The CELL NUMBER REGULATOR SlFWL5 protein regulates aerial vegetative growth in tomato, by promoting cell expansion

The FW2.2-LIKE/CELL NUMBER REGULATOR (FWL/CNR) gene family comprises PLAC8 domain-containing membrane-associated proteins and was named in reference to its founding member, the FW2.2 gene, which determines fruit size in tomato via a negative regulation on cell divisions. The function of PLAC8 domain-containing FWL/CNR proteins in plants remains largely unexplored. Only recently has the molecular and cellular mechanism of FW2.2 been described as regulating plasmodesmata-mediated cell-to-cell communication. In the present study, we provided a functional analysis of SlFWL genes in tomato, aiming at investigating any direct role in the control of organ growth. Based on a combination of molecular and cellular approaches, we selected three SlFWL proteins, namely SlFWL2, -4 and -5, which are localized at the plasma membrane. Gain- and loss-of-function transgenic plants were generated to explore their putative role as regulators of organ growth in tomato. This allowed us to shed light more specifically on the critical involvement of SlFWL5 in leaf and hypocotyl development. We show here that SlFWL5 is localized at plasmodesmata, and that it regulates leaf size and morphology, and hypocotyl growth, by promoting cell expansion, as unexpected for a CELL NUMBER REGULATOR protein. This original finding underscores further the importance of some FWL/CNR family members in growth regulation.