Abstract
Nicotianamine (NA) is an intermediate in the biosynthetic pathway of mugineic acid-family phytosiderophores (MAs), which are essential components of the iron acquisition mechanism of graminaceous plants. NA is also considered to be an essential chelator of metal cations in dicots. Thus, NA and nicotianamine synthase (NAS, EC 2.5.1.43) play important roles in Fe metabolism and homeostasis in higher plants. To evaluate the contribution of NAS to MAs biosynthesis, we introduced the barley hvnas1 gene under the control of the CaMV35S promoter into rice. Transgenic rice containing a barley HvNAS1 genomic DNA fragment was also analyzed. Although significant expression of hvnas1 driven by the CaMV35S promoter was confirmed by Northern and Western analyses, total NAS activity, endogenous NA, and 2'-deoxymugineic acid (DMA) secretion were not higher under Fe-deficient conditions compared to vector control plants. However, under Fe-sufficient conditions, the amounts of endogenous NA and secreted DMA increased slightly. No accumulation of NA due to an altered balance of nicotianamine aminotransferase (NAAT) and NAS activities was observed. These results indicate that the NAS activity may be strictly regulated.
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