UF, a WOX gene, regulates a novel phenotype of un-fused flower in tomato.

Flower formation is a basic condition for fruit set in all flowering plants. The normal stamen of tomato flower fused together to form a yellow cylinder surrounding the carpels. In this study, we identified an un-fused flower (uf) tomato mutant that is defective in petal, carpal and stamen fusion and lateral outgrowth. After RNA-seq-based BSA (BSR), the candidate region location was identified in the long arm of chromosome 3. Using map-based cloning with InDel and CAPS markers, the UF candidate gene was mapped in a 104 kb region. In this region, a WOX (WUSCHEL-related homeobox) transcription factor SlWOX1 was considered as a candidate of UF as there is a 72bp deletion in its second exon in uf mutant. The mutations of SlWOX1 generated by CRISPR/CAS9 approach under wild-type background reproduced the phenotypes of uf mutant, indicating that the SlWOX1 gene is indeed UF. Interestingly, expression analysis of organ lateral polarity determinant genes showed that abaxial genes (SlYABBY5 and SlARF4) and adaxial genes (AS and HD-ZIPIII) were significantly down-regulated in the uf mutant, which is different to that in Arabidopsis and petunia. In conclusion, this work revealed a novel function of SlWOX1 in the regulation of flower development in tomato.