Molecular studies of drought stress in plants use a variety of strategies and include different species subjected to a wide range of water deficits. Initial research has by necessity been largely descriptive, and relevant genes have been identified either by reference to physiological evidence or by differential screening. A large number of genes with a potential role in drought tolerance have been described, and major themes in the molecular response have been established. Particular areas of importance are sugar metabolism and late-embryogenesis-abundant (LEA) proteins. Studies have begun to examine mechanisms that control the gene expression, and putative regulatory pathways have been established. Recent attempts to understand gene function have utilized transgenic plants. These efforts are of clear agronomic importance.

Sucrose-phosphate synthase (SPS) is a key enzyme in the regulation of sucrose metabolism, being responsible for the synthesis of sucrose 6-phosphate from fructose 6-phosphate and uridine 5 ' -diphosphate-glucose. W e report on the isolation and characterization of cDNA clones encoding SPS from Craferosfigma planfagineum Hochst., a resurrection plant in which the accumulation of sucrose is considered to play an important role in tolerance to severe protoplastic dehydration. Two distinct classes of cDNAs encoding SPS were isolated from C. plantagineum, and are represented by the clones Cpspsl and CpspsZ. l h e transcripts corresponding to both cDNAs decrease to very low levels in dehydrating leaves of C. plantagineum. Only the Cpspsl transcript occurs in the roots, where it is present at a higher leve1 than in leaves and increases upon dehydration of the plant. Higher enzymatic activities have been determined in protein extracts of dehydrated tissues compared with untreated tissues, which correlates with an increase in protein levels. It is suggested that the overall regulation of SPS is strongly influenced by the changing composition of the cytoplasm in C. plantagineum leaves during the dehydration-rehydration cycle.

Over more than a decade molecular techniques have been applied to analyse the response of plants to drought with the objective to identify genes which contribute to drought tolerance. The studies have used a variety of experimental strategies, and they have resulted in the characterization of a large number of genes which are expressed upon dehydration. A very prominent group among these genes are the so-called Lea (=late embryogenesis abundant) genes which appear to occur ubiquitously in most higher plants. A challenge for future research is still to identify the role of the gene products in dehydration stress; it is particularly necessary to distinguish gene products with a potential in osmoprotection and those which are only involved in secondary reactions. Another area of research activities has been to elucidate the dehydration stress-triggered signal transduction and the role of ABA in this process. For this part transgenic plants have been used to evaluate promoter sequences and to characterize cis-acting regulatory promoter elements crucial for a distinct expression pattern.[331

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