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

Abstract

Cation/proton exchangers (CAXs) are a class of secondary energised ion transporter that are being implicated in an increasing range of cellular and physiological functions. CAXs are primarily Ca2+ efflux transporters that mediate the sequestration of Ca2+ from the cytosol, usually into the vacuole. Some CAX isoforms have broad substrate specificity, providing the ability to transport trace metal ions such as Mn2+ and Cd2+, as well as Ca2+. In recent years, genomic analyses have begun to uncover the expansion of CAXs within the green lineage and their presence within non‐plant species. Although there appears to be significant conservation in tertiary structure of CAX proteins, there is diversity in function of CAXs between species and individual isoforms. For example, in halophytic plants, CAXs have been recruited to play a role in salt tolerance, while in metal hyperaccumulator plants CAXs are implicated in cadmium transport and tolerance. CAX proteins are involved in various abiotic stress response pathways, in some cases as a modulator of cytosolic Ca2+ signalling, but in some situations there is evidence of CAXs acting as a pH regulator. The metal transport and abiotic stress tolerance functions of CAXs make them attractive targets for biotechnology, whether to provide mineral nutrient biofortification or toxic metal bioremediation. The study of non‐plant CAXs may also provide insight into both conserved and novel transport mechanisms and functions.

Ca2+/Cation Antiporters (CaCA): Identification, Characterization and Expression Profiling in Bread Wheat (Triticum aestivum L.)

“…The calcium signaling has been reported as an important mechanism for these kinds of stress sensing in plants (Tuteja and Sopory, 2008; Kader and Lindberg, 2010; Zhang et al, 2014a). Modulation in expression of CaCA superfamily genes such as CAXs and NCL has been earlier reported during abiotic stresses in various plant species (Wang et al, 2012; Li et al, 2016; Pittman and Hirschi, 2016b). Therefore, it becomes necessary to study the impact of these stresses on expression pattern of TaCaCA genes.…”

“…The expression analysis is a method to predict the putative function of an unknown gene (Zeng et al, 2016). Further, CaCA superfamily genes are also reported to play vital role in plant development (Shaul et al, 1999; Cheng et al, 2005; Wang et al, 2012; Pittman and Hirschi, 2016b). Therefore, their expression was analyzed in various developmental stages of T. aestivum using high throughput transcriptome data (Pingault et al, 2015).…”

“…Modulated expression of these proteins have also been observed in various plants during numerous abiotic stresses, which indicated their role in stress response (Hirschi et al, 1996, 2000; Shaul et al, 1999; Kamiya et al, 2005; Morris et al, 2008; Wang et al, 2012; Singh et al, 2015; Li et al, 2016). Functional characterization of certain CaCA proteins like AtNCL, AtCAX1, OsCAX4, SsCAX1 and others further established their role in various kinds of abiotic stress response (Mei et al, 2007; Han et al, 2012; Wang et al, 2012; Yamada et al, 2014; Li et al, 2016; Pittman and Hirschi, 2016b; Zhang et al, 2016). …”

“…The CAX proteins comprised the largest proportion of CaCA superfamily, and present in each group of organism (Shigaki et al, 2006; Manohar et al, 2011; Emery et al, 2012; Pittman and Hirschi, 2016b), except mammals and insects. These are broadly classified into type 1, type 2, and type 3 categories on the basis of phylogenetic clustering (Shigaki et al, 2006).…”

CAX1 suppresses Cd‐induced generation of reactive oxygen species in Arabidopsis halleri

“…To determine the size and composition of the CAX gene family in A. halleri , genome sequence data (Briskine et al, ) and the first A. halleri reference transcriptome (Corso et al, ; Schvartzman et al, ) were explored (see Section 2 for further details about sequence retrieval). Sequence analyses revealed that A. halleri possesses six CAX genes, as is typical for many monocots and dicots (Pittman & Hirschi, ). The A. halleri CAX genes are phylogenetically closely related to the A. thaliana ones (Figure ).…”