Khurram Bashir

Rice metal-nicotianamine transporter, OsYSL2, is required for the long-distance transport of iron and manganese.

Rice (Oryza sativa) is indispensable in the diet of most of the worlds population. Thus, it is an important target in which to alter iron (Fe) uptake and homeostasis, so as to increase Fe accumulation in the grain. We previously isolated OsYSL2, a functional iron [Fe(II)]- and manganese [Mn(II)]-nicotianamine complex transporter that is expressed in phloem cells and developing seeds. We produced RNAi (OsYSL2i) and overexpression lines (OXOsYSL2) of OsYSL2. At the vegetative stage in an OsYSL2i line, the Fe and Mn concentrations were decreased in the shoots, and the Fe concentration was increased in the roots. At the reproductive stage, positron-emitting tracer imaging system analysis revealed that Fe translocation to the shoots and seeds was suppressed in OsYSL2i. The Fe and Mn concentrations were decreased in the seeds of OsYSL2i, especially in the endosperm. Moreover, the Fe concentration in OXOsYSL2 was lower in the seeds and shoots, but higher in the roots, compared with the wild type. Furthermore, when OsYSL2 expression was driven by the sucrose transporter promoter, the Fe concentration in the polished rice was up to 4.4-fold higher compared with the wild type. These results indicate that the altered expression of OsYSL2 changes the localization of Fe, and that OsYSL2 is a critical Fe-nicotianamine transporter important for Fe translocation, especially in the shoots and endosperm.

Characterizing the role of rice NRAMP5 in Manganese, Iron and Cadmium Transport

Metals like manganese (Mn) and iron (Fe) are essential for metabolism, while cadmium (Cd) is toxic for virtually all living organisms. Understanding the transport of these metals is important for breeding better crops. We have identified that OsNRAMP5 contributes to Mn, Fe and Cd transport in rice. OsNRAMP5 expression was restricted to roots epidermis, exodermis, and outer layers of the cortex as well as in tissues around the xylem. OsNRAMP5 localized to the plasma membrane, and complemented the growth of yeast strains defective in Mn, Fe, and Cd transport. OsNRAMP5 RNAi (OsNRAMP5i) plants accumulated less Mn in the roots, and less Mn and Fe in shoots, and xylem sap. The suppression of OsNRAMP5 promoted Cd translocation to shoots, highlighting the importance of this gene for Cd phytoremediation. These data reveal that OsNRAMP5 contributes to Mn, Cd, and Fe transport in rice and is important for plant growth and development.

Cloning and Characterization of Deoxymugineic Acid Synthase Genes from Graminaceous Plants

Graminaceous plants have evolved a unique mechanism to acquire iron through the secretion of a family of small molecules, called mugineic acid family phytosiderophores (MAs). All MAs are synthesized from l-Met, sharing the same pathway from l-Met to 2′-deoxymugineic acid (DMA). DMA is synthesized through the reduction of a 3″-keto intermediate by deoxymugineic acid synthase (DMAS). We have isolated DMAS genes from rice (OsDMAS1), barley (HvDMAS1), wheat (TaD-MAS1), and maize (ZmDMAS1). Their nucleotide sequences indicate that OsDMAS1 encodes a predicted polypeptide of 318 amino acids, whereas the other three orthologs all encode predicted polypeptides of 314 amino acids and are highly homologous (82–97.5%) to each other. The DMAS proteins belong to the aldo-keto reductase superfamily 4 (AKR4) but do not fall within the existing subfamilies of AKR4 and appear to constitute a new subfamily within the AKR4 group. All of the proteins showed DMA synthesis activity in vitro. Their enzymatic activities were highest at pH 8–9, consistent with the hypothesis that DMA is synthesized in subcellular vesicles. Northern blot analysis revealed that the expression of each of the above DMAS genes is up-regulated under iron-deficient conditions in root tissue, and that of the genes OsDMAS1 and TaDMAS1 is up-regulated in shoot tissue. OsDMAS1 promoter-GUS analysis in iron-sufficient roots showed that its expression is restricted to cells participating in long distance transport and that it is highly up-regulated in the entire root under iron-deficient conditions. In shoot tissue, OsDMAS1 promoter drove expression in vascular bundles specifically under iron-deficient conditions.

A rice phenolic efflux transporter is essential for solubilizing precipitated apoplasmic iron in the plant stele

Iron deficiency is one of the major agricultural problems, as 30% of the arable land of the world is too alkaline for optimal crop production, rendering plants short of available iron despite its abundance. To take up apoplasmic precipitated iron, plants secrete phenolics such as protocatechuic acid (PCA) and caffeic acid. The molecular pathways and genes of iron uptake strategies are already characterized, whereas the molecular mechanisms of phenolics synthesis and secretion have not been clarified, and no phenolics efflux transporters have been identified in plants yet. Here we describe the identification of a phenolics efflux transporter in rice. We identified a cadmium-accumulating rice mutant in which the amount of PCA and caffeic acid in the xylem sap was dramatically reduced and hence named it phenolics efflux zero 1 (pez1). PEZ1 localized to the plasma membrane and transported PCA when expressed in Xenopus laevis oocytes. PEZ1 localized mainly in the stele of roots. In the roots of pez1, precipitated apoplasmic iron increased. The growth of PEZ1 overexpression lines was severely restricted, and these lines accumulated more iron as a result of the high solubilization of precipitated apoplasmic iron in the stele. We show that PEZ1 is responsible for an increase of PCA concentration in the xylem sap and is essential for the utilization of apoplasmic precipitated iron in the stele.

