Abstract
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 (GA3), 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. GA3 alone caused the development of seedless parthenocarpic apples. At maturity, GA3-treated apples were narrower due to reduced ovary width, indicating that GA3 induced normal growth of the hypanthium, but not the ovary. GA3-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 GA3-treated, NAA-treated, and control fruits, at 18 days after treatment and again at maturity. Overall, transcriptome analysis showed GA3-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 GA3-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 GA3 triggers the development of parthenocarpic apple fruit with morphological deviations that correlate with a number of candidate gene expression differences.
Highlights
The angiosperm fruit is a structure derived from the ovary of the flower, which functions to protect and disperse the seeds
To test whether gibberellin and auxin can cause parthenocarpic ‘Honeycrisp’ apple development, we treated flowers with these hormones and prevented pollination on these as well as a negative control (NC) to ensure developmental changes to the fruit were due to exogenous hormone treatment
To compliment the Differentially expressed gene (DEG) analyses, we performed to investigate the genetic causes of these morphological differences, we examined a number of candidate genes known to regulate either fruit acidity or fruit shape
Summary
The angiosperm fruit is a structure derived from the ovary of the flower, which functions to protect and disperse the seeds. Numerous genetic factors repress ovary development[1], and as long as they are active, the ovary will senesce and no fruit will form[2]. This default repression pathway can be overridden by fertilization signals that trigger fruit set, the first stage of fruit development[3]. Current evidence supports the major role of three fertilization-induced hormones, auxin, gibberellin (GA), and cytokinin in the regulation of fruit set[4]. In Arabidopsis, fertilization triggers an Galimba et al Horticulture Research (2019)6:41
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