The Rice Cation/H+ Exchanger Family Involved in Cd Tolerance and Transport

Abstract

Cadmium (Cd), a heavy metal toxic to humans, easily accumulates in rice grains. Rice with unacceptable Cd content has become a serious food safety problem in many rice production regions due to contaminations by industrialization and inappropriate waste management. The development of rice varieties with low grain Cd content is seen as an economic and long-term solution of this problem. The cation/H+ exchanger (CAX) family has been shown to play important roles in Cd uptake, transport and accumulation in plants. Here, we report the characterization of the rice CAX family. The six rice CAX genes all have homologous genes in Arabidopsis thaliana. Phylogenetic analysis identified two subfamilies with three rice and three Arabidopsis thaliana genes in both of them. All rice CAX genes have trans-member structures. OsCAX1a and OsCAX1c were localized in the vacuolar while OsCAX4 were localized in the plasma membrane in rice cell. The consequences of qRT-PCR analysis showed that all the six genes strongly expressed in the leaves under the different Cd treatments. Their expression in roots increased in a Cd dose-dependent manner. GUS staining assay showed that all the six rice CAX genes strongly expressed in roots, whereas OsCAX1c and OsCAX4 also strongly expressed in rice leaves. The yeast (Saccharomyces cerevisiae) cells expressing OsCAX1a, OsCAX1c and OsCAX4 grew better than those expressing the vector control on SD-Gal medium containing CdCl2. OsCAX1a and OsCAX1c enhanced while OsCAX4 reduced Cd accumulation in yeast. No auto-inhibition was found for all the rice CAX genes. Therefore, OsCAX1a, OsCAX1c and OsCAX4 are likely to involve in Cd uptake and translocation in rice, which need to be further validated.

The Cation/H+exchanger OsCAX2 is involved in cadmium tolerance and accumulation through vacuolar sequestration in rice

“…The transgenic tobacco and potato plants overexpressing AtCAX1 show Ca deficiency disorders such as yellow leaves and dwarf plants (Hirschi, 1999; Zorrilla et al, 2019). The function of OsCAX2 in Cd tolerance and transport in our study was similar to that reported for AtCAX2 , which is not unexpected since OsCAX2 is tonoplast-localized and highly homologous to AtCAX2 (Kamiya et al, 2005; Zou et al, 2021). OsCAX2- mediated tolerance does not seem to be Ca-dependent (Figure 4d and Supplementary Figure S2).…”

“…Similarly, AtCAX2 has no response to Ca (Zheng et al, 2021). Although all Arabidopsis CAX transporters are tonoplast-localized, OsCAX3 and OsCAX4 in rice are plasma member-located (Qi et al, 2005; Zou et al, 2021). The rice CAX transporters may transport Cd using the tonoplast-localized ones to remove Cd from the cytosol and releases H + , and the plasma membrane-located ones to efflux H + from the extracellular space and releases Cd.…”

“…The Cation/H + exchanger (CAX) transporters have been shown to play important roles in Cd tolerance and accumulation in Arabidopsis thaliana , and the Cd hyper-accumulators Arabidopsis halleri and Sedum alfredii (Baliardini et al, 2015; Koren’kov et al, 2007; Korenkov et al, 2007; Zou et al, 2021). Overexpression of SaCAX2h, AtCAX2 and AtCAX4 enhances Cd tolerance and improves root Cd sequestration in transgenic tobacco (Koren’kov et al, 2007; Korenkov et al, 2007; Korenkov et al, 2009; Zhang et al, 2015).…”

“…The ahcax1 and atcax3 mutants have higher Cd sensitivity and stronger ROS accumulation in Arabidopsis thaliana (Ahmadi et al, 2018; Baliardini et al, 2015; Modareszadeh et al, 2021). We recently showed that a few members of the rice CAX family have Cd transport activity in yeast (Zou et al, 2021). However, whether and how the rice CAX transporters involve in Cd tolerance and transport in rice has not been reported yet.…”

Metalloprotein-Specific or Critical Amino Acid Residues: Perspectives on Plant-Precise Detoxification and Recognition Mechanisms under Cadmium Stress

“…AtCAX4 compensates to a certain extent for the reduction of the vacuolar membrane H pump and the increase in proton leakage, which helps to improve the higher tolerance of plants to Cd [82]. OsCAX1a/OsCAX1c may also be involved in the process of Cd transfer to vacuole storage [83]. In plants, CAX is driven by the pH gradient produced by vacuolar H-ATPase and H-pyrophosphatase to drive metal ion transport.…”