A cis-Element for Root Hair Specificity Has Been Co-opted Repeatedly Through the Divergence of Upstream Fate-Determining Machineries.

Three types of root hair cell distribution patterns are observed in vascular plants. These distinctive patterning types indicate that the fate of the hair/nonhair cell of the root epidermis in each of these patterns is decided by different fate-determining machineries. The cell fate determiners ultimately control the hair or nonhair cell-specific genes so as to modulate hair morphogenesis. Here, we address the issue of whether or not the hair cell specificity of gene expression, i.e., the regulatory cis-element for hair cell specificity, has been conserved despite the divergence of the upstream fate-determining machineries. Our recent study in Plant Cell (the November issue online) reported that orthologs and paralogs of Arabidopsis expansin A7 (AtEXPA7) from angiosperms with different hair-distribution types have maintained a common cis-element (RHE for Root Hair Element) for root hair cell specificity. The study also identified the same functional RHE among root hair-specific cell wall genes other than those coding for expansins. These findings suggest that the same regulatory module of RHE and its transcription factor partner, which endows cell morphogenetic worker genes with hair cell specificity, have been coopted by different upstream fate-determining masters over angiosperm evolution.