TROPICAL FRUIT GENOMES SEQUENCING AND POSTHARVEST PRODUCT QUALITY

Abstract

The sequencing of tropical fruit genomes is entering a new phase as sequencing technology undergoes dramatic changes in speed and cost. Full genome sequencing and expressed sequence tags often coupled to microarray technology provide the possibility to determine the genes expressed and their regulation at specific stages of fruit development. Papaya is the first fleshy fruit with a climacteric ripening pattern to be sequenced. Compared to Arabidopsis and tomato, papaya has fewer genes predicted that may impact sugar accumulation, ethylene synthesis and response, respiration, chlorophyll degradation and carotenoid synthesis were predicted. Similar or fewer genes were found in papaya for the enzymes leading to volatile production than so far determined for tomato. The presence of fewer papaya make papaya a valuable and unique tool to study fruit evolution and the complex regulatory networks active in fruit ripening and quality development. Preliminary papaya microarray data showed that as ripening begins the genes associated with cellulose synthase are highly expressed in green fruit and decreased 8-fold at 30% ripe stage. In the 30% ripe fruit, many cell-wall hydrolase genes increased more than 2-fold. Four expansins increased and two declined. In addition, two of the three predicted ethylene receptors were up-regulated and the third was not expressed in either stage, auxin-related genes mostly declined, as did most GA-related genes. A slow-ripening papaya mutant line in which the ethylene-independent ripening processes precede and are partially uncoupled from ethylene-dependent processes has a single QTL with about 50 predicted genes. The microarray has been used to show how these QTL genes are expressed during ripening.