Plant Molecular Biology 36: 439–449, 1998. 439 c 1998 Kluwer Academic Publishers. Printed in Belgium. Senescence-induced RNases in tomato Amnon Lers 1 , Andrei Khalchitski 1 , Ella Lomaniec 1 , Shaul Burd 1 and Pamela J. Green 2 1 Department of Postharvest Science of Fresh Produce, The Volcani Center, P.O.Box 6, Bet Dagan 50250, Israel ( author for correspondence); 2 MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312, USA Received 25 June 1997; accepted in revised form 30 September 1997 Key words: ethylene, gene expression, leaf senescence, RNase, tomato, wounding Abstract A main feature of leaf senescence is the hydrolysis of macromolecules by hydrolases of various types, and redistribution of released materials. We have initiated a study for the characterization of RNases involved in nucleic acid catabolism during senescence. Using a PCR-based cloning approach we isolated from tomato two senescence- induced RNase cDNA clones. Each of these cDNAs hybridized to a senescence-induced transcript in northern analysis. One RNase cDNA was identical to the tomato LX RNase while the second corresponded to the LE RNase. Both LX and LE RNase genes had originally been demonstrated to be induced after phosphate starvation of tomato cell culture but nothing was known about their expression or function in plants. We observed that the expression 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 than that of LE. Low-level expression of the RNase genes was observed in flowers and artificially senescing detached leaves while no expression could be detected in stems, roots, or fruits at different ripening stages. Ethylene activated the LX gene expression in detached young leaves while LE gene expression, which could be transiently induced by wounding, appeared to be activated by abscisic acid. We suggest that the LX RNase has a role in RNA catabolism in the final stage of senescence, and LE may function during wounding as a plant defense protein. Introduction Leaf senescence is one of the most prominent develop- mental processes associated with an increase in RNase activity. This activity is thought to have a role in the organized disassembly of the cells which occurs during senescence together with the redistribution of the released materials. The catabolyzed nucleic acids become a source of mobilized phosphate that is trans- ported elsewhere in the plant. Generally, RNase activ- ity in senescing attached leaves is induced. However, due to variations between experimental systems and poor definition of leaf age, the results are somewhat variable [11]. A large number of earlier studies also report increases in RNase activity in detached leaves, a common model system for senescence [11]. Because these studies usually monitor total RNase activity which can also be induced by wounding, they should be interpreted with caution. Most data suggest that de novo synthesis of proteins is required for the induc- tion of RNase activity but definitive proof is lacking in most cases. De novo synthesis of ribonuclease was demonstrated in senescing morning flowers [8]. Both early and recent studies have examined RNase activity in tomato. McHael and Dove followed total RNase activity in attached and detached leaves of 6–8 week old Lycopersicon esculentum as it related to development and senescence [23]. In detached leaves, RNase activity increased almost linearly in older leaves, but in younger leaves it increased to a plat- eau which was maintained for 2 days before a second increase in activity occurred [23]. High light, known to delay senescence, apparently delayed the second increase in RNase activity [23]. In attached leaves, an increasing gradient in RNase activity with increasing age was observed. This increase in RNase activity was