Diverse Functions and Molecular Properties Emerging for CAX Cation/H+ Exchangers in Plants

Abstract

Steep concentration gradients of many ions are actively maintained, with lower concentrations typically located in the cytosol, and higher concentrations in organelles and outside the cell. The vacuole is an important storage organelle for many ions. The concentration gradient of cations is established across the plant tonoplast, in part, by high-capacity cation/H+ (CAX) exchange activity. While plants may not be green yeast, analysis of CAX regulation and substrate specificity has been greatly aided by utilizing yeast as an experimental tool. The basic CAX biology in ARABIDOPSIS has immediate relevance toward understanding the functional interplay between diverse transport processes. The long-range applied goals are to identify novel transporters and express them in crop plants in order to "mine" nutrients out of the soil and into plants. In doing so, this could boost the levels of essential nutrients in plants.

Calcium Pumps and Interacting BON1 Protein Modulate Calcium Signature, Stomatal Closure, and Plant Immunity

“…/H + antiporters driven by ATP and proton motive force, respectively Sze et al, 2000;Shigaki and Hirschi, 2006). Ca 2+ pumps fall into two groups: type IIA, or ECA, for ER-type Ca 2+ -ATPases, and type IIB, or ACA, for autoinhibited Ca 2+ -ATPase .…”

CAX‐ing a wide net: Cation/H+ transporters in metal remediation and abiotic stress signalling

“…Inducing enhanced CAX expression in plants also suggests that these transporters have a role in salt stress response (Shigaki & Hirschi 2006; Manohar et al . 2011a).…”

“…1996), many studies have examined the biochemical, genetic and physiological functions of these transport proteins, principally for isoforms from Arabidopsis thaliana and rice ( Oryza sativa ), and often by using the approaches of yeast heterologous expression and T‐DNA insertion mutagenesis (for review see Shigaki & Hirschi 2006; Manohar et al . 2011a).…”

Cell-Specific Vacuolar Calcium Storage Mediated by CAX1 Regulates Apoplastic Calcium Concentration, Gas Exchange, and Plant Productivity in Arabidopsis

“…The Ca 2+ /H + antiporters (CAX) have lower transport affinities for Ca 2+ than ACA proteins (CAX1, K m = 10 to 15 mM; Hirschi et al, 1996), possess an N-terminal autoinhibitory domain, and use the proton-motive force across the tonoplast to remove Ca 2+ from the cytosol when free [Ca 2+ ] cyt is significantly above resting levels. In Arabidopsis, there are six CAX genes belonging to two clades, CAX1, 3, and 4 within clade I-A and CAX2, 5, and 6 within clade I-B (Shigaki and Hirschi, 2006). Overexpression of modified CAX proteins, some with the autoinhibitory domain removed (sCAX), result in significant increases in Ca content in a variety of crops (Park et al, 2005a(Park et al, , 2005bMorris et al, 2008) but in some cases also poor growth phenotypes (reviewed in Dayod et al, 2010).…”