HOMEOBOX PROTEIN52 Mediates the Crosstalk between Ethylene and Auxin Signaling during Primary Root Elongation by Modulating Auxin Transport-Related Gene Expression

Abstract

The gaseous hormone ethylene participates in many physiological processes in plants. Ethylene-inhibited root elongation involves PIN-FORMED2 (PIN2)-mediated basipetal auxin transport, but the molecular mechanisms underlying the regulation of <i>PIN2</i> function by ethylene (and therefore auxin distribution) are poorly understood. Here, we report that the plant-specific and ethylene-responsive HD-Zip gene <i>HB52</i> is involved in ethylene-mediated inhibition of primary root elongation in <i>Arabidopsis thaliana</i> Biochemical and genetic analyses demonstrated that <i>HB52</i> is ethylene responsive and acts downstream of <i>ETHYLENE-INSENSITIVE3</i> (<i>EIN3</i>). <i>HB52</i> knockdown mutants displayed an ethylene-insensitive phenotype during primary root elongation, while its overexpression resulted in short roots, as observed in ethylene-treated plants. In addition, root auxin distribution and gravitropism were impaired in <i>HB52</i> knockdown and overexpression lines. Consistent with these findings, in vitro and in vivo binding experiments showed that HB52 regulates the expression of auxin transport-related genes, including <i>PIN2</i>, <i>WAVY ROOT GROWTH1</i> (<i>WAG1</i>), and <i>WAG2</i> by physically binding to their promoter regions. These findings suggest that HB52 functions in the ethylene-mediated inhibition of root elongation by modulating the expression of auxin transport components downstream of EIN3, revealing a mechanism in which HB52 acts as an important node in the crosstalk between ethylene and auxin signaling during plant growth and development.