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.