FTIP-Dependent STM Trafficking Regulates Shoot Meristem Development in Arabidopsis

Organogenesis in higher plants occurs in the shoot meristem, which contains pluripotent stem cells. Here, we show that two multiple C2 domain and transmembrane region proteins, FT INTERACTING PROTEIN 3 (FTIP3) and FTIP4, play an essential role in mediating proliferation and differentiation of shoot stem cells in Arabidopsis. FTIP3/4 prevent intracellular trafficking of a key regulator, SHOOTMERISTEMLESS (STM), to the plasma membrane in cells in the peripheral shoot meristem region. This facilitates STM recycling to the nucleus to maintain stem cells. Without FTIP3/4, STM localizes substantially to the plasma membrane, which promotes intercellular trafficking but compromises nuclear localization of STM. This accelerates stem cell differentiation, causing early termination of shoot apices and resulting in dwarf and bushy phenotypes. Our findings reveal a molecular framework that determines the fate of shoot stem cells and the resulting aboveground plant body.