Transcriptional Regulation of Auxin Metabolism and Ethylene Biosynthesis Activation During Apple (Malus x domestica) Fruit Maturation

Apple cultivars exhibit considerable variation in fruit maturation patterns that could influence at-harvest fruit firmness and postharvest storability. Based on the results from our previous transcriptome profiling of apple fruit maturation and well-documented auxin-ethylene crosstalk in other plant processes, the current experiment attempts to get insight into the potential crosstalk between auxin metabolism and ethylene biosynthesis during apple fruit maturation and ripening. Weekly apple fruit samples were collected to include the early maturation stages until 2 weeks after physiological maturity for both 'Minneiska' and 'Scifresh' cultivars. The expression patterns for genes with annotated functions of auxin transport, conjugation, biosynthesis, and responses were profiled by qRT-PCR along the apple fruit ripening processes and in different apple fruit tissues (seed, core, cortex, and peel). The expression profiles of both auxin metabolism and ethylene biosynthesis genes correlated with the apple fruit maturation process, but with different expression patterns and strength in each cultivar. Temporal and spatial gene expression patterns from seed to the outer fruit tissues corresponded with the center-outward ripening characteristics of apple fruit. It seems that timely reduction of biologically active auxin, in apple fruit cells of specific tissue, is critical for the activation of ethylene biosynthesis, even though auxin is needed for early fruit development. Our results suggest that the regulation of auxin transport and homeostasis may contribute to the time of activation of the ethylene biosynthesis pathway in maturing apple fruit and consequently influence the time of ripening for a specific cultivar.