Involvement of epicatechin biosynthesis in the activation of the mechanismof resistance of avocado fruits to Colletotrichum gloeosporioides

Exposure of freshly harvested avocado fruits to ethylene or CO2 increased c oncentrations of epicatechin and an antifungal diene in the peel. However, the kinetics for the accumulation of these products were greatly different for the two treatments. Following an initial increase caused by ethylene, l evels of the diene and epicatechin decreased at the same rate in treated an d nontreated fruits and were not different from the untreated controls when disease symptoms of Colletotrichum gloeosporioides first appeared. In CO2- treated fruits, on the other hand, levels of the antifungal diene and epica techin increased in two stages. An initial increase was observed soon after treatment, similar to the case with ethylene, but a second increase also o ccurred several days after the end of the first increase. Only CO2-treated fruits exhibited a reduction in fungal decay, indicating that the second in crease in epicatechin and the antifungal diene may be causally involved in the delay of C. gloeosporioides attach. Exposure of freshly-harvested avoca do fruit to ethylene or CO2 also increased phenylalanine ammonia lyase (PAL ), chalcone synthase (CHS) and flavanone 3 hydroxylase (F3H) activities. F3 H activity increased only once in ethylene-treated fruits, but also increas ed as a second peak in response to CO2 treatment. F3H activity closely foll owed the pattern of epicatechin accumulation, cDNAs clones of avocado pal, chs and f3h were used to demonstrate that changes in enzyme activities occu rred with transcriptional activation of the respective genes in ethylene an d CO2 treated fruits. Finally, fruits of avocado cultivars naturally resist ant to Fungal decay contained higher levels of epicatechin that declined sl ower during ripening. Activities of F3H were also higher in resistant than susceptible cultivars. The data are consistent with the hypothesis that hig h levels of epicatechin are required for maintenance of concentrations of t he antifungal diene sufficient to provide decay resistance during avocado f ruit ripening. (C) 1998 Academic Press.