Chitinase:: Differential induction of gene expression and enzyme activity by drought stress in the wild (Lycopersicon chilense Dun.) and cultivated (L-esculentum Mill.) tomatoes

We have previously reported the isolation of a drought: and abscisic acid (ABA)-induced chitinase gene from Lycopersicon chilense, a drought tolerant wild tomato (Chen et al., 1994. Mol. Gen. Genet. 245: 195-202). We have therefore carried out a comparative study in order to examine chitinase gene expression and enzyme activity in Lycopersicon chilense (LA2747 and LA1930) and L. esculentum (cultivar Starfire) during water stress. Both enzyme assay and northern blot analysis revealed that chitinase expression was differentially induced by drought in the two species. Higher induction of chitinase occurred in tolerant species compared to the sensitive one. Among the genotypes examined, L. chilense LA2747 which is the most drought tolerant, presented the highest level of chitinase induction, while the lowest level was found in L. esculentum, Starfire. Similar results were obtained using cell suspensions treated with ABA and mannitol. Both ABA treatment and osmotic stress induced chitinase gene expression in the cell suspensions and this induction was more pronounced in the wild tomato compared to the cultivated one. Drought induced accumulation of chitinase mRNA was investigated in L. chilense, LA2747 leaves, stems and roots. The results revealed that the chitinase gene was organ specifically expressed during drought stress. Leaves of drought stressed plants showed the highest expression and roots showed the lowest. The accumulation of chitinase during drought stress was also confirmed by in situ hybridization. Leaf water potentials (LWP) were determined in the three genotypes during drought stress. A correlation was found between the chitinase gene expression and leaf water potential during drought stress. The highest level of leaf water potential was maintained in L. chilense LA2747 which also demonstrates the highest level of chitinase expression among the three genotypes. Our results suggest that the L. chilense chitinase gene might be involved in drought tolerance of this species.