| The flower and fruit dropping is one of the common questions in grape production and the main factor affecting the industrial development. Plant hormones are closely related to the dropping of flowers and fruits. In crop production, gibberellins (GAs) have been widely used for many years to improve the qualities of grapevine berries, such as induction of seedless berry development, larger fruit size, and higher fruit number per cluster. But so far, the mechanism underlying exogenous GA regulation of fruit set and how endogenous plant hormones interact with each other during the set stage in grapevine are still unknown. In this study,’Fenghou’ grape (Vitis vinifera× V. labrusca), exhibiting a significant number of fruit drop during set stage, was used as plant material. The interaction between gibberllin and auxin or cytokinin during fruit set of grapevine were investigated at physiological, biochemical and molecular levels. In addition, the effect of exogenous application of GA on sucrose metabolism and fruit set was investigated. The main results are as follows:1. 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 x V. 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 GA3 biosynthesis inhibitor (PAC) 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. These results suggested that 4-CPA-and 6-BA-induced fruit set in grapevine require downstream gibberellin biosynthesis and GA is very important for the expansion of grape berry.2. The expression levels of VvGA20ox1, VvGA20ox2 and VvGA20ox3 in 4-CPA-and 6-BA-treated ovaries were higher than that in unpollinated ovaries between 1-14 DAA. The expression of VvGA3ox1 was much higher in pollinated 4-CPA-and 6-BA-treated ovaries than in unpollinated ovaries at 7 DAA. Expression of the VvGA3ox2 gene decreased from 1 to 21 DAA in all treatments after anthesis. In the case of VvGA3ox3, an increased transcript level was detected in pollinated and 4-CPA-induced fruits at 21 DAA. In contrast to GA biosynthesis genes, most catabolism-related VvGA2ox genes showed low expression in the fruits induced by pollination and 4-CPA and 6-BA treatment after anthesis, which was especially true for the VvGA2ox3 and VvGA2ox4 transcript levels, which were much lower in pollinated and 4-CPA- and 6-BA-induced fruits than in unpollinated ovaries at 7 DAA. These results suggested that the effect of 4-CPA and 6-BA on grape fruits set is mediated by active gibberellin through enhancement of GA biosynthesis and suppression of GA catabolism.3. GA application at 10 days before anthesis promoted fruit set of grapevine, and increased sugar content and acid invertase activity in the ovary, especially vacuolar invertase (VIN) activity during fruit set, compared with the control. However, the GA biosynthesis inhibitor paclobutrazol (PAC) had the opposite effect.4. Three Acid invertase genes were mainly expressed in the reproductive organ, whereas the expression of NI genes were almost not detected in the fruitlets or were found in all tissues. In situ hybridization showed that transcripts of VvCWIN, VvGINl and VvGIN2 were localized to the phloem, suggesting that these genes play an important role in regulating sink strength and sugar partitioning.5. Transcripts of VvGIN1 were significantly induced by GA treatment, but reduced by PAC treatment during fruit set. VvGIN2 transcripts in GA-treated fruits were also increased at 2-14 DAA, though its expression was not influenced by PAC treatment. In addition, GUS activity of tobacco transformants expressing the VvGIN1::GUS construct was higher than that of tobacco expressing the VvGIN2::GUS construct after GA treatment. These results indicted that GA can sitimulate the expression of VIN genes, thus enhancing VIN activity and carbohydrate import to promote fruit set of grapevine.In summary, application of gibberellin, auxin and cytokinin induced parthenocarpy in grapevine, although the development of parthenocarpic fruits induced by auxin and cytokinin were limited due to a less endogenous GA content during the fruit enlargement period. Auxin and cytokinin-induced fruit set in grapevine require downstream gibberellin biosynthesis. GA application at 10 days before anthesis increased sugar content and acid invertase activity in the ovary during fruit set by upregulating expression of VIN genes, expecially VvGIN1, thus promoting fruit set of grapevine. |
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