A study on the morphological process and physiological and molecular changes of flower bud differentiation in Shine Muscat grape during fruit growing season

Abstract: 【Objective】In order to provide a theoretical reference for the regulation and the further study of the flower bud differentiation mechanism of grape, the flower bud differentiation process of Shine Muscat grape and the physiological changes and relative gene expression in fruit growth stage were studied.【Methods】4- year- old Shine Muscat grape trees were used as the materials. During the growing season, twenty winter buds at the fifth node of the new shoots were randomly collected every three days from the five leaves separating stage with three repeats. The dissection of the lamella of winter buds was performed by hand under stereomicroscope to observe the morphological changes of theflower bud differentiation process. The differentiation stages of each bud were counted and recorded. The flower bud differentiation of Shine Muscat was divided into six stages, namely leaf bud stage, flat top stage, dichotomy stage, anlagen differentiation stage, inflorescence primordium differentiation stage and inflorescence branching axis differentiation stage. Each differentiation stage was determined according to the flower bud differentiation stage of over 75% of the sampled winter buds. After the inflorescence primordium differentiation stage, the samplings were carried out every 20 days until the fruit harvest. Phenological periods were recorded for each flower bud differentiation stages and sampling days after the inflorescence primordium differentiation stage. Meanwhile, twenty winter buds were collected to determine the expression of nine key flowering genes VvFT, VvSOC1, VvLFY, VvAP1, VvFUL, VvAP2, VvAP3, VvAG and VvFLC, five leaves at the fifth node of the branch were sampled for the determination of total soluble sugar, starch, mineral elements N, P, K, Ca and Mg contents with three repeats. 【Results】The results showed that the flower bud differentiation of Shine Muscat winter buds started at the sixth leaf separating stage, and entered the anlagen differentiation stage at the full-blooming stage, then entered inflorescence primordium differentiation stage at the end of flowering stage. The content of N in the leaves maintained at 2.5-3.0 g·kg^-1 from the leaf bud stage to the inflorescence primordium differentiation stage, and then reached the lowest value at the stage of 20 days after inflorescence primordium differentiation. The changes of P, K, Ca and Mg content in the leaves were the same, these elements decreased at the beginning of morphological differentiation, and then increased twice. The first peak was found at the inflorescence primordium differentiation stage for P and Mg and at the stage of 40 days after inflorescence primordium differentiation for K, and the second peak was found at the stage of 100 days after inflorescence primordium differentiation for P, K and Mg. The content of Ca in the leaves showed an overall upward trend, with a low content at the leaf bud stage and flat top stage, a significant rise at dichotomy stage, and two peaks appeared at the stage of 40 and 100 days after the inflorescence primordium differentiation. Overall, the contents of P, K, Ca and Mg in the leaves decreased significantly at the stage of 60 days after the inflorescence primordium differentiation. The flower induction genes VvFT and VvSOC1 were highly expressed during the flower buds differentiation induction stage. The expression of the VvFT reached the first peak at the flat top stage and the second peak at the stage of 20 days after the inflorescence primordium differentiation. The expression of the VvSOC1 was significantly up-regulated at the flat-top stage and reached the peak at the dichotomy stage, but kept a low level after the stage of 40 days after the inflorescence primordium differentiation. The expression of the VvLFY, VvAP1 and VvFUL were all at a low level during the early stages of flower bud differentiation. The expression of the VvLFY reached a peak at the stage of 80 days after the inflorescence primordium differentiation. The expression of the VvAP1 increased from the stage of inflorescence primordium differentiation, and reached the highest level at the stage of 20 and 100 days after the inflorescence primordium differentiation. The expression level of the VvFUL increased at the stage of the inflorescence primordium differentiation, and reached the highest level at the stage of 80 days after the inflorescence primordium differentiation. The expression of the VvAP2 reached the highest level at the inflorescence primordium differentiation stage, and showed a significant upward trend from the stage of 40 to 100 days after the inflorescence primordium differentiation. During the flower bud differentiation, the expression trends of the VvAP3 and VvAG were consistent, the expressions of the VvAP3 and VvAG were down-regulated at the beginning of the flower bud differentiation, and reached highest level at the stage of 80 days after the inflorescence primordium differentiation. The expression level of the VvFLC was significantly lower from the inflorescence primordium differentiation stage to the stage of 40 daysafter the inflorescence primordium differentiation, but exhibited a relatively high level during the stages of 60 to 100 days after the inflorescence primordium differentiation.【Conclusion】The flower bud differentiation process of Shine Muscat grape was earlier in Nanning. More organic nutrition and main mineral elements were demanded during the inflorescence branching axis differentiation stage. In production, during the stage of before and after the enlargement and softening of Shine Muscat grapes, appropriate supplements of phosphorus, potassium, calcium and magnesium fertilizers are important to promote fruit development and normal differentiation of inflorescence branching axis. The VvFT and VvSOC1 were involved in the induction of primordium and inflorescence primordium differentiation. The VvLFY, VvAP1, VvFUL, VvAP2, VvAP3 and VvAG may be involved in the inflorescence primordium differentiation and further differentiation of development of branching axis. The high expression of the VvFLC may inhibit the expression of flower induction genes, during inflorescence branching axis differentiation stage.