Auxin- and cytokinin-induced berries set in grapevine partly rely on enhanced gibberellin biosynthesis

Abstract

Gibberellins (GAs) have been widely used for many years to induce seedless grapevine and increase fruit set in production. However, the role of GAs and how they crosstalk with auxin and cytokinin during fruit set in grapevine remains unclarified. To investigate their role, GA3, 4-chlorophenoxyacetic acid (4-CPA), 6-benzylaminopurine (6-BA), and different hormone combinations were applied to unpollinated ovaries of the ‘Fenghou’ grape (Vitis vinifera × Vitis labrusca) at anthesis. The results showed that the application of 4-CPA, 6-BA, GA3, 4-CPA+GA3, 6-BA+GA3, and 4-CPA+6-BA induced parthenocarpy. Among them, 4-CPA-, 6-BA-, and 4-CPA+6-BA-induced fruits were smaller than those induced by GA3, 4-CPA+GA3, and 6-BA+GA3. Application of the GA biosynthesis inhibitor paclobutrazol reduced both the fresh weight and fruit set of 4-CPA- and 6-BA-induced fruits, but the effect of the inhibitor was reversed by the application of GA3. In contrast to unpollinated ovaries, the ovaries induced by 4-CPA and 6-BA showed enhanced accumulation of active GAs, but at a lower level than in pollinated ovaries between 7 and 21 days after anthesis (DAA). Gene analysis showed that the increase in the GA content in 4-CPA- and 6-BA-treated ovaries was the result of the upregulation of GA biosynthesis genes, such as VvGA3ox1, and suppression of GA catabolism genes, such as VvGA2ox3 and VvGA2ox4. In addition, the concentration of active IAA was enhanced in the ovaries induced by 6-BA, which was accompanied by the elevated expression of the IAA biosynthesis genes VvYUC2 and VvYUC6. These results suggested that 4-CPA- and 6-BA-induced fruit set in grapevine requires downstream gibberellin biosynthesis.