Gibberellin Application at Pre-Bloom in Grapevines Down-Regulates the Expressions of <i>VvIAA9</i> and <i>VvARF7</i>, Negative Regulators of Fruit Set Initiation, during Parthenocarpic Fruit Development

<div><p>Fruit set is initiated only after fertilization and is tightly regulated primarily by gibberellins (GAs) and auxins. The application of either of these hormones induces parthenocarpy, fruit set without fertilization, but the molecular mechanism underlying this induction is poorly understood. In the present study, we have shown that the parthenocarpic fruits induced by GA application at pre-bloom result from the interaction of GA with auxin signaling. The transcriptional levels of the putative negative regulators of fruit set initiation, including <i>Vitis auxin/indole-3-acetic acid transcription factor 9</i> (<i>VvIAA9</i>), <i>Vitis auxin response factor 7</i> (<i>VvARF7</i>), and <i>VvARF8</i> were monitored during inflorescence development in seeded diploid ‘Tamnara’ grapevines with or without GA application. Without GA application, <i>VvIAA9, VvARF7</i>, and <i>VvARF8</i> were expressed at a relatively high level before full bloom, but decreased thereafter following pollination. After GA application at 14 days before full bloom (DBF); however, the expression levels of <i>VvIAA9</i> and <i>VvARF7</i> declined at 5 DBF prior to pollination. The effects of GA application on auxin levels or auxin signaling were also analyzed by monitoring the expression patterns of auxin biosynthesis genes and auxin-responsive genes with or without GA application. Transcription levels of the auxin biosynthesis genes <i>Vitis anthranilate synthase β subunit</i> (<i>VvASB1-like</i>), <i>Vitis YUCCA2</i> (<i>VvYUC2</i>), and <i>VvYUC6</i> were not significantly changed by GA application. However, the expressions of <i>Vitis Gretchen Hagen3.2</i> (<i>VvGH3.2</i>) and <i>VvGH3.3</i>, auxin-responsive genes, were up-regulated from 2 DBF to full bloom with GA application. Furthermore, the <i>Vitis</i> GA signaling gene, <i>VvDELLA</i> was up-regulated by GA application during 12 DBF to 7 DBF, prior to down-regulation of <i>VvIAA9</i> and <i>VvARF7</i>. These results suggest that <i>VvIAA9</i> and <i>VvARF7</i> are negative regulators of fruit set initiation in grapevines, and GA signaling is integrated with auxin signaling via <i>VvDELLA</i> during parthenocarpic fruit development in grapevines.</p>