Responses to Water Deficit'

Water deficit elicits a complex of responses beginning with stress perception, which initiates a signal transduction pathway(s) and is manifested in changes at the cellular, physiological, and developmental levels. The set of responses observed depends upon severity and duration of the stress, plant genotype, developmental stage, and environmental factors providing the stress. Cellular water deficit may result from stresses such as drought, salt, and low temperature. This complexity makes it difficult to uncover the responses to water deficit that enhance stress tolerance. In recent years efforts have tumed toward isolation of genes that are induced during water deficit in order to study the function of droughtinduced gene products and the pathways that lead to gene induction. Changes in gene expression are fundamental to the responses that occur during water deficit, and they control many of the shortand long-tem responses. Studies on the molecular responses to water deficit have identified multiple changes in gene expression using twodimensional PAGE, and many genes that are water-deficitinduced have been isolated by differential screening of cDNA libraries. Functions for many of these genè products have been predicted from the deduced amino acid sequence of the genes. Genes expressed during stress are anticipated to promote cellular tolerance of dehydration through protective functions in the cytoplasm, alteration of cellular water potentia1 to promote water uptake, control of ion accumulation, and further regulation of gene expression. Although these studies are promising, it continues to be difficult to ascertain the actual function of drought-induced gene products. Expression of a gene during stress does not guarantee that a gene product promotes the ability of the plant to survive stress. The expression of some genes may result from injury or damage that occurred during stress. Other genes may be induced, but their expression does not alter stress tolerance. Yet others are required for stress tolerance and the accumulation of these gene products is an adaptive response. Complex regulatory and signaling processes, most of which are not understood, control the expression of genes during water deficit. Multiple stresses may connect into the same or a similar transduction pathway, which is evidenced by the involvement of ABA in the induction of genes induced by a number of different stresses. In addition to induction by Department of Botany and Plant Sciences, University of California, Riverside, California 92521 -01 24