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 + gradient across membranes. 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. Many cellular processes depend on specific steady-state ion concentrations within intracellular compartments. Plant nutrients are essential to numerous biochemical and physiological processes needed for plant growth and development. 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.
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Measurements of free IAA content and imaging of the auxin sensor DII-Venus, suggest that auxin accumulates in root tips of nhx1nhx2nhx3nhx4. Intracellular trafficking to the vacuole was also delayed in the mutant. 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.
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. Arabidopsis contains four vacuolar NHX isoforms known as AtNHX1 to AtNHX4. 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.
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