Mutation in Nicotianamine Aminotransferase Stimulated the Fe(II) Acquisition System and Led to Iron Accumulation in Rice

Abstract

Higher plants acquire iron (Fe) from the rhizosphere through two strategies. Strategy II, employed by graminaceous plants, involves secretion of phytosiderophores (e.g. deoxymugineic acid in rice [Oryza sativa]) by roots to solubilize Fe(III) in soil. In addition to taking up Fe in the form of Fe(III)-phytosiderophore, rice also possesses the strategy I-like system that may absorb Fe(II) directly. Through mutant screening, we isolated a rice mutant that could not grow with Fe(III)-citrate as the sole Fe source, but was able to grow when Fe(II)-EDTA was supplied. Surprisingly, the mutant accumulated more Fe and other divalent metals in roots and shoots than the wild type when both were supplied with EDTA-Fe(II) or grown under water-logged field conditions. Furthermore, the mutant had a significantly higher concentration of Fe in both unpolished and polished grains than the wild type. Using the map-based cloning method, we identified a point mutation in a gene encoding nicotianamine aminotransferase (NAAT1), which was responsible for the mutant phenotype. Because of the loss of function of NAAT1, the mutant failed to produce deoxymugineic acid and could not absorb Fe(III) efficiently. In contrast, nicotianamine, the substrate for NAAT1, accumulated markedly in roots and shoots of the mutant. Microarray analysis showed that the expression of a number of the genes involved in Fe(II) acquisition was greatly stimulated in the naat1 mutant. Our results demonstrate that disruption of deoxymugineic acid biosynthesis can stimulate Fe(II) acquisition and increase iron accumulation in rice.

Nutritionally Enhanced Staple Food Crops

“…It was concluded that Zn is highly mobile in phloem Rengel 1994, 1995;Haslett et al 2001). Similarly, the transport of iron in phloem is well studied although the transport mechanism is not well known and may involve citric acid conjugates and/or nicotianamine as a transport system with most of the studies on Fe-transport based on mutants in peas (Grusak 2000(Grusak , 2002 or Arabidopsis (Grotz and Guerinot 2006), or rice (Takahashi et al 2001;Cheng et al 2007;Masuda et al 2009). Iron-uptake through iron reductase activity (IRA) seems to be important for total seed accumulation of this mineral in common bean (Blair et al 2010a).…”

“…Rice is an exception in the sense that it possesses both systems for Fe uptake, making it especially Fe efficient (Cheng et al 2007). Plant roots and their exudates greatly facilitate the availability of various plant nutrients by bringing them in the soluble form in the soil (Neumann and R€ omheld 2002;Rengel 2002;Ryan and Graham 2002;Welch and Graham 2004).…”

“…For example, the 75 wild and weedy common beans from Jalisco and Durango state of Mexico showed large variability for seed Ca (500-7470 mg k À1 ), iron (64-280 mg k À1 ), and zinc (11-33 mg k À1 ) concentrations (Guzman-Maldonado et al 2000) or nonprogenitor Aegilop species of wheat with large variability for grain Fe and Zn (Chhuneja et al 2006). Cheng et al (2007) isolated a point mutation in a gene encoding nicotianamine aminotransferase (NAAT1), which disrupted strategy II system of rice due to the loss of a functional NAAT enzyme, and the naat1 mutant exhibited 3.8-fold higher Fe than the WT. More importantly, the Fe concentration of the polished seed of the naat1 mutant from field grown rice plants was 4.6 mg kg À1 , that is, 3.8-fold higher than the wild type approaching that of the highest naturally existing lines (Barry 2006).…”

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

“…This increased NA also enables plants to tolerate toxic concentrations of nickel (11,14). 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).…”

Iron-Binding E3 Ligase Mediates Iron Response in Plants by Targeting Basic Helix-Loop-Helix Transcription Factors

“…Under low-iron conditions, grasses such as rice (Oryza sativa) and maize (Zea mays) primarily utilize the Strategy II response, whereby they release phytosiderophores into the rhizosphere that bind to ferric iron with high affinity. Phytosiderophore-ferric iron complexes are transported into the root via membrane-localized yellow stripe and yellow stripe-like (YSL) transporters (Curie et al, 2001;Inoue et al, 2009), although several studies suggest that rice is also able to directly uptake ferrous iron (Ishimaru et al, 2006;Cheng et al, 2007).…”