A maize Ni ²⁺ /Co ²⁺ transporter <scp>ZBL1</scp> maintains cobalt and nickel homeostasis and contributes to photosynthesis

Transition metals play essential roles as coenzymes and catalysts in plants and are vital for chloroplast function and photosynthetic efficiency. Nickel (Ni) and cobalt (Co) are two important trace transition metals that are present in chloroplasts. However, the functions of Ni and Co in maize and the mechanisms regulating their homeostasis in chloroplasts remain poorly understood. In this study, we identified a maize zebra lesion (zl1) mutant characterized by leaf chlorosis, severe defects in chloroplast development, and reduced chlorophyll accumulation. Bulked segregant RNA-seq analysis and positional cloning revealed that zl1 carries a non-synonymous mutation in GRMZM2G141636, which encodes a putative nickel/cobalt transporter. Two additional non-synonymous mutations in this gene exhibit similar phenotypes and failed to complement the zl1 mutant. CRISPR-generated knockout mutants were albino and died at an early developmental stage, indicating that ZL1 is crucial for maize development. ZL1-eGFP fusion proteins localized to the chloroplasts and other plastids, and zl1 chloroplasts had significantly reduced Ni and Co levels, while supplementation with Ni²⁺ or Co²⁺ partially rescued the chlorosis phenotype, suggesting the leaf chlorosis phenotype is caused by a deficiency of Ni²⁺ or Co²⁺ in chloroplasts. RNA profiling revealed widespread misregulation of photosynthesis-related genes in the zl1 mutant. Taken together, these results highlight the crucial role of ZL1 in maize development and photosynthesis, likely through the regulation of cobalt and nickel homeostasis in chloroplasts.