Node-controlled allocation of mineral elements in Poaceae

YABBY and diverged KNOX1 genes shape nodes and internodes in the stem

Plant stems comprise nodes and internodes that specialize in solute exchange and elongation. However, their boundaries are not well defined, and how these basic units arise remains elusive. In rice with clear nodes and internodes, we found that one subclade of class I knotted1-like homeobox (KNOX1) genes for shoot meristem indeterminacy restricts node differentiation and allows internode formation by repressing YABBY genes for leaf development and genes from another node-specific KNOX1 subclade. YABBYs promote nodal vascular differentiation and limit stem elongation. YABBY and node-specific KNOX1 genes specify the pulvinus, which further elaborates the nodal structure for gravitropism. Notably, this KNOX1 subclade organization is specific to seed plants. We propose that nodes and internodes are distinct domains specified by YABBY-KNOX1 cross-regulation that diverged in early seed plants.

The OsZIP2 transporter is involved in root-to-shoot translocation and intervascular transfer of cadmium in rice

Cadmium (Cd) is a toxic metal that poses serious threats to human health. Rice is a major source of dietary Cd but how rice plants transport Cd to the grain is not fully understood. Here, we characterize the function of the ZIP (ZRT, IRT-like protein) family protein, OsZIP2, in the root-to-shoot translocation of Cd and intervascular transfer of Cd in nodes. OsZIP2 is localized at the plasma membrane and exhibited Cd2+ transport activity when heterologously expressed in yeast. OsZIP2 is strongly expressed in xylem parenchyma cells in roots and in enlarged vascular bundles in nodes. Knockout of OsZIP2 significantly enhanced root-to-shoot translocation of Cd and alleviated the inhibition of root elongation by excess Cd stress; whereas overexpression of OsZIP2 decreased Cd translocation to shoots and resulted in Cd sensitivity. Knockout of OsZIP2 increased Cd allocation to the flag leaf but decreased Cd allocation to the panicle and grain. We further reveal that the variation of OsZIP2 expression level contributes to grain Cd concentration among rice germplasms. Our results demonstrate that OsZIP2 functions in root-to-shoot translocation of Cd in roots and intervascular transfer of Cd in nodes, which can be used for breeding low Cd rice varieties.

An oligo peptide transporter family member, OsOPT7, mediates xylem unloading of Fe for its preferential distribution in rice

Iron (Fe) needs to be delivered to different organs and tissues of above-ground parts for playing its multiple physiological functions once it is taken up by the roots. However, the mechanisms underlying Fe distribution are poorly understood. We functionally characterized OsOPT7, a member of oligo peptide transporter family in terms of expression patterns, localization, transport activity and phenotypic analysis of knockdown lines. OsOPT7 was highly expressed in the nodes, especially in the uppermost node I, and its expression was upregulated by Fe-deficiency. OsOPT7 transports ferrous iron into the cells coupled with proton. Immunostaining revealed that OsOPT7 is mainly localized in the xylem parenchyma cells of the enlarged vascular bundles in the nodes and vascular tissues in the leaves. Knockdown of OsOPT7 did not affect the Fe uptake, but altered Fe distribution; less Fe was distributed to the new leaf, upper nodes and developing panicle, but more Fe was distributed to the old leaves. Furthermore, knockdown of OsOPT7 also resulted in less Fe distribution to the leaf sheath, but more Fe to the leaf blade. Taken together, OsOPT7 is involved in the xylem unloading of Fe for both long-distance distribution to the developing organs and local distribution within the leaf in rice.

