CBL1/9-CIPK23-NRAMP1 axis regulates manganese toxicity.

Manganese (Mn) is an essential micronutrient in plants. However, excessive Mn absorption in acidic soils can cause Mn toxicity, which adversely affects plant growth and crop yields. At present, acidic soils cover c. 30% of the Earth's surface. However, the mechanism underpinning Mn uptake remains largely unknown. We identified cbl1/9 and cipk23 mutants exhibiting high-Mn-sensitive phenotype through the reverse genetics method. Furthermore, we identified the CIPK23 phosphorylated NRAMP1 through a variety of protein interaction techniques and protein kinase assays. Here, we demonstrated that two calcineurin B-like proteins, CBL1/9, and their interacting kinase CIPK23 positively regulated the tolerance of Mn toxicity in Arabidopsis. The cbl1 cbl9 double mutant and cipk23 mutants exhibited high-Mn-sensitive phenotypes, which manifested as decreased primary root length, biomass, and chlorophyll concentration, and higher accumulation of Mn. In addition, CIPK23 interacted with and phosphorylated the Mn transporter NRAMP1 primarily at Ser20/22 in vitro and in vivo, and thereby induced clathrin-mediated endocytosis of NRAMP1 to reduce its distribution on the plasma membrane and enhance plant tolerance to Mn toxicity. In summary, we found that the CBL1/9-CIPK23-NRAMP1 module regulates the tolerance to high-Mn toxicity and provide insight into a mechanism of the tolerance of plants to Mn toxicity.