IQ domain protein CsSUN interacts with the Calmodulin protein CsCaM11 to regulate fruit length of cucumber

Fruit length is an important fruit quality which greatly influences the preference of consumers, but the underlying genetic mechanism regulating the fruit length in cucumber remains not very clear. In this study, the candidate gene CsSUN about the major QTL FS1.1 was cloned based on the near-isogenic lines (NILs) sun161 and SUN162. CsSUN had a 161 bp deletion in the first exon, giving rise to frameshift mutation and translation termination in the round-shape fruit of cucumber lines. The fruit length reduced by 38% to 46% when CsSUN was knocked out using CRISPR/Cas9 method in cucumber. It is found that the calcium ion levels in sun161 fruits were significantly 15.2% higher than those in SUN162. The calmodulin gene CsCaM11 was highly negatively correlated with fruit length through the regulatory networks involving the fruit length development of the NILs. To identify the function of CsCaM11, the yeast two-hybrid was carried out and it was found that CsCaM11 interacted with CsSUN, and the fruit length reduced by 5.0% to 7.5% compared with WT when CsCaM11 was overexpressed in cucumber, while the fruit length increased by 17.9% compared with WT when Cscam11 was knocked out. Further double-mutant cam11sun phenotypic analysis and subcellular co-localization of the two genes were performed, which revealed that CsCaM11 was epistatic to CsSUN, and that the interaction between them altered the subcellular distribution of CsCaM11. A model was established and elucidated that CsSUN positively regulated the fruit elongation, and CsCaM11 suppressed the fruit elongation by interacting with CsSUN protein.