The CELL NUMBER REGULATOR/FW2.2-like (CNR/FWL) gene family comprises PLAC8 domain-containing membrane-associated proteins. The CNR/FWL gene family 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 functional characterization of PLAC8 domaincontaining CNR/FWL proteins in plants is still very scarce, and only recently the molecular and cellular mode of action of FW2.2 was identified 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-offunction 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 development. We here showed that SlFWL5 is a plasmodesmata-localized protein that regulates leaf size and morphology via cell expansion, most probably through cell-to-cell communication. This original finding underscores further the importance of the CNR/FWL family members in growth regulation.Read less <