Plant cell elongation change in its rate during day and night alteration, reflecting differences in metabolism and involving cell wall remodeling that typically determine the rate and direction of cell expansion. How signal transduction is initiated by the cell wall remains elusive. Here, we show that the wall-associated kinase LENS (CELL LENGTH SUPPRESSOR) represses cell elongation in rice by restraining brassinosteroid signaling in the light, later releasing this repression via protein phosphorylation and degradation triggered by a rise in the ratio between methyl- and de-methyl-esterified pectin in the dark. LENS directly binds pectin and senses pectin changes via its extracellular domain to phosphorylate OsBRI1 at residue Thr752, thus hindering OsBRI1 interaction with its co-receptor OsSERK1/OsBAK1. We conclude that LENS is a cell wall-associated sensor that regulates cell elongation rates to adapt to the environment from the outside in, which complements the well-established inside-out signaling pathway affecting cell elongation in plants.