Root Hair-Specific EXPANSIN B Genes Have Been Selected for Graminaceae Root Hairs

Cell differentiation ultimately relies on the regulation of cell type-specific genes. For a root hair cell to undergo morphogenesis, diverse cellular processes including cell-wall loosening must occur in a root hair cell-specific manner. Previously, we identified and characterized root hair-specific cis-elements (RHE) from the genes encoding the cell wall-loosening protein EXPANSIN A (EXPA) which functions preferentially on dicot cell walls. This study reports two root hair-specific grass EXPB genes that contain RHEs. These genes are thought to encode proteins that function more efficiently on grass cell walls. The proximal promoter regions of two orthologous EXPB genes from rice (Oryza sativa; OsEXPB5) and barley (Hordeum vulgare; HvEXPB1) included RHE motifs. These promoters could direct root hair-specific expression of green fluorescent protein (GFP) in the roots of rice and Arabidopsis (Arabidopsis thaliana). Promoter deletion analyses demonstrated that the RHE motifs are necessary for root hair-specific expression of these EXPB promoters. Phylogenetic analysis of EXP protein sequences indicated that grass EXPBs are the only orthologs to these root hair-specific EXPBs, separating dicot EXPBs to distal branches of the tree. These results suggest that RHE-containing root hair-specific EXPB genes have evolved for grass-specific cell wall modification during root hair morphogenesis.