Inhibition of expression of tomato‐ripening genes at high temperature

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Abstract
Ripening tomato fruits incubated at 35°C fail to achieve normal pigmentation, soften little and show a marked decline in ethylene evolution. Labelling studies in vivo indicate that protein synthesis continues throughout incubation at 35°C although the spectrum of labelled proteins is different to that observed at 25°C. Translation of mRNAs in vitro shows traces of several ‘heat‐shock’ mRNAs at 35°C and the loss of several others normally found in fruit ripened at 25°C. Using ripening‐related cDNA clones as hybridization probes the expression of 12 ripening‐related genes was followed during incubation at 25°C and 35°C. In general, there was a marked decline in the amounts of these mRNAs following incubation of ripening fruit at 35°C. In particular, mRNA homologous to pTOM 6, a cDNA clone coding for polygalacturonase, a major cell wall degrading enzyme, showed a rapid decline following incubation at 35°C and after 72‐h at elevated temperature was undetectable. There was no recovery of expression during 120 h at 35°C and the application of exogenous ethylene did not overcome the inhibition of ripening or lead to the renewed accumulation of polygalacturonase mRNA. It is proposed that the failure to soften normally at elevated temperature is due, in part, to the suppression of polygalacturonase mRNA and that the inhibition of other facets of ripening at 35°C is due to the inhibition or reduced expression of other, as yet unidentified, ripening‐related genes.