Auxin Homeostasis and Distribution of the Auxin Efflux Carrier PIN2 Require Vacuolar NHX-Type Cation/H+ Antiporter Activity.

Shiqi Zhang,Eiji Nambara,Hiromi Tajima,Eduardo Blumwald,Elias Bassil

doi:10.3390/plants9101311

Abstract

The Arabidopsis vacuolar Na+/H+ transporters (NHXs) are important regulators of intracellular pH, Na+ and K+ homeostasis and necessary for normal plant growth, development, and stress acclimation. Arabidopsis contains four vacuolar NHX isoforms known as AtNHX1 to AtNHX4. The quadruple knockout nhx1nhx2nhx3nhx4, lacking any vacuolar NHX-type antiporter activity, displayed auxin-related phenotypes including loss of apical dominance, reduced root growth, impaired gravitropism and less sensitivity to exogenous IAA and NAA, but not to 2,4-D. In nhx1nhx2nhx3nhx4, the abundance of the auxin efflux carrier PIN2, but not PIN1, was drastically reduced at the plasma membrane and was concomitant with an increase in PIN2 labeled intracellular vesicles. Intracellular trafficking to the vacuole was also delayed in the mutant. Measurements of free IAA content and imaging of the auxin sensor DII-Venus, suggest that auxin accumulates in root tips of nhx1nhx2nhx3nhx4. Collectively, our results indicate that vacuolar NHX dependent cation/H+ antiport activity is needed for proper auxin homeostasis, likely by affecting intracellular trafficking and distribution of the PIN2 efflux carrier.

Highlights

Plant nutrients are essential to numerous biochemical and physiological processes needed for plant growth and development
The loss of vacuolar NHX activity resulted in plant growth reduction with reduced rosette diameter and decreased growth [16]
The nhx1nhx2nhx3nhx4 mutant displayed significant growth and developmental phenotypes that could be auxin-related, including the loss of shoot apical dominance, aberrant branching and reduced root growth (Figure 1). We evaluated whether such phenotypes were associated with perturbations in auxin homeostasis, by examining the growth response of the nhx1nhx2nhx3nhx4 mutant to three auxin isoforms

Summary

Introduction

Plant nutrients are essential to numerous biochemical and physiological processes needed for plant growth and development. Many cellular processes depend on specific steady-state ion concentrations within intracellular compartments. Given the fundamental importance of cellular ion homeostasis, plants contain conserved primary and secondary transport systems and sophisticated mechanisms to regulate their activities to facilitate homeostasis [1]. Primary transporters are H+ -translocating enzymes, such as H+ -ATPases on the plasma membrane (PM) and intracellular organelles and vacuolar pyrophosphatase, which use the hydrolysis of ATP and PPi to establish an electrochemical H+. Secondary transporters use the H+ gradient generated by the. One important group of secondary transporters, known as the NHX-type cation-H+ antiporters, mediate the electroneutral exchange of H+ for either Na+ or K+ and, play key roles in pH regulation, Na+ sequestration, and intracellular K+ homeostasis.

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