Homeobox transcription factor OsZHD2 promotes root meristem activity in rice by inducing ethylene biosynthesis.

Jinmi Yoon,Ki-Hong Jung,Tao Zhang,Christophe Périn,Wenzhu Yang,Charlotte Bureau,Yunfei Wu,Gynheung An,Ky Young Park,Richa Pasriga,Dabing Zhang,Yunde Zhao,Rongchen Wang,Soo Jin Wi,Lae-Hyeon Cho,Woo-Jong Hong,Kris Vissenberg

doi:10.1093/jxb/eraa209

Abstract

Root meristem activity is the most critical process influencing root development. Although several factors that regulate meristem activity have been identified in rice, studies on the enhancement of meristem activity in roots are limited. We identified a T-DNA activation tagging line of a zinc-finger homeobox gene, OsZHD2, which has longer seminal and lateral roots due to increased meristem activity. The phenotypes were confirmed in transgenic plants overexpressing OsZHD2. In addition, the overexpressing plants showed enhanced grain yield under low nutrient and paddy field conditions. OsZHD2 was preferentially expressed in the shoot apical meristem and root tips. Transcriptome analyses and quantitative real-time PCR experiments on roots from the activation tagging line and the wild type showed that genes for ethylene biosynthesis were up-regulated in the activation line. Ethylene levels were higher in the activation lines compared with the wild type. ChIP assay results suggested that OsZHD2 induces ethylene biosynthesis by controlling ACS5 directly. Treatment with ACC (1-aminocyclopropane-1-carboxylic acid), an ethylene precursor, induced the expression of the DR5 reporter at the root tip and stele, whereas treatment with an ethylene biosynthesis inhibitor, AVG (aminoethoxyvinylglycine), decreased that expression in both the wild type and the OsZHD2 overexpression line. These observations suggest that OsZHD2 enhances root meristem activity by influencing ethylene biosynthesis and, in turn, auxin.

Highlights

Root architecture influences nutrient and water uptake, anchorage, and mechanical support, interactions with microbes, and responses to various abiotic stress factors (Chen et al, 2015; Wang et al, 2018)
Analyses of transgenic rice plants carrying DR5::GUS and DR5::VENUS revealed that the expression of the DR5 reporter gene was induced following treatment with ACC, an ethylene precursor.The results suggest that OsZHD2 increases the biosynthesis of ethylene and subsequently auxin, which stimulates root growth
The results of the analyses revealed that OsZHD2 is highly expressed in the shoot apical meristem (SAM) (Fig. 2E).To evaluate whether OsZHD2 induces meristem activity, we treated seedling plants with 10 μM EdU, a thymidine analog, for 2 h to visualize the S-phase cells that actively incorporate EdU into DNA (Kotogány et al, 2010; Xu et al, 2017)

Summary

Introduction

Root architecture influences nutrient and water uptake, anchorage, and mechanical support, interactions with microbes, and responses to various abiotic stress factors (Chen et al, 2015; Wang et al, 2018). Since water and mineral supply are often limited in the soil, a plant with a more extensive root system exhibits higher performance with regard to the tolerance of drought and poor nutrient conditions (Rogers and Benfey, 2015). Several factors, including root angle, root growth rate, and root types, influence root architecture (Uga et al, 2013; Rogers and Benfey, 2015). To maintain root meristem activity, the rates of cell division and differentiation have to be coordinated (Xu et al, 2017). The interaction between cytokinin and auxin determines the size of the RAM through the regulation of the genes involved in auxin signaling and/or transport to ensure an appropriate auxin gradient (Ruzicka et al, 2007)

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Conclusion