ABSTRACT
The ripening of fleshy fruits involves changes in colour, flavour, and texture that make them attractive to eat. The process occurs through the action of enzymes that catalyse a range of distinct but coordinated biochemical changes in different cell compartments. The activities of many of the key enzymes are regulated by the controlled expression of ‘ripening genes’ which are required in order for specific facets of ripening to proceed. In climacteric fruit, such as tomato, the expression of ripening genes is stimulated by ethylene, which functions as a ripening hormone. Although ripening changes lead to improvements in quality, if particular changes such as softening, proceed too far, rapid deterioration and spoilage of fruit follows. Many ripening genes have now been cloned, including polygalacturonase and pectinesterase, involved in cell wall softening; phytoene synthase, required for carotenoid synthesis; and ACC synthase and ACC oxidase, which catalyse the production of ethylene. A number of techniques are now available for over-expressing particular genes in fruits, to change their physiological and biochemical properties. The techniques of gene silencing, either using anti-sense genes or sense-suppression, have been used successfully to reduce or inactivate the expression of specific genes and determine their function. Tomatoes genetically modified to have low polygalacturonase activity have specific benefits for fresh market or processing applications, whereas reduction in ethylene synthesis has been shown to improve quality and storage life of tomato and melon. Several genetically modified fruit products based on these findings are now on the market and others are being developed.
