SENESCENCE-INDUCED RNASES IN TOMATO

A main feature of leaf senescence is the hydrolysis of macromolecules by hydrolases of various types, and redistribution of released materia ls. We have initiated a study for the characterization of RNases invol ved in nucleic acid catabolism during senescence. Using a PCR-based cl oning approach we isolated from tomato two senescence-induced RNase cD NA clones. Each of these cDNAs hybridized to a senescence-induced tran script in northern analysis. One RNase cDNA was identical to the tomat o LX RNase while the second corresponded to the LE RNase. Both LX and LE RNase genes had originally been demonstrated to be induced after ph osphate starvation of tomato cell culture but nothing was known about their expression or function in plants. We observed that the expressio n of the LX and LE genes is induced in leaves during an advanced stage of senescence with the LX transcript level being much more induced th an that of LE. Low-level expression of the RNase genes was observed in flowers and artificially senescing detached leaves while no expressio n could be detected in stems, roots, or fruits at different ripening s tages. Ethylene activated the WC gene expression in detached young lea ves while LE gene expression, which could be transiently induced by wo unding, appeared to be activated by abscisic acid. We suggest that the LX RNase has a role in RNA catabolism in the final stage of senescenc e, and LE may function during wounding as a plant defense protein.