported. However, the same problems as encountered above may also occur. There are
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