REGULATION OF TOMATO FRUIT POLYGALACTURONASE MESSENGER-RNA ACCUMULATION BY ETHYLENE - A REEXAMINATION

Polygalacturonase (PG) is the major enzyme responsible for pectin disa ssembly in ripening fruit. Despite extensive research on the factors r egulating PC gene expression in fruit, there is conflicting evidence r egarding the role of ethylene in mediating its expression. Transgenic tomato (Lycopersicon esculentum) fruits in which endogenous ethylene p roduction was suppressed by the expression of an antisense 1-aminocycl opropane-1 -carboxylic acid (ACC) synthase gene were used to re-examin e the role of ethylene in regulating the accumulation of PC mRNA, enzy me activity, and protein during fruit ripening. Treatment of transgeni c antisense ACC synthase mature green fruit with ethylene at concentra tions as low as 0.1 to 1 mu L/L for 24 h induced PC mRNA accumulation, and this accumulation was higher at concentrations of ethylene up to 100 mu L/L. Neither PC enzyme activity nor PC protein accumulated duri ng this 24-h period of ethylene treatment, indicating that translation lags at least 24 h behind the accumulation of PC mRNA, even at high e thylene concentrations. When examined at concentrations of 10 mu L/L, PC mRNA accumulated within 6 h of ethylene treatment, indicating that the PC gene responds rapidly to ethylene. Treatment of transgenic toma to fruit with a low lever of ethylene (0.1 mu L/L) for up to 6 d induc ed levels of PC mRNA, enzyme activity, and protein after 6 d, which we re comparable to levels observed in ripening wild-type fruit. A simila r level of internal ethylene (0.15 mu L/L) was measured in transgenic antisense ACC synthase fruit that were held for 28 d after harvest. In these fruit PC mRNA, enzyme activity, and protein were detected. Coll ectively, these results suggest that PC; mRNA accumulation is ethylene regulated, and that the low threshold levels of ethylene required to promote PC mRNA accumulation may be exceeded, even in transgenic antis ense ACC synthase tomato fruit.