Endopolygalacturonase: Messenger RNA, Enzyme and Softening in the Ripening Fruit of a Range of Tomato Genotypes

Abstract

Summary Fruit from seven tomato cultivars were harvested when mature but green and allowed to ripen under conditions of regulated temperature and air flow. Fruit were analysed when pink colour was first visible at the blossom end («breaker») and 3, 6 and 9 days after breaker. Compressibility of the fruit was measured and the accumulation of the enzyme endopolygalacturonase (PG) and its messenger RNA were determined. In all genotypes, the PG mRNA abundance peaked 3 or 6 days after «breaker» and then declined. However, despite the decline in message abundance, for 4 of the 7 genotypes, the increase in PG activity between 6 and 9 days was as rapid as earlier. The two softest cultivars, Rouge de Marmande and Rutgers, accumulated most PG enzyme over 9 days and had the highest steady state levels of PG mRNA early in ripening. In Flora-Dade, a firm cultivar that accumulated over 9 days much less PG enzyme than any other cultivar, the steady state levels of PG mRNA were relatively high. Comparing rates of accumulation within cultivars or peak levels between cultivars, there was little correlation between changes in PG enzyme activity and the steady state levels of its mRNA. The correlation between enzyme activity and peak abundance of PG mRNA was much stronger when the comparison was made between populations of the genotype de Ruiters 83G38 containing the ripening-impairing genes rin or nor. In the heterozygous condition, the levels of both message and enzyme were reduced compared to the normal line; the reduction was least in the rin, intermediate in the nor and greatest in rin × nor populations. Significant levels of PG enzyme or its mRNA were not detected in populations homozygous for the mutant genes. The results indicate that the accumulation of the PG message varies in different genotypes but that post-transcriptional events other than those regulating mRNA abundance, play an important part in regulating accumulation of the PG enzyme.