Overexpression of a Gene Involved in Phytic Acid Biosynthesis Substantially Increases Phytic Acid and Total Phosphorus in Rice Seeds

The manipulation of seed phosphorus is important for seedling growth and environmental P sustainability in agriculture. The mechanism of regulating P content in seed, however, is poorly understood. To study regulation of total P, we focused on phytic acid (inositol hexakisphosphate; InsP(6)) biosynthesis-related genes, as InsP(6) is a major storage form of P in seeds. The rice (Oryza sativa L.) low phytic acid mutant lpa1-1 has been identified as a homolog of archael 2-phosphoglycerate kinase. The homolog might act as an inositol monophosphate kinase, which catalyzes a key step in InsP(6) biosynthesis. Overexpression of the homolog in transgenic rice resulted in a significant increase in total P content in seed, due to increases in InsP(6) and inorganic phosphates. On the other hand, overexpression of genes that catalyze the first and last steps of InsP(6) biosynthesis could not increase total P levels. From the experiments using developing seeds, it is suggested that the activation of InsP(6) biosynthesis in both very early and very late periods of seed development increases the influx of P from vegetative organs into seeds. This is the first report from a study attempting to elevate the P levels of seed through a transgenic approach.