R. Ardi et al., Involvement of epicatechin biosynthesis in the activation of the mechanismof resistance of avocado fruits to Colletotrichum gloeosporioides, PHYSL MOL P, 53(5-6), 1998, pp. 269-285
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.