Drought-tolerance or shock-induced genes: how to discriminate?

Cells of many glycophyte crop plants are able to respond to a graduaI water deficit by a set of biochemical and metabolic adjustments to cope with the new cellular water balance. However, when a change in the cellular water content is sudden, it may cause cellular damage and ultimately to cell death. In either case, sets of genes are inhibited or activated. In acclimated cells, the function of the activated gene products is associated with the recovery of a normal cellular metabolism, but it is rather aimed at repairing cellular damage in water-shocked cells. To identify genes associated with the plant ability to acclimate to drought, our laboratory has characterized a potato cell population exposed to increasing concentrations of PEG 8000, to mimic graduaI water deficit. DDRT-PCR has been used extensively for cloning genes differentially expressed in response to graduaI water deficit. mRNA populations of acclimated cells were compared with those of PEG- shocked cells. The comparison was also made with mRNAs of ABA- treated cells, based on the findings that ABA accumulates in shocked cells but not in acclimated cells. A set of cDNAs specific for each mechanism was identified and cloned. Genes under study include genes involved in the maintenance of membrane stability and physical state under water stress (desaturases), as well as in Ca2+ mediated signal transduction pathway (calmodulins).