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ARS Home » Northeast Area » Kearneysville, West Virginia » Appalachian Fruit Research Laboratory » Innovative Fruit Production, Improvement, and Protection » Research » Publications at this Location » Publication #357798

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

item GALIMBA, KELSEY - University Of Maryland
item Bullock, Daniel - Dan
item Dardick, Christopher - Chris
item LIU, ZHONGCHI - University Of Maryland
item Callahan, Ann

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2019
Publication Date: 4/6/2019
Citation: Galimba, K.D., Bullock, D.G., Dardick, C.D., Liu, Z., Callahan, A.M. 2019. Gibberellic acid induced parthenocarpic ‘Honeycrisp’ apples (Malus domestica) exhibit reduced ovary width and lower acidity. Horticulture Research.

Interpretive Summary: Plant hormones can be used to increase apple fruit set, size and shape, because they somehow either mimic or bypass the normal pollination/fertilization signals. It’s not really known how they do this, and there is some evidence to suggest they may also have detrimental side effects to fruit quality. We applied a number of hormones to apple flowers, to study their effects. We found that one hormone, gibberellic acid, caused fruit to develop even though they weren’t pollinated, and the resulting fruit were very similar to pollinated fruit, though they were thinner and slightly less acidic. Looking closer, we determined that the thinness of the GA3 fruit was due to them having thinner ovaries (cores), but similar hypanthiums (the fleshy edible part of the apple). Next, we looked at gene expression, focusing on genes that had different expression patterns between GA3 and hand-pollinated fruits. We found a number of genes in the cell division and cell expansion pathways that might be responsible for the thin cores. We also found that one gene, which has been shown to be low in low-acid apple varieties, is also lower in the less-acidic GA3 fruit. These finding are important because although a number of studies have examined the effect on hormones on various aspects of apple fruit development, none have gone as in-depth to determine morphological or genetic responses. Pinpointing exact changes in the fruit and determining the genes involved helps characterize the function of these hormones in apples and also gives critical insight into how hormone applications might be used more efficiently.

Technical 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 in negatively affect other aspects of fruit quality. We applied gibberellin (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 thinner, due to thinner ovary width, and were also less acidic than hand-pollinated controls, but had similar firmness, starch and sugar content. In order to determine the genetic cause of ovary width and acidity differences, we performed transcriptome analysis on hypanthium, ovary and ovule/seed tissues from GA3-treated, NAA-treated and control fruits, at 18 days after treatment (DAT), and again at maturity (132 DAT). Overall, transcriptome analysis showed GA3-treated and hand-pollinated fruit were the most similar in RNA expression profiles. Early expression differences in putative cell division, cytokinin degradation and cell wall modification genes in GA3-treated ovaries are likely involved in the observed fruit shape differences, while early expression differences in the acidity gene Ma1 is likely responsible for the changes in pH. Taken together, our results indicate that GA3 is sufficient to trigger the development of parthenocarpic apple fruit with few deviations from normal fruit morphology, which correlate with a number of candidate gene expression differences.