Three rice nicotianamine synthase genes, OsNAS1, OsNAS2, and OsNAS3 are expressed in cells involved in long‐distance transport of iron and differentially regulated by iron

Abstract

SummaryNicotianamine (NA), a chelator of metals, is ubiquitously present in higher plants. In graminaceous plants, NA is a biosynthetic precursor of phytosiderophores and is thus a crucial component for iron (Fe) acquisition. Here, we show that three rice NA synthase (NAS) genes, OsNAS1, OsNAS2, and OsNAS3 are expressed in cells involved in long-distance transport of Fe and that the three genes are differentially regulated by Fe. OsNAS1 and OsNAS2 transcripts were detected in Fe-suf®cient roots but not in leaves, and levels of both increased markedly in both roots and leaves in response to Fe de®ciency. In contrast, the OsNAS3 transcript was present in leaves but was very low in roots of Fe-suf®cient plants. Further, OsNAS3 expression was induced in roots but was suppressed in leaves in response to Fe de®ciency. Promoter±GUS analysis revealed that OsNAS1 and OsNAS2 were expressed in Fe-suf®cient roots in companion cells and pericycle cells adjacent to the protoxylem. With Fe de®ciency, OsNAS1 and OsNAS2 expression extended to all root cells along with an increase in phytosiderophore secretion. In Fe-de®cient plants, OsNAS1 and OsNAS2 were expressed in the vascular bundles of green leaves and in all cells of leaves showing severe chlorosis. OsNAS3 expression was restricted to the pericycle and companion cells of the roots, and in companion cells of leaves irrespective of Fe status. These results strongly suggested that NAS and NA play an important role in long-distance transport of Fe in rice plants, in addition to their roles in phytosiderophore secretion from roots.

Iron fortification of rice seeds through activation of the nicotianamine synthase gene

“…In rice, NA is synthesized by 3 genes that are differentially regulated by Fe (16). Similar expression patterns and the very close genomic location of OsNAS1 and OsNAS2 on the same chromosome suggest their functional redundancy and recent duplication (16). In contrast, OsNAS3 expression is induced in roots but suppressed in leaves when Fe is lacking.…”

“…This discrepancy was likely due to differences in expression patterns. OsNAS3 has been reported to be expressed in the pericycle and companion cells (16), whereas ectopic expression of OsNAS3 by the ubiquitin promoter caused a general increase in transcripts. Therefore, maintaining the endogenous expression pattern may be critical for efficiently elevating metal contents.…”

“…Disruption of the rice nicotianamine aminotransferase (NAAT1) enzyme, which uses NA as the substrate, results in a significant elevation of Fe concentrations in both seedlings (by 15.8% in shoots and 41.8% in roots) and seeds (1.8-fold) (15). Among the 3 OsNAS genes present in rice, OsNAS1 and OsNAS2 transcripts are markedly elevated in both roots and leaves in response to Fe deficiency, whereas OsNAS3 expression is induced in roots but suppressed in leaves when the Fe supply is inadequate (16).…”

Increase in Iron and Zinc Concentrations in Rice Grains Via the Introduction of Barley Genes Involved in Phytosiderophore Synthesis

“…Under Fe-sufficient conditions, rice genes involved in MAs biosynthesis, including OsNAS1, OsNAS2, OsNAAT1, and OsDMAS1, are expressed in the portions of roots that are involved in long-distance transport [2,9,10]. Thus, DMA might help to transport Fe over long distances in Fesufficient conditions.…”

Cell-Type Specificity of the Expression of Os BOR1, a Rice Efflux Boron Transporter Gene, Is Regulated in Response to Boron Availability for Efficient Boron Uptake and Xylem Loading

“…Histochemical staining was performed according to Inoue et al (2003), with the following modifications. The sections were incubated at 378C for 60 min in GUS reaction buffer (Inoue et al, 2003). After staining, the sections were washed in 70% ethanol and held in 70% ethanol until observation.…”