Understanding phloem's role in long-distance transport and accumulation of arsenic (As) in rice: toward low-As-accumulating grain development

MAIN CONCLUSION

This review highlights the roles of phloem in the long-distance transport and accumulation of As in rice plants, facilitating the formulation of new strategies to reduce the grain As content. Rice is a staple diet for a significant proportion of the global population. As toxicity is a major issue affecting the rice productivity and quality worldwide. Phloem tissues of rice plants play vital roles in As speciation, long-distance transport, and unloading, thereby controlling the As accumulation in rice grains. Phloem transport accounts for a significant proportion of As transport to grains, ranging from 54 to 100% depending on the species [inorganic arsenate (As(V)), arsenite (As(III)), or organic dimethylarsinic acid (DMA(V)]. However, the specific mechanism of As transport through phloem leading to its accumulation in grains remains unknown. Therefore, understanding the molecular mechanism of phloem-mediated As transport is necessary to determine the roles of phloem in long-distance As transport and subsequently reduce the grain As content via biotechnological interventions. This review discusses the roles of phloem tissues in the long-distance transport and accumulation of As in rice grains. This review also highlights the biotechnological approaches using critical genetic factors involved in nodal accumulation, vacuolar sequestration, and cellular efflux of As in phloem- or phloem-associated tissues. Furthermore, the limitations of existing transgenic techniques are outlined to facilitate the formulation of novel strategies for the development of rice with reduced grain As content.

Knockout of a rice K5.2 gene increases Ca accumulation in the grain

Rice is a staple food for half of the world's population, but it is a poor dietary source of calcium (Ca) due to the low concentration. It is an important issue to boost Ca concentration in this grain to improve Ca deficiency risk, but the mechanisms underlying Ca accumulation are poorly understood. Here, we obtained a rice (Oryza sativa) mutant with high shoot Ca accumulation. The mutant exhibited 26%-53% higher Ca in shoots than did wild-type rice (WT) at different Ca supplies. Ca concentration in the xylem sap was 36% higher in the mutant than in the WT. There was no difference in agronomic traits between the WT and mutant, but the mutant showed 25% higher Ca in the polished grain compared with the WT. Map-based cloning combined with a complementation test revealed that the mutant phenotype was caused by an 18-bp deletion of a gene, OsK5.2, belonging to the Shaker-like K+ channel family. OsK5.2 was highly expressed in the mature region of the roots and its expression in the roots was not affected by Ca levels, but upregulated by low K. Immunostaining showed that OsK5.2 was mainly expressed in the pericycle of the roots. Taken together, our results revealed a novel role for OsK5.2 in Ca translocation in rice, and will be a good target for Ca biofortification in rice.

New insights into the key role of node I in thallium accumulation in seed of coix (Coix lacryma-jobi L.)

The mechanisms underlying the distribution of many toxic metal(loid)s in shoots and metal(loid) transport to grains have been well documented in the quest for food safety but there remains a lack of knowledge on thallium (Tl) accumulation in food crops. Here, field investigations combined with a glasshouse pot experiment were conducted to investigate the characteristics of Tl distribution and accumulation in coix, a major food crop in south Guizhou province, China, and the role of node I in restricting Tl transport to the seed. Fourteen percent of coix seed samples collected from the Lanmuchang Tl-As-Hg mine contained higher Tl concentrations than the recommended limit for foods and feedstuffs in Germany (0.5 mg kg-1), with the highest exceedance rate of the metal(loid)s determined, when grown in soils surrounding the mine with a very high Tl concentration of 0.07-89.5 mg kg-1 and a general low pH of 4.19-6.48. Thallium concentrations were higher in coix nodes than in internodes, followed by roots and grains. The Tl translocation factors from node I to grains were 0.01-0.21 and were the lowest of any translocation factors between different tissues. Node I is therefore the key tissue restricting Tl transport to coix grains. Thallium was localized mainly in the diffuse vascular bundles (DVBs) in node I. The co-localization of Tl and sulfur in the DVBs and Tl contamination-induced phytochelatin (PC) accumulation indicate that Tl storage in the DVBs involving complexation with PCs in node I is an important process in Tl accumulation in coix grains. Moreover, the area of DVBs in node I increased with increasing soil Tl pollution level, providing more channels for Tl transport to the panicles and grains and thereby acting as a key factor restricting Tl transport to the grains. These results provide new insights into the key role of node I in Tl accumulation in coix grains and indicate key points to minimize Tl accumulation in grains for food safety.

