The nucleotide sequence of 1e16, a tomato (Lycopersicon esculentum Mill.) gene induced by drought stress and regulated by abscisic acid specifically in aerial vegetative tissue, is presented. The single open reading frame contained within the gene has the capacity to encode a polypeptide of 12.7 kilodaltons and is interrupted by a small intron. The predicted polypeptide is rich in leucine, glycine, and alanine and has an isoelectric point of 8.7. The amino terminus is hydrophobic and characteristic of signal sequences that target polypeptides for export from the cytoplasm. There is homology (47.2% identity) between the amino terminus of the LE16 polypeptide and the corresponding amino terminal domain of the maize phospholipid transfer protein. 1e16 was expressed in drought-stressed leaf, petiole, and stem tissue and to a much lower extent in the pericarp of mature green tomato fruit and developing seeds. No expression was detected in the pericarp of red fruit or in drought-stressed roots. Expression of 1e16 was also induced in leaf tissue by a variety of other abiotic stresses including polyethylene glycol-mediated water deficit, salinity, cold stress, and heat stress. None of these stresses or direct applications of abscisic acid induced the expression of 1e16 in the roots of the same plants. The unique expression characteristics of this gene indicates that novel regulatory mechanisms, in addition to endogenous abscisic acid, are involved in controlling gene expression.During a period of drought stress, a plant undergoes a number of physiological and metabolic changes together with an increase in the biosynthesis of the plant hormone ABA (38). Because the accumulation of ABA is an early event after the onset of drought stress, it has been postulated that ABA acts to coordinate the plant's response to water deficit (16). These responses include the closure of stomata, resulting in a limitation of the transpiration rate, a decrease in the water potential of the plant, and decreased photosynthesis (2). Changes in gene expression have been observed after periods of water deficit as indicated by the accumulation of novel polypeptides (3,12). In addition, a number of ABA-regulated genes have been isolated and characterized (for review, see ref. 32). Whereas the majority of these genes are expressed in embryos of developing seeds when the levels of ABA also rise, a few are also expressed in the wilting vegetative portions of the plant.

The cDNAs, pLE4 and pLE25, represent mRNAs that accumulate in response to water deficit and elevated levels of endogenous abscisic acid in detached leaves of drought-stressed tomato (Lycoperskcon esculentum Mill., cv Ailsa Craig) (A Cohen, EA Bray [1990] Planta 182: 27-33). DNA sequence analysis of pLE4 and pLE25 showed that the deduced polypeptides were 13.9 and 9.3 kilodaltons, respectively. Each polypeptide was hydrophilic, cysteine-and tryptophan-free, and found to be similar to previously identified proteins that accumulate during the late stages of embryogenesis. pLE4 and pLE25 mRNA accumulated in a similar organ-specific pattem in response to specific abiotic stresses. Yet, expression pattems of the corresponding genes in response to developmental cues were not similar. pLE25 mRNA accumulated to much higher levels in developing seeds than in droughtstressed vegetative organs. pLE4 mRNA accumulated predominantiy in drought-stressed leaves. The similarities and differences in the accumulation characteristics of these two mRNAs indicates that more than one mechanism exists for the regulation of their corresponding genes.

In response to water deficit, endogenous abscisic acid (ABA) accumulates in plants. This ABA serves as a signal for a multitude of processes, including regulation of gene expression. ABA accumulated in response to water deficit signals cellular as well as whole plant responses playing a role in the pattern of gene expression throughout the plant. Although the fimction of genes regulated by ABA during stress are currently poorly understood, a number of these genes may permit the plant to adapt to environmental stress.List of abbreviations: ABA, abscisic acid; GUS, 13-glucuronidase; RWC, relative water content; wt, wild type

Elevated levels of abscisic acid (ABA) are required for the expression of several genes during water deficit. The expression patterns of the ABA‐requiring genes, le4, le16, le25 and his1‐s, differed during a 4 d period of water deficit and a 24 h recovery period in intact tomato plants. During water deficit, a strong correlation between ABA content and mRNA accumulation was observed for le16, with only a moderate or poor correlation observed between the accumulation of ABA and the other mRNAs. The regulation of le4 and le16 can be accounted for by an increase in transcription rate, while components of post‐transcriptional regulation are implicated in the regulation of the le25 and his1‐s genes. A strong correlation between leaf ABA content and the transcription rate of le4 and le16 during the stress and recovery periods was observed. Protein synthesis was not necessary for the expression of the four genes. However, the mRNA content of le25 increased in the presence of translation inhibitors plus ABA, implicating inhibition of a transcriptional repressor. The regulation of gene expression in response to stress is complex; although ABA was required for the regulation of the genes in this study, there must be additional control mechanisms that interact with components of the ABA regulatory machinery.

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