Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2009
Publication Date: 11/2/2009
Citation: Waters, B.M., Uauy, C., Dubcovsky, J., Grusak, M.A. 2009. Wheat (Triticum aestivum) NAM proteins regulate the translocation of iron, zinc, and nitrogen compounds from vegetative tissues to grain. Journal of Experimental Botany. 60(15):4263-4274. Interpretive Summary: Wheat is a crop of major importance and together with other staple cereals they supply the bulk of calories and nutrients in the diets of a large proportion of the world's population. Cereals are inherently low in protein and mineral micronutrients such as iron and zinc. Wheat breeders are interested in improving the nutritional quality of wheat as this could benefit the nutritional status of millions of people. Previous studies have shown that a regulatory protein, derived from the gene NAM-B1 from wild emmer wheat, was associated with increased grain protein, iron, and zinc concentrations. It was concluded that this protein increased the mobilization of iron, zinc, and amino acids from leaves to developing seeds in wheat, but no detailed mineral data were available to confirm this conclusion. In the present study, we conducted whole-plant mineral analyses in wheat lines differing in the expression of the NAM-B1 gene, in order to determine the basis for elevated grain nutrient concentrations. Plants were grown on varying levels of iron and zinc, and plants were harvested at different stages of reproductive development for subsequent mineral analysis. Our detailed study confirmed that a major effect of the protein product of NAM genes is to increase the movement of nutrients from vegetative tissues to developing grains. This information will allow us to identify other genes relevant to nutrient delivery from leaves, and will provide new targets for breeders to improve the nutritional quality of wheat and other cereal crops.
Technical Abstract: The NAM-B1 gene is a NAC transcription factor that affects grain nutrient concentrations in wheat (Triticum aestivum). An RNAi line with reduced expression of NAM genes has lower grain protein, iron (Fe), and zinc (Zn) concentrations. To determine whether decreased remobilization, lower plant uptake, or decreased partitioning to grain are responsible for this phenotype, mineral dynamics were quantified in wheat tissues throughout grain development. Control and RNAi wheat were grown in potting mix and hydroponics. Mineral (Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn) and nitrogen (N) contents of organs were determined at regular intervals to quantify net remobilization from vegetative tissues and accumulation of nutrients in grain. Total nutrient accumulation was similar between lines, but grain Fe, Zn, and N were at lower concentrations in the NAM knockdown line. In potting mix, net remobilization of N, Fe, and Zn from vegetative tissues was impaired in the RNAi line. In hydroponics with ample nutrients, net remobilization was not observed, but grain Fe and Zn contents and concentrations remained lower in the RNAi line. When Fe or Zn was withheld post-anthesis, both lines demonstrated remobilization. These results suggest that a major effect of NAM genes is increased efflux of nutrients from vegetative tissues and higher partitioning of nutrients to grain.