Stem cutting: A novel introduction site for transporting water-insoluble particles into tomato (Solanum lycopersicum) seedlings

The introduction of exogenous particles into plants has promising applications in agriculture and biotechnology. Nanoparticles can be transported into plants through foliar application or root uptake. However, both methods have limitations in terms of the size of the particles (<40 nm) that can be transported due to the barriers of the cell wall and cuticle. In the present study, we proposed a novel method to deliver particles of up to 110 nm into plants by cutting the stem of tomato seedlings. We demonstrated for the first time, using water-insoluble silica colloids, that not only nanoparticles but also submicron particles can be transported toward the leaves when the plant stem is used as the entry point of particles. Thirty-five-day-old tomato seedlings were used as the target plants. When the cut stem seedlings were immersed in the colloidal particle suspension for up to 24 h, significant particle accumulation was observed in the nodes and leaves. The relatively low particle concentrations (10 mg/L) allowed effective transport throughout the plants. Silica particles with average diameters of 10 nm and 110 nm were both well transported and moved through the stem. Even after the particles entered the plant, adventitious roots were formed, resulting in the formation of whole plants with roots, stems, and leaves. This method can be applied not only to tomatoes but also to other food crops for various applications in plant biotechnology.

Heat shock-inducible clonal analysis reveals the stepwise establishment of cell fate in the rice stem

The stem, consisting of nodes and internodes, is the shoot axis, which supports aboveground organs and connects them to roots. In contrast to other organs, developmental processes of the stem remain elusive, especially those initiating nodes and internodes. By introducing an intron into the Cre recombinase gene, we established a heat shock-inducible clonal analysis system in a single binary vector and applied it to the stem in the flag leaf phytomer of rice (Oryza sativa). With detailed characterizations of stem structure and development, we show that cell fate acquisition for each domain of the stem occurs stepwise. Cell fate for a single phytomer was established in the shoot apical meristem (SAM) by one plastochron before leaf initiation. Cells destined for the foot (nonelongating domain at the stem base) also started emerging before leaf initiation. Cell fate acquisition for the node began just before leaf initiation at the flank of the SAM, separating cell lineages for leaves and stems. Subsequently, cell fates for the axillary bud were established in early leaf primordia. Finally, cells committed to the internode emerged from, at most, a few cell tiers of the 12- to 25-cell stage stem epidermis. Thus, internode cell fate is established last during stem development. This study provides the groundwork to unveil underlying molecular mechanisms in stem development and a valuable tool for clonal analysis, which can be applied to various species.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

An intracellular transporter OsNRAMP7 is required for distribution and accumulation of iron into rice grains

Iron (Fe) is an essential micronutrient for plant growth and human health. Plants have evolved an efficient transport system for absorbing and redistributing Fe from the soil to other organs; however, the molecular mechanisms underlying Fe loading into grains are poorly understood. Our study shows that OsNRAMP7, a member of the natural resistance-associated macrophage protein (NRAMP) family, is a rice Fe transporter that localizes to the Golgi and trans-Golgi network (TGN). OsNRAMP7 was highly expressed in leaf blade, node I, pollen, and vascular tissues of almost tissues at the rice flowering stage. OsNRAMP7 knockdown by RNA interference (RNAi) increased Fe accumulation in the flag leaf blade, but decreased the Fe concentration in node I and rice grains. In addition, the knockdown of OsNRAMP7 also reduced grain fertility, pollen viability, and grain Fe concentration in the paddy fields; OsNRAMP7 overexpression significantly promoted Fe accumulation in the grains. Thus, our results suggest that OsNRAMP7 is required for the distribution and accumulation of Fe in rice grains and its overexpression could be a novel strategy for Fe biofortification in staple food crops.

