Biotic Stress/Applied Plant Bio

Abstract

CS-8-5 - Vacuolar uptake of Na+ and K+ by tonoplast localized NHX-type cation/H+ antiporters

Sunday, July 15
4:38 PM - 4:58 PM

Na+/H+ (NHX-type) antiporters are important regulators of intracellular ion homeostasis and critical to growth and stress acclimation. In Arabidopsis, four distinct NHX isoforms, named NHX1 to NHX4, localize to the tonoplast. Previously we showed that NHX1 and NHX2 mediate vacuolar K+/H+ and Na+/H+ exchange and control vacuolar pH, K+ and Na+ homeostasis. Given the importance of NHX-type cation transport to vacuole cation homeostasis, a reverse functional genetic approach was used to examine the concerted role of all tonoplast NHXs on vacuolar Na+ and K+ homeostasis by comparing multiple T-DNA insertion knockout lacking all combinations of NHX1 to NHX4, including a quadruple knockout lacking any vacuolar NHX activity. Vacuolar NHX knockouts have unique and complex growth phenotypes, including reduced cell expansion, flower abnormalities, aberrant branching, and differential responses to K+ and Na+. Using immobilized, intact vacuoles loaded with cation selective dyes, we measured directly the vacuolar uptake of K+ and Na+. Kinetic analysis of cation transport indicates that NHX1, NHX2 are the main contributors to vacuolar K+ and Na+ uptake, while NHX3 and NHX4 different in Na+/K+ selectivity. The lack of any vacuolar NHX activity resulted in negligible K+ uptake, acidic vacuoles and reduced but not abolished vacuolar Na+ uptake. Notably exposure to moderate amounts of NaCl improved growth in some nhx knockouts, unlike K+ that was deleterious. These data resolve the Na+ and K+ transport kinetics of vacuolar NHXs, and show that certain isoforms have unique cation selectivities and contribute differentially to vacuolar ion homeostasis. Results also point to the existence of an alternate vacuolar Na+ conductive pathway(s) that is not likely to be H+-coupled. The implications of vacuolar NHX function to growth, development and stress acclimation will be discussed. 


 

Co-Authors

Eduardo Blumwald – Universiy of California

Elias Bassil

Assistant Professor
university of florida

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CS-8-5 - Vacuolar uptake of Na+ and K+ by tonoplast localized NHX-type cation/H+ antiporters



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