The tight regulators of fruit set initiation, gibberellin (GA) and auxin, have been applied for decades to induce parthenocarpy, fruit set without fertilization. The integration of GA and auxin signaling mediated by either GA or auxin application during parthenocarpy has been actively reported in tomato, and recently we reported that GA application at pre-bloom also activating auxin signaling and down-regulated negative regulators of fruit set initiation in grapevines. However, the activation of auxin signaling upon GA application without up-regulation of auxin biosynthesis is still unclear. In this study, expression patterns of three auxin efflux transporter genes, VvPIN1a, VvPIN2 and VvPIN4, were monitored during inflorescence development in ‘Tamnara’ grapevines with or without GA application. Without GA application, transcription levels of VvPIN1a and VvPIN4 gradually increased from 14 days before full bloom (DBF) to 2 and 5 days after full bloom (DAF), respectively, except down-regulation of VvPIN1a during 5 DBF to full bloom. However, VvPIN2 expression declined steadily after peaking at 10 DBF. With GA application, VvPIN1a did not show significantly different expression patterns when compared to no GA application, with the exception of 4-fold up-regulation at full bloom, but transcription of VvPIN4 was reduced between 5 and 2 DBF. In addition, VvPIN2 was down-regulated between 12 and 10 DBF by more than 50% compared to levels in the absence of GA application. These reductions of both VvPIN2 and VvPIN4 with GA application prior to pollination suggest that GA application might regulate auxin distribution, instead of auxin biosynthesis, to activating auxin signaling during parthenocarpic fruit initiation.

Gibberellic acid (GA) is a well-characterized plant hormone, which plays a critical role in various plant growth and development. including stem elongation, floral indcution and seed development. GA is known to cause enlargement of ripening fruits and, especially in grapevines, GA shows a unique function: the induction of seedlessness in seeded grape varieties. However, despite extensive previous studies about GA, there has been no clear verification of the mechanism that induces seedlessness in grapes. To understand how GA treatment results in artificial parthenocarpy of seeded grapes at molecular levels, we analyzed transcriptional changes in seeded grapes with and without GA application in various inflorescence developmental stages using RNA-seq. At 14 days before flowering (DBF), seeded grapes were treated with 100 ppm GA and clusters were collected at three developmental stages: 7 DBF, full bloom, and 5 days after flowering (DAF). Of a total of 28,974 genes that were mapped to grape genome reference sequences, 7,013 and 9,064 genes were up- and down-regulated, respectively, in the GA-treated grape as compared to the non-GA-treated control at 7 DBF, full bloom, and 5 DAF. Clustering analysis revealed that these genes could be grouped into 9 clusters with different expression patterns. We also carried out functional annotation based on gene ontology categories. There were significant differences in the expression of the GA and auxin-related gene families. These findings expand our understanding of the complex molecular and cellular mechanisms of GA-induced parthenocarpy of grapes and provide a foundation for future studies on seed development in grapevines.