DIFFERENTIAL REGULATION OF POLYGALACTURONASE AND PECTIN METHYLESTERASE GENE-EXPRESSION DURING AND AFTER HEAT-STRESS IN RIPENING TOMATO (LYCOPERSICON-ESCULENTUM MILL) FRUITS

The effects of extended heat stress on polygalacturonase (PG; EC 3.2.1 .15) and pectin methylesterase (PME; EC 3.1.1.11) gene expression at m RNA, protein and activity levels in ripening tomato fruits were invest igated. Steady state levels of PG mRNA declined at temperatures of 27 degrees C and above, and a marked reduction in PG protein and activity was observed at temperatures of 32 degrees C and above. Exogenous eth ylene treatment did not reverse heat stress-induced inhibition of PG g ene expression. Transfer of heat-stressed fruits to 20 degrees C partl y restored PG mRNA accumulation, but the rate of PG mRNA accumulation declined exponentially with duration of heat stress. Heat stress-induc ed inhibition of PME mRNA accumulation was recoverable even after 14 d ays of heat stress. In fruits held at 34 degrees C, both PG and PME pr otein and activity continued to accumulate for about 4 days, but there after PG protein and activity declined while little change was observe d in PME protein and activity. In spite of increases in mRNA levels of both PG and PME during the recovery of heat-stressed fruit at 20 degr ees C, levels of PG protein and activity declined in fruits heat-stres sed for four or more days while PME protein and activity levels remain ed unchanged. Collectively, these data suggest that PG gene expression is being gradually and irreversibly shut off during heat stress, whil e PME gene expression is much less sensitive to heat stress.