ROOT HAIR DEFECTIVE SIX-LIKE Class I Genes Promote Root Hair Development in the Grass Brachypodium distachyon.

Chul Min Kim,Liam Dolan,Li-Jia Qu

doi:10.1371/journal.pgen.1006211

Abstract

Genes encoding ROOT HAIR DEFECTIVE SIX-LIKE (RSL) class I basic helix loop helix proteins are expressed in future root hair cells of the Arabidopsis thaliana root meristem where they positively regulate root hair cell development. Here we show that there are three RSL class I protein coding genes in the Brachypodium distachyon genome, BdRSL1, BdRSL2 and BdRSL3, and each is expressed in developing root hair cells after the asymmetric cell division that forms root hair cells and hairless epidermal cells. Expression of BdRSL class I genes is sufficient for root hair cell development: ectopic overexpression of any of the three RSL class I genes induces the development of root hairs in every cell of the root epidermis. Expression of BdRSL class I genes in root hairless Arabidopsis thaliana root hair defective 6 (Atrhd6) Atrsl1 double mutants, devoid of RSL class I function, restores root hair development indicating that the function of these proteins has been conserved. However, neither AtRSL nor BdRSL class I genes is sufficient for root hair development in A. thaliana. These data demonstrate that the spatial pattern of class I RSL activity can account for the pattern of root hair cell differentiation in B. distachyon. However, the spatial pattern of class I RSL activity cannot account for the spatial pattern of root hair cells in A. thaliana. Taken together these data indicate that that the functions of RSL class I proteins have been conserved among most angiosperms—monocots and eudicots—despite the dramatically different patterns of root hair cell development.

Highlights

Root hairs are filamentous extensions of epidermal cells that extend the absorbing surface of roots into the surrounding soil
Root hairs are tubular extensions that extend from specialized cells in the root surface
The spatial arrangement of root hair cells in grasses is very different from cresses like Arabidopsis thaliana

Summary

Introduction

Root hairs are filamentous extensions of epidermal cells that extend the absorbing surface of roots into the surrounding soil. They play essential functions in nutrient acquisition and are important for the uptake of nutrients with limited soil mobility such as phosphate [1,2]. In other taxa— including the cress family, the Brassicaeae—cell files comprising only root hair cells are flanked by two or more files that contain only hairless epidermal cells [8]. In Brassiceae files of root hair cells are located between a pair of underling cortical cells while hairless epidermal cell files develop over single cortical cells [9,10]. In the Poaceae the two epidermal cell types develop in any position relative to underlying cortical cells [4,7]

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Conclusion