Gene Expression Profiles for Two Auxin Transporters during Apple Fruit Maturation and Ripening

The maturation and ripening patterns of apple varieties differ greatly due to their long history of cultivation, self-incompatible nature and the high-level heterozygosity of the apple genome. The ripening season across elite apple cultivars can span more than three months. Apple maturation and ripening is a tightly regulated genetic process which can result in considerable variation in fruit quality and storability. Ethylene biosynthesis and its response play a central role in apple ripening; auxin metabolism may also critically affect apple fruit maturation patterns. While climacteric ethylene production is the primary indicator of ripening for either early or late-ripening cultivars, auxin may control the timing of activating climacteric ethylene biosynthesis through its cross-talk with ethylene pathways. Previous transcriptome analysis on apple cultivar ‘Honeycrisp’ and ‘Cripps Pink’ suggested that auxin transport and availability of auxin may be a key factor regulating the different ripening patterns and quality attributes between these two cultivars. In this study, the gene expression patterns of two auxin efflux carrier encoding genes, PIN 1 (MDP0000138035, Pin-formed protein) and AECFP (MDP0000344085; Auxin Efflux Carrier Family Protein), were characterized during apple maturation and ripening in six apple cultivars.  Using a qRT-PCR method, their gene expression profiles of relative transcript abundance and dynamic changes showed distinguishable features and correlated with the ripening season of the 6 cultivars investigated. Tissue-specificity expression indicated that the seed and core region are highly expressed sites for both genes, with lower expression in cortex (flesh) and peel tissues. The potential direction of auxin transportation (from core to outer cortex) seems to correspond with the ethylene production and starch clearing patterns progressing from the inner core outward. To get more insight of genotype-specific  regulation mechanisms for these two genes, the genomic copies including the promoter region, 5’UTR, 3’UTR, introns and exons were cloned and their sequence features were analyzed.