Receptor-like protein kinase required for cell elongation during vegetative growth, mostly in a brassinosteroid-(BR-) independent manner. Mediates the response of growing plant cells to the perturbation of cellulose synthesis and may act as a cell-wall-integrity sensor. Controls ectopic-lignin accumulation in cellulose-deficient mutant backgrounds. Expressed in most vegetative tissues, including leaves, stems and roots, primarily in expanding cells and vascular tissue. By brassinosteroids (BR). Autophosphorylated. No visible phenotype; due to redundancy with HERK1. Herk1 and the1 double mutants are stunted. In the1-4, shorter hypocotyls without brassinolide (BL) treatment. In the1-3, partially restored hypocotyl growth defect of prc1-8 and of other cellulose-deficient mutants. Semidominant suppressor of the cellulose-deficient mutant procuste1-1. This protein was called 'Theseus' after the Greek mythological figure Theseus, who slew the brigand Procustes. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.