Abstract text: Cellulose and auxin are both crucial for plant growth, but their interaction remains unclear. We show that auxin regulates cellulose biosynthesis by directly modulating the phosphorylation of cellulose synthase (CESA). In Arabidopsis seedlings, inhibiting auxin biosynthesis reduces hypocotyl length, cellulose content, and CESA complex velocity—effects reversed by indole-3-acetic acid (IAA). Notably, we identified the plasma membrane protein kinases PMK1 and PMK2 as direct CESA3 interactors and that these kinases phosphorylated CESA3. Phosphorylation site mutants (CESA3D and CESA3A) revealed that CESA3D fully rescued the null mutant cesa3-3 phenotype, while CESA3A caused stunted hypocotyls, reduced cellulose, impaired CESA activity, and nearly abolished auxin responsiveness. Null pmk1/2 mutants phenocopied CESA3A, indicating that auxin controls cellulose synthesis through the PMKs. Our results outline a direct mechanism for how auxin coordinates cell wall relaxation and synthesis through the phosphorylation of proton pumps^[1] and the CESAs.

[1] Lin, W., Zhou, X., Tang, W. et al. TMK-based cell-surface auxin signalling activates cell-wall acidification. Nature 599, 278–282 (2021).