Novel Regulation of Vegetative Storage Protein Genes.

Until recently, storage protein research has been largely restricted to the seed-specific reserve proteins, which provide nutrients to seedlings during their early growth. Their abundance, and the potential for improving seed nutritional quality, were early incentives to investigate seed storage proteins, and they continue to be intensively studied as a plant model for tissue-specific and developmentally regulated genes. Recent research has shown that non-seed tissues also synthesize proteins that are distinct from the seed reserves but have the characteristics of storage proteins. That is, they are expressed abundantly in specific cells, are localized in vacuoles (the progenitors of protein bodies in seeds), and are degraded at a later time for the nutritional needs of other organs. It is perhaps not surprising that vegetative tissues contain reserve proteins since plants assimilate, translocate, and reutilize nutrients extensively throughout their life cycle (for reviews, see Pate, 1980; Simpson, 1986). This review focuses on a soybean vegetative storage protein (VSP) that is particularly important for the temporary storage of nitrogen. Unlike the seed proteins, whose massive accumulation comes at the end of a developmental stage, the VSP genes are highly modulated throughout development according to the immediate need to store surplus nitrogen or amino acids. They are particularly intriguing and attractive for further studies because their expression is enhanced by severa1 different stimuli, including seed pod removal, high nitrogen nutrition, and wounding. Jasmonic acid, a nove1 plant compound and a possible endogenous gene regulator, also induces VSP gene expression. As with their seed counterparts, the VSP genes may be important targets for increasing plant productivity or nutritional quality through genetic engineering.