A Firm Focus on Tropical Fruit Ripening

Development of fruits, including tropical fruits, follows several patterns. Mature fruit then undergo ripening which is a coordinated process typically involving changes in pigmentation, flavour, respiration, surface waxes and texture. The latter, ripening-related fruit softening, is reviewed in depth. Changes in the cell walls of fruits underpin this softening but it is important to appreciate these biochemical changes may not occur uniformly throughout the wall but in discrete zones. To understand the basis of this phenomenon, studies have either examined modifications in the polysaccharides that comprise the cell wall, i.e. the substrates, or the enzymes that catalyse such changes, in either case looking for correlations between these and softening. Molecular biological approaches have allowed a more detailed understanding of how such enzymes are regulated but more importantly using genetic engineering techniques it has been possible to silence or overexpress specific genes and more directly test the role of such proteins in fruit softening. Using the tomato model system the roles of potential mediators of fruit softening are examined; endo-beta-1,4-glucanase, pectin methylesterase, polygalacturonase, pectate lyase, xyloglucan endotransglucosylase/hydrolase, endo-beta-1,4-mannanase, beta-galactosidase, a-arabinosidase, expansins and hydroxyl radicals. A similar review is made of ripening-related softening in the major tropical fruits, avocado, banana, mango and papaya. In all cases, both pectins and hemicelluloses are targeted, often using different enzymes. With such diversity of fruit development, it is not that surprising diverse mechanisms of softening come into play.