The EXS Domain of PHO1 Participates in the Response of Shoots to Phosphate Deficiency via a Root-to-Shoot Signal.

Stefanie Wege,Evangelia Vogiatzaki,Ji-Yul Jung,Yves Poirier,Isabel Aller,Andreas J Meyer,Sylvain Pradervand,Ghazanfar Abbas Khan

doi:10.1104/pp.15.00975

Stefanie Wege, Evangelia Vogiatzaki + Show 6 more

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https://doi.org/10.1104/pp.15.00975

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Abstract

The response of shoots to phosphate (Pi) deficiency implicates long-distance communication between roots and shoots, but the participating components are poorly understood. We have studied the topology of the Arabidopsis (Arabidopsis thaliana) PHOSPHATE1 (PHO1) Pi exporter and defined the functions of its different domains in Pi homeostasis and signaling. The results indicate that the amino and carboxyl termini of PHO1 are both oriented toward the cytosol and that the protein spans the membrane twice in the EXS domain, resulting in a total of six transmembrane α-helices. Using transient expression in Nicotiana benthamiana leaf, we demonstrated that the EXS domain of PHO1 is essential for Pi export activity and proper localization to the Golgi and trans-Golgi network, although the EXS domain by itself cannot mediate Pi export. In contrast, removal of the amino-terminal hydrophilic SPX domain does not affect the Pi export capacity of the truncated PHO1 in N. benthamiana. While the Arabidopsis pho1 mutant has low shoot Pi and shows all the hallmarks associated with Pi deficiency, including poor shoot growth and overexpression of numerous Pi deficiency-responsive genes, expression of only the EXS domain of PHO1 in the roots of the pho1 mutant results in a remarkable improvement of shoot growth despite low shoot Pi. Transcriptomic analysis of pho1 expressing the EXS domain indicates an attenuation of the Pi signaling cascade and the up-regulation of genes involved in cell wall synthesis and the synthesis or response to several phytohormones in leaves as well as an altered expression of genes responsive to abscisic acid in roots.

Highlights

The response of shoots to phosphate (Pi) deficiency implicates long-distance communication between roots and shoots, but the participating components are poorly understood
We concentrated our effort to study the topology of the EXS domain, while investigation of the N-terminal domain was limited to determining whether the hydrophilic SPX domain was in the cytosol or the Golgi/trans-Golgi network (TGN) lumen
For localization of the SPX domain, we used a method based on bimolecular fluorescence complementation (BiFC) in transiently transformed N. benthamiana leaves that is adapted for membrane proteins located in the endoplasmic reticulum (ER)/ Golgi (Zamyatnin et al, 2006; Sparkes et al, 2010)

Summary

Introduction

The response of shoots to phosphate (Pi) deficiency implicates long-distance communication between roots and shoots, but the participating components are poorly understood. Using transient expression in Nicotiana benthamiana leaf, we demonstrated that the EXS domain of PHO1 is essential for Pi export activity and proper localization to the Golgi and trans-Golgi network, the EXS domain by itself cannot mediate Pi export. In addition to PHO1, plants have a number of SPX-containing proteins, including NLA1, a protein involved in ubiquitination of the PHT1 Pi transporter (Kant et al, 2011; Lin et al, 2013), and the SPX1, SPX2, and SPX4 proteins that bind to and modulate the activity of PHR1, the primary transcription factor mediating the response to Pi deficiency in plants (Lv et al, 2014; Puga et al, 2014; Wang et al, 2014). Expression of only the EXS domain in roots of the pho mutant resulted in the stimulation of shoot growth and the attenuation of the Pi deficiency signaling cascade, revealing an important role for the EXS domain in modulating the Pi deficiency response via a long-distance root-to-shoot signal

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