CsFUL1 and auxin participate in the CsCLV3-CsWUS mediated carpel number variation in cucumber (Cucumis sativus L.)

The carpel number (CN) is an important fruit trait affecting fruit shape, size, and internal quality in cucumber. CsCLAVATA3 (CsCLV3) was previously showed to be the simply inherited gene responsible for carpel number variation in cucumber, but the molecular mechanism of CsCLV3 regulating carpel number remains elusive. Here, we found that the expression of CsCLV3 was negatively correlated with carpel number variation in different cucumber lines. Knock down of CsCLV3 by RNAi led to increased number of petals and carpels, suggesting that CsCLV3 functions as a negative regulator for floral organ number in cucumber. WUSCHEL (WUS) has been well characterized to promote CLV3-expressing stem cell activity in a non-cell autonomous manner to regulate meristem maintenance and floral organ number. However, here we found the expression region of CsCLV3 overlaps with CsWUS in the basal domain of meristem, and CsCLV3 interact with CsWUS at the protein level through binding to the WUS-box motif. Overexpression of CsFUL1, a FRUITFULL-like MADS-box gene involved in fruit length regulation, resulted in increased number of floral organs in cucumber. Biochemical analyses indicated that CsFUL1 can directly bind to CsWUS promoter to stimulate its expression. Further, we found that auxin participates in carpel number variation in cucumber through physical interaction of AUXIN RESPONSE FACTOR 14 (CsARF14) and CsWUS. Therefore, CsFUL1 and CsARF14 are two new players in the WUS-CLV pathway in determining carpel number variation in cucumber. Overall design: Fruits at anthesis day of 32X and GFC cucumber plants were used for RNA-seq analysis. Three biological replicates were performed for each sample.