Differential accumulation of phytoalexins in tomato leaves but not in fruits after inoculation with virulent and avirulent races of Cladosporium fulvum

Abstract

Leaves of near-isogenic lines of tomato, carrying resistance gene Cf4 or Cf5 to Cladosporium fulvum, were both inoculated with race 4 and 5 of this fungus. Accumulation of two phytoalexins occurred more rapidly in the two incompatible combinations (Cf4 + race 5; Cf5 race 4) than in the two compatible ones (Cf4 + race 4; Cf5 + race 5) in the whole leaf as well as in the lower epidermis. Hyphal growth in incompatible combinations was inhibited 3 to 5 days after inoculation, shortly after stomatal penetration. At this time, accumulation of phytoalexins was observed only in incompatible interactions. Following prolonged incubation phytoalexins also accumulated in compatible interactions, when there was already abundant fungal growth. The identity of the phytoalexins is not yet known. Inoculation of pericarp tissue of tomato fruits of both near-isogenic lines with Cladosporium fulvum resulted in a very rapid accumulation of rishitin and four unidentified phytoalexins, two of which were identical to those occurring in tomato leaves with respect to their relative retention and R F value. The concentration of rishitin, the main phytoalexin in tomato fruit, reached its maximum 1 to 2 days after inoculation and decreased later. In contrast to tomato leaves, in fruits no differential accumulation of phytoalexins in incompatible and compatible interactions was observed at an early stage of infection. The phytoalexins in tomato leaves and fruits accumulated only after interaction with fungal material; they did not accumulate in wounded controls or after treatment with HgCl 2 and terramycin. For this reason, they cannot be regarded as general stress metabolites.