Small kernel2 Encodes a Glutaminase in Vitamin B₆ Biosynthesis Essential for Maize Seed Development

Vitamin B₆, an essential cofactor for a range of biochemical reactions and a potent antioxidant, plays important roles in plant growth, development, and stress tolerance. Vitamin B₆ deficiency causes embryo lethality in Arabidopsis (Arabidopsis thaliana), but the specific role of vitamin B₆ biosynthesis in endosperm development has not been fully addressed, especially in monocot crops, where endosperm constitutes the major portion of the grain. Through molecular characterization of a small kernel2 (smk2) mutant in maize, we reveal that vitamin B₆ has differential effects on embryogenesis and endosperm development in maize. The B₆ vitamer pyridoxal 5'-phosphate (PLP) is drastically reduced in both the smk2 embryo and the endosperm. However, whereas embryogenesis of the smk2 mutant is arrested at the transition stage, endosperm formation is nearly normal. Cloning reveals that Smk2 encodes the glutaminase subunit of the PLP synthase complex involved in vitamin B₆ biosynthesis de novo. Smk2 partially complements the Arabidopsis vitamin B₆-deficient mutant pdx2.1 and Saccharomyces cerevisiae pyridoxine auxotrophic mutant MML21. Smk2 is constitutively expressed in the maize plant, including developing embryos. Analysis of B₆ vitamers indicates that the endosperm accumulates a large amount of pyridoxamine 5'-phosphate (PMP). These results indicate that vitamin B₆ is essential to embryogenesis but has a reduced role in endosperm development in maize. The vitamin B₆ required for seed development is synthesized in the seed, and the endosperm accumulates PMP probably as a storage form of vitamin B₆.