Intracellularly Localized PIN-FORMED8 Promotes Lateral Root Emergence in Arabidopsis.

Hyodong Lee,Minho Park,Richard Dongwook Lee,Hyung-Taeg Cho,Anindya Ganguly

doi:10.3389/fpls.2019.01808

Abstract

PIN-FORMED (PIN) auxin efflux carriers with a long central hydrophilic loop (long PINs) have been implicated in organogenesis. However, the role of short hydrophilic loop PINs (short PINs) in organogenesis is largely unknown. In this study, we investigated the role of a short PIN, PIN8, in lateral root (LR) development in Arabidopsis thaliana. The loss-of-function mutation in PIN8 significantly decreased LR density, mostly by affecting the emergence stage. PIN8 showed a sporadic expression pattern along the root vascular cells in the phloem, where the PIN8 protein predominantly localized to intracellular compartments. During LR primordium development, PIN8 was expressed at the late stage. Plasma membrane (PM)-localized long PINs suppressed LR formation when expressed in the PIN8 domain. Conversely, an auxin influx carrier, AUX1, restored the wild-type (WT) LR density when expressed in the PIN8 domain of the pin8 mutant root. Moreover, LR emergence was considerably inhibited when AXR2-1, the dominant negative form of Aux/IAA7, compromised auxin signaling in the PIN8 domain. Consistent with these observations, the expression of many genes implicated in late LR development was suppressed in the pin8 mutant compared with the WT. Our results suggest that the intracellularly localized PIN8 affects LR development most likely by modulating intracellular auxin translocation. Thus, the function of PIN8 is distinctive from that of PM-localized long PINs, where they generate local auxin gradients for organogenesis by conducting cell-to-cell auxin reflux.

Highlights

Auxin plays a critical role in plant growth and development by forming local concentration gradients
The expression of PIN8 was not detected in this region before stage VIII of lateral root (LR) development (Figures 2B, C), suggesting that PIN8 is involved in LR primordium development mainly at a late stage
In our gene expression analysis, most of the genes engaged in early LR development did not show a significant difference in expression levels between the WT and pin8 mutant (Supplementary Figure S10), the expression of GATA23 was suppressed in the mutant background (Figure 6C). These results suggest that the PIN8-mediated change in auxin homeostasis in phloem cells is involved in the transcriptional regulation of a battery of genes involved in LR development at the emergence stage

Summary

Introduction

Auxin plays a critical role in plant growth and development by forming local concentration gradients. PIN8 was shown to be expressed in the leaf vein and the pollen, where it is localized to intracellular compartments (Dal Bosco et al, 2012; Ding et al, 2012; Sawchuk et al, 2013; Verna et al, 2015). When ectopically expressed, PIN8 shows intracellular or PM localization in the root epidermal cells, depending on the developmental stage (Ganguly et al, 2014). Whenever it is localized predominantly to the PM, PIN8, like other long PINs, shows an obvious auxin-exporting activity in the Arabidopsis root hair system and tobacco suspension cells (Ganguly et al, 2010). It is conceivable that, depending on the cell type and developmental stage, short PINs are able to regulate intracellular auxin homeostasis and facilitate intercellular auxin transport

Methods

Results

Conclusion