Genomics Approaches to Understanding Ripening Control and Fruit Quality in Tomato

The maturation and ripening of fleshy fruits contributes a major component of human diets, nutrition and agricultural activity. While ripening brings about highly desirable changes in fruit character and chemistry in terms of flavor, appearance, texture and nutrition, the advanced stages of ripening lead to sub-optimal fruit quality and eventually post-harvest loss. Fruit biologists have studied numerous fruiting species with the intent of identifying strategies and technologies toward improving desirable ripening attributes while minimizing those with negative consequences. Tomato has emerged as a model for fleshy fruit ripening, in part due to its ease of use as a model system resulting from facilitating attributes including simple genetics, numerous characterized mutants, cross-fertile wild germplasm to promote genetic studies and routine transformation technology. In the last decade, the tomato system has been further complemented with molecular and genomic tools including dense genetic maps, large EST collections and the recently initiated genome sequencing effort. The isolation of genes corresponding to several previously described tomato ripening mutations has led to considerable advancement of a genetic regulatory model of fruit ripening while emerging genomics technologies promise exceptional opportunities for continued advancement of this field. INTRODUCTION Fleshy fruits including berries, stone, pome and numerous additional temperate and tropical fruits undergo a ripening process in which the biochemistry, physiology and structure of the organ are developmentally and biochemically altered to influence appearance, texture, flavor and aroma to attract seed dispersing organisms (reviewed in Seymour et al., 1993). While the specific regulatory and biochemical programs resulting in ripening phenomena vary among species, changes typically include: a) modification of visible pigmentation usually through alteration of chlorophyll, carotenoid, and/or flavonoid synthesis and accumulation; b) textural modification, often leading to softening, via alteration of cell turgor and cell wall metabolism and/or structure; c) modification of sugars, acids, and volatile chemistry leading to significant modifications in nutritional quality, flavor and aroma characteristics; and d) generally enhanced susceptibility to opportunistic pathogens (at least in part due to loss of cell wall structural integrity). In sum, these attributes define the ripening of most fleshy fruits and lead to enhanced fruit quality while simultaneously representing a production and distribution challenge in terms of increased post-harvest decay. While fruit species are classically defined physiologically on the basis of the presence (climacteric) or absence (non-climacteric) of elevated respiration and synthesis of the gaseous hormone ethylene at the onset of ripening (Lelievre et al.. 1997), fruit displaying both ripening physiologies typically follow the general developmental changes outlined above. Ripening is studied in numerous species though tomato has clearly developed as the most widely used model system for ripening due to the availability of extensive compatible germplasm, informative ripening mutants, comprehensive genetic maps and marker systems, efficient transformation and the recent development of genomics tools including large insert BAC libraries, extensive EST collections and an ongoing genome Proc. IS on Biotechnol. Temp. Fruit Crops & Trop. Species Eds. R.F. Litz and R. Scorza 63 Ada lirt. 735, ISI IS 2007