Studies on the role of polygalacturonase isoenzymes in tomato fruit softening and ripening
Changes in the activity of polygalacturonase during the development and/or ripening of tomato fruit of a wide range of genotypes have been followed. Normal commercial cultivars have three forms of the enzyme; the isoenzyme having the highest molecular weight, PG1, appeared as fruit began to change colour, and its rate of accumulation was reflected in the rate of fruit softening. This early rate of softening was closely related to the eventual degree of softness shown by the fully ripe fruit. PG1 was also the predominant isoenzyme during the senescence of slow-ripening (Never ripe; Longkeeper, Spanish Winter) genotypes. Trace levels of this isoenzyme were also found in aged ripening-resistant (ripening inhibitor, non-ripening and Alcobaca) tomato genotypes. The negligible degree of softening of these lines was reflected by their low polygalacturonase activity. Two smaller isoenzymes of polygalacturonase, PG2A and PG2B, appeared as normally ripening fruit turned orange and then red in colour, so that in ripe fruit up to 95% of the total polygalacturonase activity occurred in these forms. Neither the rate of accumulation nor the total activity of PG2 correlated with the extent of fruit softening at this period of ripening, and there appeared to be a supra-abundance of the enzyme. Loss of cell wall material during ripening was correlated with the degree of fruit softening, but differences in this property shown by fruit from a range of cultivars could not be explained by altered susceptibility of their cell walls to attack by exogenous polygalacturonase extracted from red fruit. It is hypothesised that PG1 degrades the middle lamella of the tissue leading to rapid fruit softening. The high activity of PG2 produced by fruit during the later stages of ripening is seen as eroding the primary cell wall more extensively, causing a release of soluble wall components, but not producing the increase in loss of fruit firmness that might be expected. Normal tomato fruit ripening involves autocatalytic ethylene production and exponential polygalacturonase synthesis accompanied by pigment transformations and the adjustment of fruit composition to produce an acceptable flavour. However, in this study of wild species and salt-induced ripening of various genotypes, instances have been found in which low levels of ethylene production and very limited polygalacturonase do eventually result in partial ripening of the fruit.