Potassium transporter OsHAK18 mediates potassium and sodium circulation and sugar translocation in rice

High-affinity potassium (K+) transporter (HAK)/K+ uptake permease (KUP)/K+ transporter (KT) have been identified in all genome-sequenced terrestrial plants. They play an important role in K+ acquisition and translocation and in enhancing salt tolerance. Here, we report that plasma membrane-located OsHAK18 functions in K+ and sodium (Na+) circulation and sugar translocation in rice (Oryza sativa). OsHAK18 was expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K+ concentration in phloem sap and roots but increased K+ accumulation in the shoot of both 'Nipponbare' and 'Zhonghua11' cultivars, while overexpression (OX) of OsHAK18 driven by its endogenous promoter increased K+ concentration in phloem sap and roots and promoted Na+ retrieval from the shoot to the root under salt stress. Split-root experimental analysis of rubidium (Rb+) uptake and circulation indicated that OsHAK18-OX promoted Rb+ translocation from the shoot to the root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and oppositely affected the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX dramatically increased grain yield and physiological K+ utilization efficiency. Our results suggest that-unlike other OsHAKs analyzed heretofore-OsHAK18 is critical for K+ and Na+ recirculation from the shoot to the root and enhances the source-to-sink translocation of photo-assimilates.

Three-dimensional visualization of the vascular bundle in a branched bamboo node

Bamboo is a natural vascular bundle (VB) reinforced composite material used in more than 10 fields such as construction and furniture. The nodes in bamboo are crucial to its mechanical properties, but understanding of its performance is limited by lack of knowledge about the three-dimensional (3D) structure of the node. This work aimed to non-destructively identify the multi-dimensional characteristics of the VB in a bamboo branched node (BN) using X-ray microtomography (µCT). The VB was segmented from the BN using deep learning combined with the Watershed algorithm. The 3D model reconstruction and characterization of the VB were also conducted. It was found that the structure of VBs showed significant changes along the height of the BN. The VBs formed a complex 3D structure, VBs of the culm are connected with those of the branch, and the connectivity of the conducting tissue and fibers was 88.91% and 99.95%, respectively. The conducting tissue and the fibers had similar shapes but varying thicknesses, which enabled VBs to perform both water transport and mechanical support functions. The volumes fraction of parenchyma, fibers, and conducting tissue in the BN were 61.3%, 35.3%, and 3.4%, respectively, but the tissue proportion of the different heights of the BN varied from each other. The nodal ridge was a mechanical weak point of the BN, with a maximum fibers proportion of 43.8%. This study contributes to understanding the relationship of VBs between the branch and the culm. It provides a structural perspective for understanding the mechanical properties of BN and a theoretical basis for optimizing bamboo utilization efficiency.

ZIP Genes Are Involved in the Retransfer of Zinc Ions during the Senescence of Zinc-Deficient Rice Leaves

Rice lacks sufficient amounts of zinc despite its vitality for human health. Leaf senescence enables redistribution of nutrients to other organs, yet Zn retransfer during deficiency is often overlooked. In this hydroponic experiment, we studied the effect of Zn deficiency on rice seedlings, focusing on the fourth leaf under control and deficient conditions. Growth phenotype analysis showed that the growth of rice nodal roots was inhibited in Zn deficiency, and the fourth leaf exhibited accelerated senescence and increased Zn ion transfer. Analyzing differentially expressed genes showed that Zn deficiency regulates more ZIP family genes involved in Zn ion retransfer. OsZIP3 upregulation under Zn-deficient conditions may not be induced by Zn deficiency, whereas OsZIP4 is only induced during Zn deficiency. Gene ontology enrichment analysis showed that Zn-deficient leaves mobilized more biological pathways (BPs) during aging, and the enrichment function differed from that of normal aging leaves. The most apparent "zinc ion transport" BP was stronger than that of normal senescence, possibly due to Zn-deficient leaves mobilizing large amounts of BP related to lipid metabolism during senescence. These results provide a basis for further functional analyses of genes and the study of trace element transfer during rice leaf senescence.

