Fig. 6. Scheme summarizing the events from pollination to fruit set, and the effect of preventing pollination and the auxin response.

A Pollination occurred at anthesis, then the pollen tubes were observed at 1 DPA, while fertilization markers were detected from 2 DAP onward. Cell number and length increased significantly from 6 to 10 DAP, and from 10 to 12 DAP, leading to fruit set at 12 DAP. Since exploratory RNA-seq suggested intense gene variation in IAA pathway, we explored the expression of auxin-related genes. From 0 to 2 DAP and from 2 to 6 DAP the expression of the auxin biosynthetic genes VvYUCCA10 and TAR2 decreased, respectively, possibly as the auxin response had already been triggered, which is consistent with the increase in the expression of the auxin-responsive genes VvGH3.5, VvSAUR50, and VvIAA11 from 2 to 6 DAP. The gene VvGH3.5 encodes a putative amido synthetase, which could also contribute to the control of the IAA levels after the auxin response activation. Dynamic metabolism of auxin is revealed by the presence of IPyA and IAM precursors, and the conjugates IAA-Asp, IAA-Glu, and OxIAA-Glu from pollination to fruit set. B When the auxin response inhibitor IAA-Trp was applied at 1 DAP, cell number and length decreased at 12 DAP compared with control fruits, and the expression of VvYUCCA10 was higher, which suggests that the auxin response inhibition might be counteracted with more auxin production. C A similar effect occurred when pistils remained unpollinated, where fruit set was even more impaired. Integrating A, B, and C, we propose that most of the fruit sizing effect of pollination is mediated by auxin during the grapevine fruit formation. Numbers within the fruits indicate the time after pollination (0, 1, 2, 6, 10, and 12 days after pollination, DAP)