The rice mitochondrial iron transporter is essential for plant growth

In plants, iron (Fe) is essential for mitochondrial electron transport, heme, and Fe-Sulphur (Fe-S) cluster synthesis; however, plant mitochondrial Fe transporters have not been identified. Here we show, identify and characterize the rice mitochondrial Fe transporter (MIT). Based on a transfer DNA library screen, we identified a rice line showing symptoms of Fe deficiency while accumulating high shoot levels of Fe. Homozygous knockout of MIT in this line resulted in a lethal phenotype. MIT localized to the mitochondria and complemented the growth of Δmrs3Δmrs4 yeast defective in mitochondrial Fe transport. The growth of MIT-knockdown (mit-2) plants was also significantly impaired despite abundant Fe accumulation. Further, the decrease in the activity of the mitochondrial and cytosolic Fe-S enzyme, aconitase, indicated that Fe-S cluster synthesis is affected in mit-2 plants. These results indicate that MIT is a mitochondrial Fe transporter essential for rice growth and development.

Iron Uptake and Loading into Rice Grains

Iron (Fe) is an important micronutrient for living organisms. Fe deficiency severely impairs plant growth and is a widespread human dietary problem, with particularly high numbers of affected children and females. Rice (Oryza sativa) is a source of energy for more than half of the world’s population. Thus, understanding the mechanisms of Fe uptake and translocation in rice is of utmost importance in the development of rice varieties that are tolerant to low Fe availability and with high seed levels of Fe. In recent years, the mechanisms underlying Fe transport and homeostasis have been revealed, providing opportunities to increase the Fe content of rice grain. As excess Fe is toxic to cells, plants have developed sophisticated mechanisms to control Fe flow, making it difficult to alter Fe transport. Thus, choosing appropriate chelators and Fe transporters driven by appropriate promoters seems to be the key in developing rice that is tolerant to low Fe availability and which accumulates high grain levels of Fe. Many recent studies have been aimed at increasing the Fe content of rice. Here, we summarize these efforts and review recent progress in understanding the mechanisms of Fe transport.

Zn Uptake and Translocation in Rice Plants

Zinc (Zn) is an essential micronutrient with numerous cellular functions in plants, and its deficiency represents one of the most serious problems in human nutrition worldwide. Zn deficiency causes a decrease in plant growth and yield. On the other hand, Zn could be toxic if excess amounts are accumulated. Therefore, the uptake and transport of Zn must be strictly regulated. In this review, the dominant fluxes of Zn in soil–root–shoot translocation in rice plants (Oryza sativa) are described, including Zn uptake from soils in the form of Zn2+ and Zn-DMA at the root surface, and Zn translocation to shoots. Knowledge of these fluxes could be helpful to formulate genetic and physiologic strategies to address the widespread problem of Zn-limited crop growth.

The road to micronutrient biofortification of rice: progress and prospects

Biofortification (increasing the contents of vitamins and minerals through plant breeding or biotechnology) of food crops with micronutrient elements has the potential to combat widespread micronutrient deficiencies in humans. Rice (Oryza sativa L.) feeds more than half of the world’s population and is used as a staple food in many parts of Asia. As in other plants, micronutrient transport in rice is controlled at several stages, including uptake from soil, transport from root to shoot, careful control of subcellular micronutrient transport, and finally, and most importantly, transport to seeds. To enhance micronutrient accumulation in rice seeds, we need to understand and carefully regulate all of these processes. During the last decade, numerous attempts such as increasing the contents/expression of genes encoding metal chelators (mostly phytosiderophores) and metal transporters; Fe storage protein ferritin and phytase were successfully undertaken to significantly increase the micronutrient content of rice. However, despite the rapid progress in biofortification of rice, the commercialization of biofortified crops has not yet been achieved. Here, we briefly review the progress in biofortification of rice with micronutrient elements (Fe, Zn, and Mn) and discuss future prospects to mitigate widespread micronutrient deficiencies in humans.

Molecular mechanisms of zinc uptake and translocation in rice

BackgroundZinc (Zn) is an essential micronutrient for plants and humans, involved in protein, nucleic acid, carbohydrate, and lipid metabolism. In addition, Zn is critical to the control of gene transcription and the coordination of other biological processes.ScopeZn deficiency is one of the most serious problems in plant and human nutrition. Like other plants, rice plant acquires Zn from soil and transports it to vegetative tissue as well as seed through a number of transporters which are strictly regulated. Several members of the Zn-regulated transporters and iron (Fe)-regulated transporter-like protein (ZIP) gene family have been characterized and shown to be involved in metal uptake and transport in rice. The most characterized members of this family in rice are OsZIP1, OsZIP3, OsZIP4, OsZIP5, and OsZIP8, however little is known about the expression of these genes through different growth stages of rice.ConclusionHere we discuss the molecular mechanisms of Zn transport in rice as an essential advance for understanding and manipulating the Zn absorption and translocation in rice. OsZIP1 and OsZIP3 seems important for Zn uptake from soil, OsZIP4, OsZIP5 and OsZIP8 for root to shoot translocation, while OsZIP4 and OsZIP8 seems particularly important for Zn transport to seed.

Signals from chloroplasts and mitochondria for iron homeostasis regulation

Iron (Fe) is an essential element for human nutrition. Given that plants represent a major dietary source of Fe worldwide, it is crucial to understand plant Fe homeostasis fully. A major breakthrough in the understanding of Fe sensing and signaling was the identification of several transcription factor cascades regulating Fe homeostasis. However, the mechanisms of activation of these cascades still remain to be elucidated. In this opinion, we focus on the possible roles of mitochondria and chloroplasts as cellular Fe sensing and signaling sites, offering a new perspective on the integrated regulation of Fe homeostasis and its interplay with cellular metabolism.