The SOS4 pyridoxal kinase is required for maintenance of vitamin B6-mediated processes in chloroplasts

Vitamin B6 (pyridoxal 5′-phosphate and its vitamers) is an important cofactor in numerous enzymatic reactions. In spite of its importance, the consequences of altering vitamin B6 content on plant growth and development are not well understood. This study compares two mutants for vitamin B6-metabolizing enzymes in Arabidopsis thaliana: a pdx1.3 mutant in the de novo synthesis pathway and a salvage pathway sos4 mutant that accumulates more vitamin B6. We show that despite a difference in total B6 content in leaf tissue, both mutants share similar phenotypes, including chlorosis, decreased size, altered chloroplast ultrastructure, and root sensitivity to sucrose. Assay of B6 vitamer content from isolated chloroplasts showed that, despite differing B6 vitamer content in whole leaf tissue, both mutants share a common deficiency in total and phosphorylated vitamers in chloroplasts. One of the splice variants of the SOS4 proteins was shown to be located in the chloroplast. Our data indicate that some of the phenotypic consequences shared between the pdx1.3 and sos4 mutants are due to B6 deficiency in chloroplasts, and show that SOS4 is required for maintenance of phosphorylated B6 vitamer concentrations in chloroplasts. Further, our data are consistent with a diffusion model for transport of vitamin B6 into chloroplasts.