Multiple AUX/IAA–ARF modules regulate lateral root formation: the role ofArabidopsisSHY2/IAA3-mediated auxin signalling

Tatsuaki Goh,Tetsuro Mimura,Yuji Kamiya,Hidehiro Fukaki,Hiroyuki Kasahara

doi:10.1098/rstb.2011.0232

Abstract

In Arabidopsis thaliana, lateral root (LR) formation is regulated by multiple auxin/indole-3-acetic acid (Aux/IAA)–AUXIN RESPONSE FACTOR (ARF) modules: (i) the IAA28–ARFs module regulates LR founder cell specification; (ii) the SOLITARY-ROOT (SLR)/IAA14–ARF7–ARF19 module regulates nuclear migration and asymmetric cell divisions of the LR founder cells for LR initiation; and (iii) the BODENLOS/IAA12–MONOPTEROS/ARF5 module also regulates LR initiation and organogenesis. The number of Aux/IAA–ARF modules involved in LR formation remains unknown. In this study, we isolated the shy2-101 mutant, a gain-of-function allele of short hypocotyl2/suppressor of hy2 (shy2)/iaa3 in the Columbia accession. We demonstrated that the shy2-101 mutation not only strongly inhibits LR primordium development and emergence but also significantly increases the number of LR initiation sites with the activation of LATERAL ORGAN BOUNDARIES-DOMAIN16/ASYMMETRIC LEAVES2-LIKE18, a target gene of the SLR/IAA14–ARF7–ARF19 module. Genetic analysis revealed that enhanced LR initiation in shy2-101 depended on the SLR/IAA14–ARF7–ARF19 module. We also showed that the shy2 roots contain higher levels of endogenous IAA. These observations indicate that the SHY2/IAA3–ARF-signalling module regulates not only LR primordium development and emergence after SLR/IAA14–ARF7–ARF19 module-dependent LR initiation but also inhibits LR initiation by affecting auxin homeostasis, suggesting that multiple Aux/IAA–ARF modules cooperatively regulate the developmental steps during LR formation.

Highlights

In vascular plants, branched root systems arise through the production of lateral and adventitious roots, thereby allowing the plants to absorb water and nutrients from the soil and to sustain the aerial shoots [1]
We demonstrated that the shy2-101/ iaa3 mutation strongly inhibited lateral root (LR) primordium development and LR emergence as observed in the shy2-2 mutant in the Landsberg erecta (Ler) accession background, but the shy2-101/iaa3 mutation significantly increased LR initiation sites with the activation of LBD16/ASL18, a target gene of the SLR/IAA14 – ARF7 – ARF19 module
Our results indicate the critical role of the SUPPRESSOR OF HY2 (SHY2)/IAA3– AUXIN RESPONSE FACTOR (ARF) module in LR formation after SLR/IAA14– ARF7 – ARF19dependent LR initiation, suggesting that multiple auxin/indole-3-acetic acid (Aux/IAA) – ARF-signalling modules cooperatively regulate the developmental steps during LR formation

Summary

INTRODUCTION

In vascular plants, branched root systems arise through the production of lateral and adventitious roots, thereby allowing the plants to absorb water and nutrients from the soil and to sustain the aerial shoots [1]. (d) The shy mutations strongly elevate endogenous indole-3-acetic acid level in the roots Our genetic evidence that enhanced LR initiation in the shy101 mutant depends on the SLR/IAA14 – ARF7 – ARF19 auxin-signalling module suggests the possibility that the shy mutant might contain higher auxin levels in the roots, thereby increasing the number of LR initiation sites To clarify this point, we measured endogenous IAA levels in the roots of 7-day-old wild-type and shy seedlings by LC – ESI – MS/MS analysis (see §2). The shy101 roots had a much higher IAA level than shy, suggesting differences in the genetic background between the Ler and Col accessions (figure 4) These results indicate that the shy gain-of-function mutations increased endogenous auxin levels in roots, strongly suggesting that elevated auxin levels promoted LR initiation and resulted in the increased number of LR initiation sites in the shy101 mutant

DISCUSSION

Findings

LR emergence

CONCLUSIONS AND OUTLOOK