Involved in the signaling pathway that regulates cell wall function, including cellulose biosynthesis, likely via an 1-aminocyclopropane-1-carboxylic acid (ACC)-mediated signal (a precursor of ethylene). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with the ACC synthases ACS5 and ACS9 but not ACS2, via the kinase domain. Expressed in the root meristem and elongation zone, and in hypocotyls of etiolated seedlings. Autophosphorylated. No visible phenotype. Fei1 and fei2 double mutants exhibit disrupted anisotropic expansion (e.g. during hypocotyl elongation), impaired synthesis of cell wall polymers, and abnormal cellulose biosynthesis. 'Fei' means fat in Chinese. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.