SlKNUCKLES regulates floral meristem activity and controls fruit size in Solanum lycopersicum

Timed termination of floral meristem (FM) is crucial for proper development of floral organs and fruits. In Solanum lycopersicum, CLAVATA3 (CLV3)-WUSCHEL (WUS) feedback regulation maintains FM homeostasis in early stage of floral buds. It is known that the zinc finger protein SlKNUCKLES (SlKNU) functions to promote FM determinacy by directly repressing the stem cell identity gene SlWUS. However, how the robust FM activity is suppressed to secure fruit development is not fully understood in tomato. Here, we demonstrate that SlKNU also directly represses the stem cell marker gene SlCLV3 and the receptor gene SlCLV1 for FM determinacy control. Besides, loss-of-function mutants of SlKNU generated by CRISPR-Cas9 show increased fruit size of tomato. Moreover, overexpression of SlKNU attenuates the activities of the shoot apical meristem (SAM) and FM in Arabidopsis, but normal carpel development is still maintained. Hence, although the function of KNU in tomato and Arabidopsis may diverge during evolution, the role of KNU for FM determinacy and fruit size control is conserved and may potentially be useful for enhancing fruit yield of tomato.