2018 ASHS Annual Conference

Fruit set and development are dependent on the plant hormones auxin and gibberellin, which can create parthenocarpic, seedless fruit in many species when applied ectopically. In the apple fruit, a pome, these hormones trigger the enlargement of the hypanthium, a cup-like structure which surrounds the ovary and forms the majority of the fruit flesh. Sprays containing these hormones are available commercially with the purpose of improving apple fruit set, size and shape, but results may vary. This variability, in combination with reports that hormone sprays may cause lower fruit quality at maturity, highlight the need for research in this area. We applied gibberellic acid (GA₃), synthetic auxin (NAA) and both, in addition to the auxin-transport inhibitor NPA to ‘Honeycrisp’ apple flowers. Our results showed that GA₃ applications alone caused the development of parthenocarpic apples. At maturity, GA₃-treated apples had a slightly different shape but were similar in quality to hand-pollinated controls, though they were less acidic. This contradicts previous reports of accelerated ripening leading to quality issues at harvest. We performed RNA-sequencing analysis on hypanthium, ovary and ovule/seed tissues from GA₃-treated, NAA-treated and control fruits, and found that while GA₃-treated parthenocarpic and hand-pollinated fruit were the most similar in RNA expression profiles, interesting differences do occur. We focused on the apple GA20ox, GA3ox and GA2ox orthologs, which have been shown in other species to synthesize or degrade bioactive GA. We found that many members of these families exhibit unique responses to pollination, GA₃ and/or NAA treatment, which may play a role in the morphological differences we observed. Taken together, our data have led to a preliminary characterization of the roles of these genes and expands upon the function of GA in apple fruit development.