The CsHEC1-CsOVATE module contributes to fruit neck length variation via modulating auxin biosynthesis in cucumber

Fruit neck is the proximal portion of the fruit with undesirable taste that has detrimental effects on fruit shape and commercial value in cucumber. Despite the dramatic variations in fruit neck length of cucumber germplasms, the genes and regulatory mechanisms underlying fruit neck elongation remain mysterious. In this study, we found that Cucumis sativus HECATE1 (CsHEC1) was highly expressed in fruit neck. Knockout of CsHEC1 resulted in shortened fruit neck and decreased auxin accumulation, whereas overexpression of CsHEC1 displayed the opposite effects, suggesting that CsHEC1 positively regulated fruit neck length by modulating local auxin level. Further analysis showed that CsHEC1 directly bound to the promoter of the auxin biosynthesis gene YUCCA4 (CsYUC4) and activated its expression. Enhanced expression of CsYUC4 resulted in elongated fruit neck and elevated auxin content. Moreover, knockout of CsOVATE resulted in longer fruit neck and higher auxin. Genetic and biochemical data showed that CsOVATE physically interacted with CsHEC1 to antagonize its function by attenuating the CsHEC1-mediated CsYUC4 transcriptional activation. In cucumber germplasms, the expression of CsHEC1 and CsYUC4 positively correlated with fruit neck length, while that of CsOVATE showed a negative correlation. Together, our results revealed a CsHEC1-CsOVATE regulatory module that confers fruit neck length variation via CsYUC4-mediated auxin biosynthesis in cucumber.