INVOLVEMENT OF REACTIVE OXYGEN SPECIES, GLUTATHIONE METABOLISM, AND LIPID-PEROXIDATION IN THE CF-GENE-DEPENDENT DEFENSE RESPONSE OF TOMATO COTYLEDONS INDUCED BY RACE-SPECIFIC ELICITORS OF CLADOSPORIUM-FULVUM

The chronological order of responses to Cladosporium fulvum (Cooke) (C f) race-specific elicitors was assessed in cotyledons of three near-is ogenic tomato (Lycopersicon esculentum Mill.) lines carrying either Cf -9 or Cf-2 or no Cf gene. The responses observed were dependent on the presence of a Cf gene, Avr-gene product dose injected, and the relati ve humidity (RH) of the growth chamber. At ambient RH, superoxide form ation and lipid peroxidation occurred after 2 h (Cf9) and 4 h (Cf2). A t elevated RH (98%) and at lower avirulence elicitor dose, Cf-Avr-depe ndent lipid peroxidation was considerably attenuated. Significant elec trolyte leakage occurred by 18 h but only at the lower RH. Total gluta thione levels began to increase 2 to 4 h and 4 to 8 h after challenge of Cf9 and Cf2 cells, respectively, and by 48 h reached 665 and 570% o f initial levels. A large proportion of this accumulation (87%) was as oxidized glutathione. When the RH was increased to 98%, increases in glutathione levels were strongly attenuated. Increased lipoxygenase en zyme activity was detected 8 h postchallenge in either incompatible in teraction. These results indicate that the activation of the Cf-Avr-me diated defense response results in severe oxidative stress.