The Density and Length of Root Hairs Are Enhanced in Response to Cadmium and Arsenic by Modulating Gene Expressions Involved in Fate Determination and Morphogenesis of Root Hairs in Arabidopsis.

Ramin Bahmani,Jin A Kim,Dong G Kim,Seongbin Hwang

doi:10.3389/fpls.2016.01763

Abstract

Root hairs are tubular outgrowths that originate from epidermal cells. Exposure of Arabidopsis to cadmium (Cd) and arsenic [arsenite, As(III)] increases root hair density and length. To examine the underlying mechanism, we measured the expression of genes involved in fate determination and morphogenesis of root hairs. Cd and As(III) downregulated TTG1 and GL2 (negative regulators of fate determination) and upregulated GEM (positive regulator), suggesting that root hair fate determination is stimulated by Cd and As(III). Cd and As(III) increased the transcript levels of genes involved in root hair initiation (RHD6 and AXR2) and root hair elongation (AUX1, AXR1, ETR1, and EIN2) except CTR1. DR5::GUS transgenic Arabidopsis showed a higher DR5 expression in the root tip, suggesting that Cd and As(III) increased the auxin content in the root tip. Knockdown of TTG1 in Arabidopsis resulted in increased root hair density and decreased root hair length compared with the control (Col-0) on 1/2 MS media. This phenotype may be attributed to the downregulation of GL2 and CTR1 and upregulation of RHD6. By contrast, gem mutant plants displayed a decrease in root hair density and length with reduced expression of RHD6, AXR2, AUX1, AXR1, ETR1, CTR1, and EIN2. Taken together, our results indicate that fate determination, initiation, and elongation of root hairs are stimulated in response to Cd and As(III) through the modulation of the expression of genes involved in these processes in Arabidopsis.

Highlights

The metalloid arsenic (As) and the heavy metal cadmium (Cd) are highly toxic elements present in the environment, and they pose a considerable risk to living organisms owing to their accumulation in the soil and water (Godt et al, 2006; Zhao et al, 2009)
Cd and As(III) markedly downregulated TTG1 expression compared with that in the control, providing an explanation for the increment in root hair density induced by Cd and As(III), since TTG1 is a negative regulator of root hair differentiation (Figures 3A,B)
These results indicated that the Cd or As(III) induced increase in root hair density was probably caused by the inhibition of GL2 and TTG1 gene expression

Summary

Introduction

The metalloid arsenic (As) and the heavy metal cadmium (Cd) are highly toxic elements present in the environment, and they pose a considerable risk to living organisms owing to their accumulation in the soil and water (Godt et al, 2006; Zhao et al, 2009). Typical toxic effects of Cd on plants are the inhibition of primary root growth and increased lateral root density (Hu et al, 2013). The influence of Cd on root hairs has not been researched in detail, studies have shown that Cd at specific concentrations (that inhibit root growth by about 50%) increases root hair density and length in various plant species including radish (Vitoria et al, 2003), barley (Durcekova et al, 2007), sorghum (Kuriakose and Prasad, 2008), and Rhode grass (Kopittke et al, 2010). In Arabidopsis (Arabidopsis thaliana), root anatomy is well determined, and the different steps of root hair development including fate determination, initiation, bulge formation, and tip growth have been defined (Schiefelbein and Somerville, 1990; Dolan et al, 1993; Galway et al, 1994). Epidermal cells located over the anticlinal junction between two underlying cortical cells differentiate into trichoblast cells that develop root hairs, while epidermal cells that lie above the outer periclinal cortical cells become non-hair cell atrichoblasts (Masucci et al, 1996; Dolan, 2006)

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