COBL9 and COBL7 synergistically regulate root hair tip growth via controlling apical cellulose deposition.

Root hairs are cylindrical extensions of root epidermal cells that are important for the acquisition of water and minerals, interactions between plant and microbes. The deposition of cell wall materials in the tip enables root hairs to maintain elongation constantly. To date, our knowledge of the regulators that connect the architecture of cell wall and the root hair development remains very limited. Here, we demonstrated that COBL9 and COBL7, two genes of COBRA-Like family in Arabidopsis as well as their counterparts in rice, OsBC1L1 and OsBC1L8, regulate root hair growth. Single mutant cobl9, double mutants cobl7 cobl9 and double mutants osbc1l1 osbc1l8 all displayed prematurely terminated root hair elongation, though at varying levels. COBL7-YFP and COBL9-YFP accumulate prominently in the growing tips of newly emerged root hairs. Furthermore, cobl9, cobl7 cobl9 and osbc1l1 osbc1l8 mutants were defective in the enrichment of cellulose in the tips of the growing root hairs. We also discovered that overexpression of COBL9 could promote root hair elongation and salinity tolerance. Taken together, these results provide compelling evidence that the polarized COBL7 and COBL9 in the tip of the emerging root hairs have conserved roles in regulating root hair development and stress adaptation in dicots and monocots.