Phenotypic and genetic dissection of the contents of important metallic elements in hybrid rice grown in cadmium-contaminated paddy fields

Rice (Oryza sativa L.) is a staple food that feeds over half of the world's population, and the contents of metallic elements in rice grain play important roles in human nutrition. In this study, the contents of important metallic elements were determined by ICP-OES, and included cadmium (Cd), zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), nickel (Ni), calcium (Ca), and magnesium (Mg) in brown rice, in the first node from the top (Node 1), in the second node from the top (Node 2), and in roots of 55 hybrids and their parental lines. The heritability of metallic element contents (MECs), the general combining ability (GCA) for MEC, and the correlation between MECs in different organs/tissues of hybrids were also analyzed. The results indicated that: (1) there was a positive correlation between the contents of Cd and Zn in nodes and roots, but a negative correlation between the contents of Cd and Zn in brown rice of the hybrids(2) the GCA for MECs can be used to evaluate the ability of the parental lines to improve the metal contents in brown rice of the hybrids(3) the contents of Cd, Zn, Ca, and Mg in brown rice were mainly affected by additive genetic effects(4) the restorer lines R2292 and R2265 can be used to cultivate hybrids with high Zn and low Cd contents in the brown rice.

Inhibition Roles of Calcium in Cadmium Uptake and Translocation in Rice: A Review

Cadmium (Cd) contamination in rice grains is posing a significant threat to global food security. To restrict the transport of Cd in the soil-rice system, an efficient way is to use the ionomics strategy. Since calcium (Ca) and Cd have similar ionic radii, their uptake and translocation may be linked in multiple aspects in rice. However, the underlying antagonistic mechanisms are still not fully understood. Therefore, we first summarized the current knowledge on the physiological and molecular footprints of Cd translocation in plants and then explored the potential antagonistic points between Ca and Cd in rice, including exchange adsorption on roots, plant cell-wall composition, co-transporter gene expression, and transpiration inhibition. This review provides suggestions for Ca/Cd interaction studies on rice and introduces ionomics research as a means of better controlling the accumulation of Cd in plants.

Metallochaperone OsHIPP9 is involved in the retention of cadmium and copper in rice

Metallochaperones are a unique class of proteins that play crucial roles in metal homoeostasis and detoxification. However, few metallochaperones have been functionally characterised in rice. Heterologous expression of Heavy metal-associated Isoprenylated Plant Protein 9 (OsHIPP9), a metallochaperone, altered yeast tolerance to cadmium (Cd) and copper (Cu). We investigated the physiological role of OsHIPP9 in rice. OsHIPP9 was primarily expressed in the root exodermis and xylem region of enlarged vascular bundles (EVB) at nodes. KO of OsHIPP9 increased the Cd concentrations of the upper nodes and panicle, but decreased Cd in expanded leaves. KO of OsHIPP9 decreased Cu uptake and accumulation in rice. Constitutive OX of OsHIPP9 increased Cd and Cu accumulation in aboveground tissues and brown rice. OsHIPP9 showed binding capacity for Cd and Cu. We propose that OsHIPP9 has dual metallochaperone roles, chelating Cd in the xylem region of EVB for Cd retention in the nodes and chelating Cu in rice roots to aid Cu uptake.

