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

Abstract

Excessive cadmium (Cd) in rice grains is a serious food safety problem. The development of Cd-safe varieties requires the identification of germplasms and genes with major effect on Cd accumulation but without negative effects on other important traits. Here, we reported that OsCAX2, a member of the rice Cation/H+ exchanger (CAX) family, is an important Cd transporter. OsCAX2 encodes a tonoplast-localized protein and is strongly upregulated by Cd, mainly expresses in root exodermis, parenchyma in cortex, endodermis and stele cells. Depletion of OsCAX2 resulted in enhanced Cd sensitivity and root-to-shoot translocation in rice, while overexpression of OsCAX2 significantly increased Cd tolerance and reduced Cd transport by promoting root Cd influx and vacuolar storage, which ultimately reduced Cd transport via xylem. OsCAX2 also had significant effects on tissues/organs distribution of Cd but had no effects on grain yield and agronomic traits. Importantly, the OsCAX2 overexpressing lines had more than 70% lower grain Cd accumulation, increased iron (Fe), zinc (Zn) and manganese (Mn) and reduced copper (Cu) accumulation. Therefore, OsCAX2 is an ideal gene for developing Cd-safe rice varieties via transgenic approach.

Effect of Seed Priming with Cations on Cadmium Accumulation in Wheat Seedings under Cadmium-Contaminated Weakly Alkaline Soil

“…Under Cd stress, several metal transporter proteins participate in the absorption, transportation, and chelation of Cd by plants. These proteins include the Zn-regulated transporter/Fe-regulated transporter-like protein (ZIP) family, the cation exchanger (CAX) family, , the natural resistance-associated macrophage protein (NRAMP) family, the heavy-metal ATPase (HMA) family, the ATP-binding cassette (ABC) transporter family, and the yellow stripe-like transporter (YSL) family . These transmembrane transporters play a crucial role in regulating the distribution of metals within different compartments of plants.…”

Inhibition Roles of Calcium in Cadmium Uptake and Translocation in Rice: A Review

“…Similarly, the rice OsCAXs family has six members, namely OsCAX1a, OsCAX1b, OsCAX1c, OsCAX2, OsCAX3 and OsCAX4 [132]. Among them, the loss of OsCAX2 function resulted in increasing Cd toxicity, while the upregulation of its expression significantly inhibited Cd accumulation in rice [133]. Preliminary evidence from a yeast heterologous system showed that OsCAX1a, OsCAX1c and OsCAX4 had Cd transport activity in rice [132].…”