The Arabidopsis Na+/H+ Antiporters NHX1 and NHX2 Control Vacuolar pH and K+ Homeostasis to Regulate Growth, Flower Development, and Reproduction  

Abstract

Intracellular Na + /H + (NHX) antiporters have important roles in cellular pH and Na + , K + homeostasis. The six Arabidopsis thaliana intracellular NHX members are divided into two groups, endosomal (NHX5 and NHX6) and vacuolar (NHX1 to NHX4). Of the vacuolar members, NHX1 has been characterized functionally, but the remaining members have largely unknown roles. Using reverse genetics, we show that, unlike the single knockouts nhx1 or nhx2, the double knockout nhx1 nhx2 had significantly reduced growth, smaller cells, shorter hypocotyls in etiolated seedlings and abnormal stamens in mature flowers. Filaments of nhx1 nhx2 did not elongate and lacked the ability to dehisce and release pollen, resulting in a near lack of silique formation. Pollen viability and germination was not affected. Quantification of vacuolar pH and intravacuolar K + concentrations indicated that nhx1 nhx2 vacuoles were more acidic and accumulated only 30% of the wild-type K + concentration, highlighting the roles of NHX1 and NHX2 in mediating vacuolar K + /H + exchange. Growth under added Na + , but not K + , partly rescued the flower and growth phenotypes. Our results demonstrate the roles of NHX1 and NHX2 in regulating intravacuolar K + and pH, which are essential to cell expansion and flower development.

Tonoplast calcium sensors CBL2 and CBL3 control plant growth and ion homeostasis through regulating V-ATPase activity in Arabidopsis

“…These phenotypes are probably independent of mis-regulation of V-ATPase because vha-a2 vha-a3 did not show obvious phenotypes in response to external K + perturbations [34]. K + translocation into the lumen can be mediated by vacuolar NHX-type K + /H + antiporters such as NHX1 [51][52][53]. Relevant to this finding, the activity of NHX1 is shown to be regulated by another calcium sensor, calmodulin [54].…”

Salt acclimation processes in wheat

“…AtNHX1 and AtNHX2 are highly expressed in many tissues, while AtNHX3 and AtNHX4 are exclusively expressed in flowers and roots (Aharon et al, 2003, Yokoi et al, 2002. Recent results also suggested that NHX1 and NHX2 are vacuolar K + /H + exchangers that control vacuolar pH and K+ homeostasis, they are essential for active K + uptake at the tonoplast, osmotic adjustment and turgor regulation, and that they play a unique role in stomatal function (Bassil et al, 2011, Barragan et al, 2012.…”

Ectopic expression of Arabidopsis thaliana Na+(K+)/H+ antiporter gene, AtNHX5, enhances soybean salt tolerance

“…In the phylogenetic tree, AtNHX5 and 6 constituted a separate phylogenetic branch different from that formed by AtNHX1 to 4 (Yokoi et al, 2002;Aharon et al, 2003). Furthermore, AtNHX5 and 6 localize in the Golgi and trans-Golgi network (Bassil et al, 2011a), whereas AtNHX1 to 4 localize in the vacuole and tonoplast (Yokoi et al, 2002;Bassil et al, 2011b;Barragán et al, 2012). These data suggest existence of possibly different mechanisms by which AtNHX1 and 5 maintain the intracellular ion homeostasis and enhance the plant salt stress tolerance.…”

“…Among these eight NHXs, AtNHX1 is the most widely studied NHX; the roles of other NHXs remain largely unknown. Studies have revealed that AtNHX1 and 2 localize in the vacuole and tonoplast and have an important role in maintaining the intracellular pH and K + homeostasis (Yokoi et al, 2002;Bassil et al, 2011b;Barragán et al, 2012). Furthermore, atnhx1 and atnhx2 double knockout mutants displayed high sensitivity to salt stress, whereas overexpression of these genes increased the salt tolerance of the plants (Apse et al, 1999;Yokoi et al, 2002;Bassil et al, 2011b;Barragán et al, 2012).…”

“…Studies have revealed that AtNHX1 and 2 localize in the vacuole and tonoplast and have an important role in maintaining the intracellular pH and K + homeostasis (Yokoi et al, 2002;Bassil et al, 2011b;Barragán et al, 2012). Furthermore, atnhx1 and atnhx2 double knockout mutants displayed high sensitivity to salt stress, whereas overexpression of these genes increased the salt tolerance of the plants (Apse et al, 1999;Yokoi et al, 2002;Bassil et al, 2011b;Barragán et al, 2012). Interestingly, ectopic expression of AtNHX1 can enhance the salt stress tolerance of other plants, such as tomato, kiwifruit, wheat, tobacco, buckwheat, and peanut (Chen et al, 2008;Leidi et al, 2010;Zhou et al, 2011;Banjara et al, 2012;Moghaieb et al, 2014).…”