Characterizing the remobilization flux of cadmium from pre-anthesis vegetative pools in rice during grain filling using an improved stable isotope labeling method

A clear understanding of the allocation of Cd to grains is essential to manage the level of Cd in cereal diets effectively. Yet, debate remains over whether and how the pre-anthesis pools contribute to grain Cd accumulation, resulting in uncertainty regarding the need to control plant Cd uptake during vegetative growth. To this end, rice seedlings were exposed to ¹¹¹Cd labeled solution until tillering, transplanted to unlabeled soils, and grown under open-air conditions. The remobilization of Cd derived from pre-anthesis vegetative pools was studied through the fluxes of ¹¹¹Cd-enriched label among organs during grain filling. The ¹¹¹Cd label was continuously allocated to the grain after anthesis. The lower leaves remobilized the Cd label during the earlier stage of grain development, which was allocated almost equally to the grains and husks + rachis. During the final stage, the Cd label was strongly remobilized from the roots and, less importantly, the internodes, which was strongly allocated to the nodes and, to a less extent, the grains. The results show that the pre-anthesis vegetative pools are an important source of Cd in rice grains. The lower leaves, internodes, and roots are the source organs, whereas the husks + rachis and nodes are the sinks competing with the grain for the remobilized Cd. This study provides insight into understanding the ecophysiological mechanism of Cd remobilization and setting agronomic measures for lowering grain Cd levels.

Cadmium isotope fractionation and gene expression evidence for tracking sources of Cd in grains during grain filling in a soil-rice system

Grain filling is the key period that causes excess cadmium (Cd) accumulation in rice grains. Nevertheless, uncertainties remain in distinguishing the multiple sources of Cd enrichment in grains. To better understand the transport and redistribution of Cd to grains upon drainage and flooding during grain filling, Cd isotope ratios and Cd-related gene expression were investigated in pot experiments. The results showed that the Cd isotopes in rice plants were much lighter than those in soil solutions (∆^114/110Cd_{rice-soil solution} = -0.36 to -0.63 ‰) but moderately heavier than those in Fe plaques (∆^114/110Cd_{rice-Fe plaque} = 0.13 to 0.24 ‰). Calculations revealed that Fe plaque might serve as the source of Cd in rice (69.2 % to 82.6 %), particularly upon flooding at the grain filling stage (82.6 %). Drainage at the grain filling stage yielded a larger extent of negative fractionation from node I to the flag leaves (∆^114/110Cd_{flag leaves-node I} = -0.82 ± 0.03 ‰), rachises (∆^114/110Cd_{rachises-node I} = -0.41 ± 0.04 ‰) and husks (∆^114/110Cd_{rachises-node I} = -0.30 ± 0.02 ‰), and significantly upregulated the OsLCT1 (phloem loading) and CAL1 (Cd-binding and xylem loading) genes in node I relative to that upon flooding. These results suggest that phloem loading of Cd into grains and transport of Cd-CAL1 complexes to flag leaves, rachises and husks were simultaneously facilitated. Upon flooding of grain filling, the positive fractionation from the leaves, rachises and husks to the grains (∆^114/110Cd_{flag leaves/rachises/husks-node I} = 0.21 to 0.29 ‰) is less pronounced than those upon drainage (∆^114/110Cd_{flag leaves/rachises/husks-node I} = 0.27 to 0.80 ‰). The CAL1 gene in flag leaves is down-regulated relative to that upon drainage. Thus, the supply of Cd from the leaves, rachises and husks to the grains is facilitated during flooding. These findings demonstrate that the excess Cd was purposefully transported to grain via xylem-to-phloem within nodes I upon the drainage during grain filling, and the expression of genes responsible for encoding ligands and transporters together with isotope fractionation could be used to tracking the source of Cd transported to rice grain.

Molecular mechanisms and genetic improvement of low-phosphorus tolerance in rice

Phosphorus (P) is a macronutrient required for plant growth and reproduction. Orthophosphate (Pi), the preferred P form for plant uptake, is easily fixed in the soil, making it unavailable to plants. Limited phosphate rock resources, low phosphate fertilizer use efficiency and high demands for green agriculture production make it important to clarify the molecular mechanisms underlying plant responses to P deficiency and to improve plant phosphate efficiency in crops. Over the past 20 years, tremendous progress has been made in understanding the regulatory mechanisms of the plant P starvation response. Here, we systematically review current research on the mechanisms of Pi acquisition, transport and distribution from the rhizosphere to the shoot; Pi redistribution and reuse during reproductive growth; and the molecular mechanisms of arbuscular mycorrhizal symbiosis in rice (Oryza sativa L.) under Pi deficiency. Furthermore, we discuss several strategies for boosting P utilization efficiency and yield in rice.

