Gibberellic acid induced parthenocarpic ‘Honeycrisp’ apples (Malus domestica) exhibit reduced ovary width and lower acidity

Fruit set and development are dependent on auxin, gibberellin, and cytokinin, which cause parthenocarpic development in many species when applied ectopically. Commercial sprays containing these hormones are used to improve apple fruit set, size, and shape, but have been implicated negatively in other aspects of fruit quality. We applied gibberellic acid (GA(3)), synthetic auxin (NAA), and the auxin-transport inhibitor NPA to ‘Honeycrisp’ apple flowers. Fruit retention and size were quantified throughout development, and seed number and fruit quality parameters were measured at maturity. GA(3) alone caused the development of seedless parthenocarpic apples. At maturity, GA(3)-treated apples were narrower due to reduced ovary width, indicating that GA(3) induced normal growth of the hypanthium, but not the ovary. GA(3)-treated fruits were also less acidic than hand-pollinated controls, but had similar firmness, starch, and sugar content. To further understand the regulation of parthenocarpy, we performed tissue-specific transcriptome analysis on GA(3)-treated, NAA-treated, and control fruits, at 18 days after treatment and again at maturity. Overall, transcriptome analysis showed GA(3)-treated and hand-pollinated fruits were highly similar in RNA expression profiles. Early expression differences in putative cell division, cytokinin degradation, and cell wall modification genes in GA(3)-treated ovaries correlated with the observed shape differences, while early expression differences in the acidity gene Ma1 may be responsible for the changes in pH. Taken together, our results indicate that GA(3) triggers the development of parthenocarpic apple fruit with morphological deviations that correlate with a number of candidate gene expression differences.