Zhou, K.; Hepler, N. K.; Jia, M.; Cosgrove, D. J.

Plant cell wall enlargement is fundamental to crop productivity and its sensitivity to drought (1). Tip growth and diffuse growth are contrasting wall enlargement patterns often proposed to be limited by different processes: localized secretion and remodeling of pectins for tip growth versus loosening and sliding of cellulosic networks by -expansins (EXPAs) for diffuse growth (2,3). Here, we knocked out root-hair specific EXPA7 and EXPA18 in Arabidopsis, abolishing root-hair tip growth which was restored by complementation with genes from some, but not all, expansin clades. Notably, EXPA13 and EXPA20 failed to complement; they belong to two ancient clades lacking a highly conserved Asp considered essential for expansin activity. Mutation of this Asp in EXPA7 confirmed its requirement for wall enlargement. EXPA-mCherry fusions revealed widely contrasting patterns of subcellular trafficking and wall-binding for different EXPAs. The results demonstrate an essential EXPA requirement for root-hair tip growth and uncover a greater diversity of expansin functions than previously recognized.