Nodes play a major role in cadmium (Cd) storage and redistribution in low-Cd-accumulating rice (Oryza sativa L.) cultivars

Rice cadmium (Cd) contamination is one of the critical agricultural issues. Breeding of low-Cd-accumulating cultivar is an effective approach to reduce Cd bioaccumulation in rice. To investigate the molecular mechanism underlying Cd transport in rice, the functions of nodes in Cd transport are explored. The results show that different nodes have different functions of Cd transport in the rice plant and the physiological structure of the first node under panicle (N1) determine the Cd accumulation in the brown rice. The upper nodes can redistribute the Cd transport in aboveground tissues. The expressions of Cd-efflux transporter genes (OsLCT1 and OsHMA2) located on the plasma-membrane are the main factors affecting the Cd transport form node to brown rice, which are more depended on the node functions but not the node Cd concentrations. Lower expressions of OsLCT1 and OsHMA2 in N1 result in lower Cd transport from node to brown rice. The size of vascular-bundle (VB) areas in the junctional node with the flag leaf can determine the expression of OsHMA2 and the expression of OsLCT1 positively correlated with the Cd transport ability of first node (N1). The expressions of OsVIT2 and OsABCC1 cannot allow Cd to be immobilized into the vacuoles in node. The VB structure and Cd transporter gene expression level of N1 proved that the Cd concentration of N1 can be used as an important indicator for screening low-Cd-accumulating cultivars. The major implication is that selecting or breeding cultivars with lower Cd accumulations in N1 could be an effective strategy to reduce Cd accumulation in rice grains.

Genetic Regulation Mechanism of Cadmium Accumulation and Its Utilization in Rice Breeding

Cadmium (Cd) is a heavy metal whose pollution in rice fields leads to varying degrees of Cd accumulation in rice. Furthermore, the long-term consumption of Cd-contaminated rice is harmful to human health. Therefore, it is of great theoretical significance and application value to clarify the genetic regulation mechanism of Cd accumulation in rice and cultivate rice varieties with low Cd accumulation for the safe use of Cd-contaminated soils. This review summarizes the effects of Cd on rice growth, yield, and quality; the physiological and molecular mechanisms of Cd absorption in the roots, loading, and transport of Cd in the xylem, the distribution of Cd in nodes, redistribution of Cd in leaves, and accumulation of Cd in the grains; the regulation mechanism of the Cd stress response; and the breeding of rice with low Cd accumulation. Future directions on the genetic regulation of Cd in rice and application are also discussed. This review provides a theoretical basis for studies exploring the genetic regulation of Cd stress in rice. It also offers a basis for formulating effective strategies to reduce the Cd content in rice.

