The cucurbit family, including melon, translocates the galactosyl-sucrose oligosaccharides, raffinose and stachyose, in addition to sucrose, from the source leaves to the fruit sink. The metabolism of these photoassimilates in the fruit sink controls fruit growth and development, including the horticulturally important phenomenon of sucrose accumulation, which determines melon fruit sweetness. During this research project we have characterized the complete pathway of galactosyl sucrose metabolism in developing fruit, from before anthesis until maturity. We have also compared the metabolic pathway in scurose accumulating genotypes, as compared to non-accumulating genotypes. Furthermore, we studied the pathway in different fruit tissues, in response to pollination, and also analyzed the response of the individual steps of the pathway to perturbations such as low temperature and leaf removal. The results of our studies have led to the conclusion that generally galactosyl-sucrose metabolism functions as a coordinately controlled pathway. In one case, as an immediate response to the absence of pollination, the activity of a single enzyme, UDPglu pyrophosphorylase, was drastically reduced. However, during young fruit development, sucrose accumulation, and in response to perturbations of the system, groups of enzymes, rather than single enzymes, respond in a concerted manner. Our research has characterized in detail the initial enzymes of galactosyl-sucrose metabolism, including the galactosidases, galactokinase and the UDPgal- and UDPglu pyrophosphorylases. We have discovered a novel alkaline a-galactoside which hydrolyzes both stachyose and reaffinose and thereby may have solved the dilemma of cytosolic-sucrose metabolism, since prior to this research there was no known alkaline a-galactosidase capable of hydrolyzing raffinose.
The Contribution of Sucrose Metabolism Enzymes to Sucrose Accumulation in Cucumis melo
