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

Abstract

The physiological role and mechanism of nutrient storage within vacuoles of specific cell types is poorly understood. Transcript profiles from Arabidopsis thaliana leaf cells differing in calcium concentration ([Ca], epidermis <10 mM versus mesophyll >60 mM) were compared using a microarray screen and single-cell quantitative PCR. Three tonoplast-localized Ca 2+ transporters, CAX1 (Ca 2+ /H + -antiporter), ACA4, and ACA11 (Ca 2+ -ATPases), were identified as preferentially expressed in Ca-rich mesophyll. Analysis of respective loss-of-function mutants demonstrated that only a mutant that lacked expression of both CAX1 and CAX3, a gene ectopically expressed in leaves upon knockout of CAX1, had reduced mesophyll [Ca]. Reduced capacity for mesophyll Ca accumulation resulted in reduced cell wall extensibility, stomatal aperture, transpiration, CO 2 assimilation, and leaf growth rate; increased transcript abundance of other Ca 2+ transporter genes; altered expression of cell wall-modifying proteins, including members of the pectinmethylesterase, expansin, cellulose synthase, and polygalacturonase families; and higher pectin concentrations and thicker cell walls. We demonstrate that these phenotypes result from altered apoplastic free [Ca 2+ ], which is threefold greater in cax1/cax3 than in wild-type plants. We establish CAX1 as a key regulator of apoplastic [Ca 2+ ] through compartmentation into mesophyll vacuoles, a mechanism essential for optimal plant function and productivity.

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

“…CAXs appear to be a fulcrum for modulating ion homeostasis, pH and redox levels within plant cells (Conn et al . 2011; Cho et al . 2012).…”

“…In mesophyll cells, CAXs appear to be especially important in maintaining Ca 2+ homeostasis as well as stomatal conductance (Conn et al . 2011). These phenotypes suggest that altered CAX expression misappropriates Ca 2+ within plant cells.…”

“…Furthermore, transcriptomic microarray data of the double mutant further document what one would expect, a multitude of changes in gene expression (Conn et al . 2011), thus further challenging the mechanistic interpretation of these phenotypes.…”

“…2008; Conn et al . 2011). Evidence has also linked CAX activity with apoplastic pH homeostasis in guard cells (Cho et al .…”

Vacuolar Ca2+/H+ Transport Activity Is Required for Systemic Phosphate Homeostasis Involving Shoot-to-Root Signaling in Arabidopsis

“…Elemental analysis has also revealed that impairment of CAX1 and CAX3 caused dramatic alterations in the shoot ionome, namely elevated levels of phosphate (PO 4 3-; P i ), manganese (Mn 2+ ), and zinc (Zn 2+ ) and decreased Ca 2+ and magnesium (Mg 2+ ) ion concentrations (Cheng et al, 2005). A recent study further showed that the leaf apoplastic free Ca 2+ concentration ([Ca 2+ ]) of cax1/cax3 was 3-fold greater than that of the wild type, accounting for the phenotypes of reduced cell wall extensibility, stomatal aperture, transpiration, CO 2 assimilation, and leaf growth rate (Conn et al, 2011b). Despite no evidence yet of a direct involvement of Ca 2+ in P i signaling, the observation of an increased accumulation of P i in the cax1/cax3 mutant has provided the first link between Ca 2+ and P i homeostasis and thus drew our attention and interest to investigate the potential role of CAX1 and CAX3 in P i signaling.…”