Characterization of Expression of Drought- and Abscisic Acid-Regulated Tomato Genes in the Drought-Resistant Species Lycopersicon pennellii

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Abstract
A number of genes are induced by drought stress, and some of these genes are regulated by the plant hormone abscisic acid (ABA). In tomato (Lycopersicon esculentum), four genes have been identified and isolated that require elevated levels of endogenous ABA for expression: le4, le16, le20, and le25. To gain a better understanding of the role of these genes during stress, their expression has been studied in the drought-resistant relative of tomato, Lycopersicon pennellii. It was determined that homologous genes to all four of the L. esculentum genes were present in the L. pennellii genome. Studies were undertaken to compare the expression characteristics of these genes in L. esculentum, L. pennellii, and their F1. Using two methods of water-deficit imposition, whole plants to which water was withheld and detached leaves that were wilted to 88% of their original fresh weight, it was demonstrated that transcripts of these genes accumulated in L. pennellii in response to water deficit. In general, the increase occurred after a longer period of water deficit in L. pennellii than in tomato. As in drought-sensitive species, ABA levels were elevated by drought stress in L. pennellii, although the levels were reduced compared with those in tomato. All four tomato genes were responsive to ABA in L. esculentum and the F1, but only three of the four genes (le16, le20, and le25) were induced in response to exogenous application of ABA in L. pennellii. The patterns of expression of these genes in L. pennellii are generally similar to that of L esculentum; therefore, it is suggested that these genes play a similar, yet undefined, role in both genotypes rather than being genes that are responsible for the greater drought resistance of L. pennellii.