The IQ67-domain protein IQD1 regulates fruit shape through complex multiprotein interactions in pepper (Capsicum annuum L.).

Natural genetic variation can be used to improve important crop agronomic traits, and understanding the genetic basis of natural variation in fruit shape can help breeders develop pepper varieties that meet market demand. In this study, we identified a QTL controlling fruit length-width ratio by conventional genetic mapping, encoding a previously uncharacterized gene CaIQD1. Reduced CaIQD1 expression resulted in short and wide fruits in pepper, whereas heterologous overexpression of CaIQD1 resulted in narrower fruits in tomato. Further experiments suggested that CaIQD1 regulates fruit shape in pepper by affecting cell proliferation, expansion and morphological changes. CaIQD1 also has a direct protein interaction with CaOFP20 in CaTRM-like-CaOFP20. Reduced CaOFP20 expression caused pepper fruits to become elongated and curved, whereas reduced CaTRM-like expression led to the formation of rounder fruits. These gene expression changes had a significant effect on the expression of genes related to the cell cycle and cell expansion. The CaTRM-like-CaOFP20-CaIQD1 module may thus represent a conserved regulatory pathway for controlling pepper fruit shape. CaIQD1 also showed direct interactions with the pepper calmodulin CaCaM7, the tubulin CaMAP70-2 and the microtubule motor protein CaKLCR1, suggesting that the regulation of fruit shape by CaIQD1 is related to changes in microtubule dynamics mediated by Ca-CaM. We also found that CaIQD1 interacts with several homologues of genes that typically regulate fruit shape in other plant species. In summary, our results show that CaIQD1 acts as a core hub in regulating pepper fruit shape through interactions with multiple proteins.