C7(P32) AND C6(P34) PR PROTEINS INDUCED IN TOMATO LEAVES BY CITRUS EXOCORTIS VIROID INFECTION ARE CHITINASES

Two chitinases induced in tomato leaves (Lycopersicon esculentum Mill cv. Rutgers) by citrus exocortis viroid (CEV) infection were purified. Their molecular masses determined by sodium dodecyl sulfate-gel electrophoresis were 32 kDa and 34 kDa and by filtration through Sephadex G-100 were 23 kDa and 25 kDa, respectively. These chitinases (P32 and P34) have been shown to be identical to the tomato pathogenesis-related proteins C7 and C6. They were purified in three stages: ammonium sulphate fractionation, chitin affinity chromatography and CM-Sepharose chromatography. The characterization of P32, the major component of the CEV-induced chitinase activity, revealed a basic protein (pI, 8·5), with a low optimum pH of 5·2 for hydrolytic activity, a high thermal stability and resistance to proteolytic degradation. These tomato chitinases have been shown to be serologically related to the tobacco chitinases. Our results lend further support to the hypothesis that enhanced chitinase activity is a component of a general mechanism of response to pathogens and other stress agents. © 1990.