| text |
Rice and other graminaceous plants synthesize and secrete natural Fe chelators, mugineic acid family phytosiderophores (MAs) from their roots to solubilize Fe in the rhizosphere (Takagi 1976, Römheld and Marchner 1986; Mori 1999). Methionine serves as a precursor for biosynthesis of S-adenosylmethionine (SAM) which further synthesizes nicotianamine (NA) in both graminaceous and non-graminaceous plants. In rice, NA is converted into a 3?-keto intermediate and then deoxymugineic acid. DMAs are then secreted to the rhizosphere and solubilize rhizospheric Fe(III). NA is synthesized in all plant species and is involved in metal translocation in plants but not secreted to the rhizosphere. The genes that encode key enzymes for MA biosynthesis are SAM synthase genes, NA synthase (NAS) genes, NA aminotransferase (NAAT) and deoxymugineic acid synthase (DMAS). To meet the increased demand for methionine, the methionine cycle is vigorously induced along with the biosynthesis of MAs (Ma et al., 1995). MAs play a role in the internal distribution of iron in graminaceous plants. Endogenous MAs were detected in the shoots of barley and rice, and the amount of MAs increased dramatically under iron deficiency. Presence of DMA is observed in rice phloem and xylem sap. The genes involved in DMA biosynthesis in rice are co-expressed in phloem companion cells in roots and leaves.
|
