<i>Small kernel2</i> Encodes a Glutaminase in Vitamin B₆ Biosynthesis Essential for Maize Seed Development

Abstract

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 (<i>Arabidopsis thaliana</i>), 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 <i>small kernel2</i> (<i>smk2</i>) 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 <i>smk2</i> embryo and the endosperm. However, whereas embryogenesis of the <i>smk2</i> mutant is arrested at the transition stage, endosperm formation is nearly normal. Cloning reveals that <i>Smk2</i> encodes the glutaminase subunit of the PLP synthase complex involved in vitamin B₆ biosynthesis de novo. <i>Smk2</i> partially complements the Arabidopsis vitamin B₆-deficient mutant <i>pdx2.1</i> and <i>Saccharomyces cerevisiae</i> pyridoxine auxotrophic mutant MML21. <i>Smk2</i> 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.