The characterization of LeNUC1, a nuclease associated with leaf senescence of tomato

Induction of nuclease and RNase activities, together with decreases in nucleic acid content are considered to be characteristics of senescence in higher plants. However, little is known about the specific identities or functions of the enzymes involved or the mechanisms controlling their activation. Here we report the identification of a 41-kDa-tomato nuclease, LeNUC1, which is specifically induced during tomato leaf senescence but not in ripening fruits. LeNUC1 is a glycoprotein, which can degrade both RNA and DNA and has optimal activity at pH 7.5-8. EDTA inhibits the activity of LeNUC1, while the addition of Co2+ or Mn2+ can restore its activity in the presence of the chelating agent. Interestingly, the activity of LeNUC1 is also induced in young leaves upon treatment with ethylene, which is known to be a senescence-promoting hormone in tomato. Constitutive activity of a 39-kDa nuclease, LeNUC2, similar in its biochemical requirements to LeNUC1, was also detected. LeNUC2 is not induced by ethylene and does not seem to be glycosylated. Based on their characteristics, LeNUC1 and LeNUC2 can be classified as Nuclease I enzymes. LeNUC1 may be involved in nucleic acid metabolism during tomato leaf senescence.