Plant NHX cation/proton antiporters

Abstract

Although physiological and biochemical data since long suggested that Na(+)/H(+) and K(+)/H(+) antiporters are involved in intracellular ion and pH regulation in plants, it has taken a long time to identify genes encoding antiporters that could fulfil these roles. Genome sequencing projects have now shown that plants contain a very large number of putative Cation/Proton antiporters, the function of which is only beginning to be studied. The intracellular NHX transporters constitute the first Cation/Proton exchanger family studied in plants. The founding member, AtNHX1, was identified as an important salt tolerance determinant and suggested to catalyze Na(+) accumulation in vacuoles. It is, however, becoming increasingly clear, that this gene and other members of the family also play crucial roles in pH regulation and K(+) homeostasis, regulating processes from vesicle trafficking and cell expansion to plant development.

Ion Exchangers NHX1 and NHX2 Mediate Active Potassium Uptake into Vacuoles to Regulate Cell Turgor and Stomatal Function in Arabidopsis

“…The discovery that vacuolar NHX proteins were capable of exchanging Na + and H + across the tonoplast led to the now widespread view that NHX proteins mediate this critical process in plants faced with a saline environment (Apse et al, 1999;Gaxiola et al, 1999;Blumwald, 2000;Quintero et al, 2000). However, this notion has been recently challenged based on the biochemistry of NHX proteins, which do not discriminate between Na + and K + or have a preference for K + transport (Venema et al, 2002;Rodríguez-Rosales et al, 2009;Jiang et al, 2010). The lack of correlative evidence between greater salt tolerance and the enhancement of Na + accumulation in different plant species overexpressing NHX proteins from various sources has also been pointed out (Rodríguez-Rosales et al, 2009;Jiang et al, 2010).…”

“…However, this notion has been recently challenged based on the biochemistry of NHX proteins, which do not discriminate between Na + and K + or have a preference for K + transport (Venema et al, 2002;Rodríguez-Rosales et al, 2009;Jiang et al, 2010). The lack of correlative evidence between greater salt tolerance and the enhancement of Na + accumulation in different plant species overexpressing NHX proteins from various sources has also been pointed out (Rodríguez-Rosales et al, 2009;Jiang et al, 2010). Recently, Bassil et al (2011b) have shown that NHX1 and NHX2 proteins play a comparatively greater role in K + homeostasis than in Na + sequestration.…”

pH Regulation by NHX-Type Antiporters Is Required for Receptor-Mediated Protein Trafficking to the Vacuole in Arabidopsis

“…NHX-type Na + (K + )/H + exchangers are particularly important for the regulation of pH and ion homeostasis and have been implicated in a wide variety of physiological processes, including cell volume and expansion, osmotic adjustment, and stress responses (Rodríguez-Rosales et al, 2009;Bassil et al, 2012). They operate by exchanging luminal H + for Na + or K + and, therefore, regulate monovalent cation homeostasis in addition to functioning as "H + leaks" to fine-tune luminal pH by countering the acidity generated by the H + pumps (Orlowski and Grinstein, 2011).…”

Reduced Tonoplast Fast-Activating and Slow-Activating Channel Activity Is Essential for Conferring Salinity Tolerance in a Facultative Halophyte, Quinoa

“…The classic view is that this sequestration is achieved by tonoplast Na + /H + antiporters (Barkla et al, 1995;Flowers and Colmer, 2008), a process energized by both vacuolar H + pumps: ATPase (Ayala et al, 1996;Vera-Estrella et al, 1999;Wang et al, 2001) and pyrophosphatase (Parks et al, 2002;Vera-Estrella et al, 2005;Guo et al, 2006;Krebs et al, 2010). However, recent studies have added more complexity to the relationship between Na + /H + antiporters and vacuolar Na + sequestration, assigning a role to the transporter in the regulation of K + and H + homeostasis (for review, see Rodríguez-Rosales et al, 2009;Jiang et al, 2010; Bassil et al, 2011 (Yamaguchi et al, 2005). Consequently, other transporters, in addition to and different from NHX, are likely to be involved in vacuolar Na + sequestration.…”