Preparation for Harvest: Contributions of Unripe Fruit Chloroplasts and Preformed Defenses for Ripe Fruit Quality

POWELL ALT; BLANCO-ULATE B; VINCENTI E; VICENTE AR; LABAVITCH JM

Lemessos

Congreso; Postharvest Unlimited; 2014

ISHS

Regulated partially by transcription factors, fruit development establishes structures and metabolism that are reconfigured later as other transcription factors and hormones activate and regulate ripening programs that impact the quality of harvested fruit.  Chloroplast elaboration in unripe fruit determines the nutrient quality of ripened fruit and the association of anti-pathogen proteins in the cell wall matrix determines the integrity of harvested and stored fruit. The fruit of most angiosperm plants begin development as green organs with chloroplasts containing chlorophyll and structures typical of photosynthesizing organs.  In tomato (Solanum lycopersicum) with its prototype fleshy fruit, genes controlling the elaboration of chloroplasts in fruit have been identified.  While the distribution of chloroplasts is not uniform over the tomato fruit sphere, expression of a Golden 2-like transcription factor, GLK2, specifies fruit chloroplast  development.  The elaboration of chloroplasts when the fruit are green determines the lycopene and sugar contents of ripe fruit.  Carotenoids and sugars contribute significantly to ripe fruit flavor and nutrient qualities.  Unlike in senescing leaves where chloroplasts are dissolved and their contents are recycled as nutrients, fruit chloroplasts are remodeled into chromoplasts where nutrients of harvested fruit are synthesized.  By understanding how GLK2 uniquely regulates chloroplasts in fruit, we have the opportunity to improve ripened fruit quality without altering photosynthetic and chloroplast functions in the rest of the plant. In developing fruit, proteins that inhibit virulence functions of microbial pathogens accumulate in the extracellular matrix prior to encounters with rot-causing pathogens.  Polygalacturonase inhibiting proteins (PGIPs) inhibit the pathogen, but the not the fruit ripening enzymes that hydrolyze pectins in the cell wall matrix.  Expression of PGIPs reduces the susceptibility of tomato fruit to Botrytis cinerea, which causes grey mold on most harvested and stored fruit. These examples illustrate the importance of events prior to ripening for the quality of harvested fruit.