Study On The Roles Of COBRA-LIKE 7 And 9 In Regulating Root Hair Development In Arabidopsis

Root hairs are cylindrical extensions of root epidermal cells that are important to the acquisition of water and minerals and the interactions between plant and microbes.Many transcription factors as well as ion gradients,extracellular pH and reactive oxygen species have been documented to regulate the initiation and tip growth of root hair in plant.However,our knowledge of the connections between the architecture of plant cell wall and the root hair formation and development remains very limited.Mutations in COBL9,a member of COBLA-Like(COBL)gene family have been reported to result in shorter root hair in Arabidopsis.In this study,we found that COBL9,together with one of its close homologs,COBL7 but not COBL8,regulate the development of root hairs through modulating the polymer composition of cell wall.We first demonstrated the high expression of gene COBL9 and its two close homologous gene,COBL7 and COBL8 in root hair.When examined the subcellular localizations of COBL7,COBL8 and COBL9,we observed that COBL7 and COBL9 accumulate prominently at the tip of newly emerged root hair,whereas in the mature hairs,the fluorescence signals of COBL7-YFP and COBL9-YFP distribute rather uniformly at the cell periphery.The spatio-temporal localization patterns of COBL7 and COBL9 are reminiscent of the pattern of cellulose deposition.Intriguingly,neither is the initiation of COBL8 similar to those of COBL7 and COBL9,nor does its subcellular localization show appreciable change during root hair growth.Single mutant of COBL7 results in a moderate inhibition of root hair growth,whereas COBL8 mutant does not affect the form of root hairs,hence conceivably it is the knock-down of COBL7,not COBL8 that greatly aggravates the development defects of root hairs in cobl9 plants.In contrast to the approximately 30%cell burst and prematurely terminated root hair elongation in cobl9,nearly all hairs in double mutant cob17/cob19 go burst and stop growing,suggesting COBL7 synergistically regulates the development of root hair with COBL9.Moreover,we observed that in the misshaped root hairs of both the single cobl9 and the double cobl7/cobl9 mutants there is no enrichment of the cellulose at the tip of young root hairs in contrast to that is tightly associated with the root hair in the wild-type plants,indicating COBL7 and COBL9 are required for the cellulose synthesis,in line with the observations made on several other members of the COBL family.In summary,our work reveals that the polarized COBL7 and COBL9 at the tip of the emerging root hairs have a key role to play in regulating cellulose synthesis and governing the root hair development in Arabidopsis.