Abstract

A tomato plant (Lycopersicon esculentum Mill.) with necrotic leaf spots mimicking disease lesions was singled out in progeny under selection in Moscow (breeding material of Ignatova Svetlana). The progeny from spontaneous selfing of such a plant (V20368), in the presence of increasing temperature and high light intensity, exhibited spontaneous necrotic lesions on the leaves, with acropetal progression (autonecrosis). A similar phenotype, described in 1948 by Langford, appeared to be associated with the Cf-2 resistance gene, introgressed from L. pimpinellifolium. Recently, Kruger et al. (2002) demonstrated that the Cf-2 effect depends on a second gene (Rcr3^pim) encoding a cysteine protease, and that autonecrosis is activated by the contemporary presence of Cf-2 and the L. esculentum allele Rcr3^esc. In this work we characterised the V20368 mutant and verified that autonecrosis is caused by the presence of an interaction between Cf-2 and Rcr3^esc. When the environmental conditions are favourable, this interaction triggers an oxidative burst, as evidenced by a strong increase in H₂O₂ production and activities of catalase (EC 1.11.1.6), peroxidase (EC 1.11.1.7) and ATPase (EC 3.6.1.3). In addition, by grafting the necrotic mutant on the cv. Riogrande and vice versa, we proved that the necrotic phenotype is not associated with the movement of a signal molecule, since the autonecrosis was not transmitted across the grafting point. Finally, the interaction between Cf-2 and Rcr3^esc appeared to lower the threshold of stress perception, as evidenced by an increased sensitivity to the insecticide Fenthion.