Handbook of Plant and Crop Physiology

Removal of the carboxyl group, the 1 -hydroxyl, and the 4 -carbonyl had the greatest reduction in gene
expression [126]. The ABA molecule is still recognized for gene induction if the 1 -hydroxyl is removed
(Figure 3).
In another attempt to determine what is required for ABA regulation of gene expression, protein syn-
thesis has been inhibited by the application of cycloheximide to determine if proteins must be synthesized
for ABA action. Interestingly, the requirement for protein synthesis in the response to ABA is dependent
on the gene studied. Protein synthesis is required for ABA induction of rd22but not for rd29[88]. It was
also found that ABA induction did not require protein synthesis for rab16[9]. Therefore, there are at least
two different pathways of gene induction in response to ABA.

V. FUTURE DIRECTIONS AND POSSIBLE AGRICULTURAL BENEFITS

Much progress has been made in the identification, isolation, and characterization of genes induced by
different abiotic stresses. Studies on the isolation of genes and the regulation of specific genes have indi-
cated that there are many similarities between stresses that result in cellular dehydration. Many of the
genes that are induced by these stresses are also induced by ABA application, and several of these have
been shown to require elevated levels of ABA for expression. Many of the genes induced during stress
are predicted to play a protective role through direct protection of cellular contents or by altering the cel-
lular water content.
The major challenge of the future is to obtain biochemical and genetic evidence that these gene prod-
ucts function in stress tolerance, improving the adaptability of plants to the environment. If adaptive gene
products are characterized, these may have promise for use in the development of crop plants with in-
creased stress tolerance. The use of different protective traits in combination, such as enhanced osmotic
adjustment and overproduction of a hydrophilic gene product, may improve the chances of developing
transgenic crop plants with an agricultural benefit.

REFERENCES

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10. TJ Close, RD Fenton, A Yang, R Asghar, DA DeMason, DE Crone, NC Meyer, F Moonan. In: TJ Close, EA
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748 BRAY

Figure 3 Structure of ()-abscisic acid. Modifications made to the molecule to study the molecular struc-
ture required for gene regulation are shown shaded. (From Ref. 126.)