Transcriptomic Changes Associated with Postharvest Susceptibility of an ABA-Deficient Mutant of Oranges to Non-Chilling Peel Pitting

'Pinalate' orange is a spontaneous mutant from the Navel orange ('Navelate'), producing distinctive yellow-colored fruit which are also deficient in abscisic acid (ABA). Fruit of this mutant are very prone to dehydration and much more susceptible to develop non-chilling peel pitting than fruit of its parental 'Navelate'. Therefore, 'Pinalate' mature fruit is a valuable experimental system to address transcriptomic studies aimed to understand the molecular basis of this physiological disorder and to analyze the potential role of ABA and dehydration in the incidence of the disorder. We have evaluated changes in peel damage development, ABA levels and global changes in gene expression occurring in the outer part of the peel (flavedo) of 'Pinalate' and 'Navelate' fruit stored at 12°C. ABA content in the flavedo of the mutant fruit was about 10-fold lower during the whole storage period, while the development of peel damage and the rate of dehydration were higher. Global changes in gene expression in the flavedo of 'Pinalate' and 'Navelate' fruit stored at 12°C were evaluated by using a cDNA microarray containing 20000 unigenes generated by the Spanish 'Citrus Functional Genomics Project'. Major changes in the number of differentially expressed genes occurred by one week of storage in 'Navelate' fruits as compared to freshly harvested fruits, the number of up-regulated genes (1090) being lower than that of down-regulated genes (1694). By this period, all the genes showing differential expression (392) in 'Pinalate' fruits were down-regulated. Gene ontology analysis revealed that the biological processes related to defence response and biotic stimuli were repressed by one week of storage in 'Navelate' and by three weeks in 'Pinalate' fruits, while water deprivation and inorganic cation transport processes were only induced in 'Navelate' fruit. The results also showed that biological processes related to aromatic compounds and carbohydrate biosynthetic processes were repressed by 3 weeks in the ABA-deficient mutant.