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

Exposure of freshly harvested avocado fruits to ethylene or CO sub(2) increased concentrations 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, levels of the diene and epicatechin decreased at the same rate in treated and non-treated fruits and were not different from the untreated controls when disease symptoms of Colletotrichum gloeosporioides first appeared. In CO sub(2)-treated fruits, on the other hand, levels of the antifungal diene and epicatechin increased in two stages. An initial increase was observed soon after treatment, similar to the case with ethylene, but a second increase also occurred several days after the end of the first increase. Only CO sub(2)-treated fruits exhibited a reduction in fungal decay, indicating that the second increase in epicatechin and the antifungal diene may be causally involved in the delay of C. gloeosporioides attack. Exposure of freshly-harvested avocado fruit to ethylene or CO sub(2) also increased phenylalanine ammonia lyase (PAL), chalcone synthase (CHS) and flavanone 3 hydroxylase (F3H) activities. F3H activity increased only once in ethylene-treated fruits, but also increased as a second peak in response to CO sub(2) treatment. F3H activity closely followed the pattern of epicatechin accumulation. cDNAs clones of avocado pal, chs and f3h were used to demonstrate that changes in enzyme activities occurred with transcriptional activation of the respective genes in ethylene and CO sub(2) treated fruits. Finally, fruits of avocado cultivars naturally resistant to fungal decay contained higher levels of epicatechin that declined slower during ripening. Activities of F3H were also higher in resistant than susceptible cultivars. The data are consistent with the hypothesis that high levels of epicatechin are required for maintenance of concentrations of the antifungal diene sufficient to provide decay resistance during avocado fruit ripening.