Protein kinase MtCIPK12 modulates iron reduction in Medicago truncatula by regulating riboflavin biosynthesis.

Iron (Fe) is an essential micronutrient, and deficiency of available Fe is one of the most important limiting factors for plant growth. In some species including Medicago truncatula, iron deficiency results in accumulation of riboflavin, a response associated with iron acquisition. However, how the plant's Fe status is integrated to tune riboflavin biosynthesis and how riboflavin levels affect iron acquisition and utilization remains largely unexplored. We report that protein kinase CIPK12 regulates ferric reduction by accumulation of riboflavin and its derivatives in roots of M. truncatula via physiological and molecular characterization of its mutants and over-expressing materials. Mutations in CIPK12 enhance iron accumulation and improve photosynthetic efficiency, whereas over-expression of CIPK12 shows the opposite phenotypes. The Calcineurin B-like proteins CBL3 and CBL8 interact with CIPK12, which negatively regulates the expression of genes encoding key enzymes in the riboflavin biosynthesis pathway. CIPK12 negatively regulates iron acquisition by suppressing accumulation of riboflavin and its derivatives in roots, which in turn influences ferric reduction activity by riboflavin-dependent electron transport under iron deficiency. Our findings uncover a new regulatory mechanism by which CIPK12 regulates riboflavin biosynthesis and iron-deficiency responses in plants. This article is protected by copyright. All rights reserved.