Abstract text: Cellulose microfibrils are extruded into the cell wall by membrane-integrated Cellulose Synthases (CESA), which multimerize to form Cellulose Synthase Complexes (CSC). CESAs are synthesized in the endoplasmic reticulum and trafficked to the plasma membrane via the trans-Golgi network. We identified the paralogous integral membrane proteins SHOU4 and SHOU4L (SHOU4/4L) as negative regulators of CESA exocytosis in Arabidopsis thaliana. The double shou4shou4l mutant exhibits increased CESA exocytosis and elevated levels of non-crystalline cellulose, resulting in a partially infertile, dwarf plant with twisted roots and fewer root hairs. SHOU4/4L directly interact with the large cytoplasmic loop of multiple CESAs involved in primary cell wall biosynthesis. Recent work and publicly available data show that SHOU4/4L are differentially phosphorylated by several kinases, including the cell wall integrity sensor FERONIA and the pathogen-activated kinase BIK1, as well as in response to multiple signals, including ethylene and brassinosteroids. We hypothesize that SHOU4/4L integrate developmental and environmental signals to control cellulose biosynthesis by regulating the number of CSCs at the plasma membrane. To elucidate the molecular mechanisms by which SHOU4/4L regulate CSC exocytosis, we are investigating SHOU4/4L protein interactors, characterizing the effects of phosphorylation on SHOU4/4L activity, and interrogating the cortical microtubule array’s role in SHOU4/4L’s function.