A tale of two metals: Biofortification of rice grains with iron and zinc

Iron (Fe) and zinc (Zn) are essential micronutrients needed by virtually all living organisms, including plants and humans, for proper growth and development. Due to its capacity to easily exchange electrons, Fe is important for electron transport in mitochondria and chloroplasts. Fe is also necessary for chlorophyll synthesis. Zn is a cofactor for several proteins, including Zn-finger transcription factors and redox metabolism enzymes such as copper/Zn superoxide dismutases. In humans, Fe participates in oxygen transport, electron transport, and cell division whereas Zn is involved in nucleic acid metabolism, apoptosis, immunity, and reproduction. Rice (Oryza sativa L.) is one of the major staple food crops, feeding over half of the world's population. However, Fe and Zn concentrations are low in rice grains, especially in the endosperm, which is consumed as white rice. Populations relying heavily on rice and other cereals are prone to Fe and Zn deficiency. One of the most cost-effective solutions to this problem is biofortification, which increases the nutritional value of crops, mainly in their edible organs, without yield reductions. In recent years, several approaches were applied to enhance the accumulation of Fe and Zn in rice seeds, especially in the endosperm. Here, we summarize these attempts involving transgenics and mutant lines, which resulted in Fe and/or Zn biofortification in rice grains. We review rice plant manipulations using ferritin genes, metal transporters, changes in the nicotianamine/phytosiderophore pathway (including biosynthetic genes and transporters), regulators of Fe deficiency responses, and other mutants/overexpressing lines used in gene characterization that resulted in Fe/Zn concentration changes in seeds. This review also discusses research gaps and proposes possible future directions that could be important to increase the concentration and bioavailability of Fe and Zn in rice seeds without the accumulation of deleterious elements. We also emphasize the need for a better understanding of metal homeostasis in rice, the importance of evaluating yield components of plants containing transgenes/mutations under field conditions, and the potential of identifying genes that can be manipulated by gene editing and other nontransgenic approaches.

Novel rice iron biofortification approaches using expression of ZmYS1 and OsTOM1 controlled by tissue-specific promoters

Intrinsic improvement of iron (Fe) concentration in rice grains, called rice Fe biofortification, is a promising countermeasure against widespread human Fe deficiency. In this study, two novel rice Fe biofortification approaches are reported. The first approach (Y approach) involved the expression of maize YELLOW STRIPE 1 controlled by the HEAVY METAL ATPASE 2 promoter. The Y approach increased the polished grain Fe concentrations up to 4.8-fold compared with the non-transgenic (NT) line. The second approach (T approach) involved the expression of rice TRANSPORTER OF MUGINEIC ACID 1 controlled by the FERRIC REDUCTASE DEFECTIVE LIKE 1 promoter. The T approach increased the polished grain Fe concentrations by up to 3.2-fold. No synergistic increases in the polished grain Fe concentrations were observed when Y and T approaches were combined (YT approach). However, the polished grain Fe concentrations further increased by 5.1- to 9.3-fold compared with the NT line, when YT approach was combined with the endosperm-specific expression of FERRITIN (YTF approach), or when YTF approach was combined with the constitutive expression of NICOTIANAMINE SYNTHASE (YTFN approach). Total grain weight per plant in most Y, T, YT, and YTFN lines was comparable to that in the NT line, while it was significantly decreased in most YTF lines. The novel approaches reported in this study expand the portfolio of genetic engineering strategies that can be used for Fe biofortification in rice.

Effects of Cd uptake, translocation and redistribution in different hybrid rice varieties on grain Cd concentration

In order to identify the key transport process that determines the Cd concentration in brown rice, this study used 21 hybrid rice varieties as experimental materials and conducted field experiments in Qiyang (cadmium-contaminated site) and Yongding (low-cadmium site). Cd concentrations in 8 organs were measured, and bioconcentration factors and transfer factor were further calculated. The results showed that the Cd concentrations of the organs related to the xylem transport were as follows: root > node > stem > leaf sheath > leaf. In the phloem, the Cd concentrations were as follows: rachis > brown rice > rice husk. And the results of the correlation analysis found that Cd concentration between brown rice and root showed a significant positive correlation in Cd-contaminated site, but no significant correlation in low-cadmium site. Meanwhile, at both experimental sites, the Cd concentration of brown rice showed the most significant correlation with the phloem transfer factor from leaf and leaf sheath to brown rice. Principal Component Analysis (PCA) and stepwise regression analysis likewise found that Cd concentration in leaf and leaf sheath and their phloem transport of Cd to brown rice were significantly and positively correlated with Cd concentration in brown rice. The above results showed that the transport of leaf and leaf sheath to brown rice was a key process, and played a more important role in the accumulation of cadmium in brown rice than in root.

Effect of foliage applied chitosan-based silicon nanoparticles on arsenic uptake and translocation in rice (Oryza sativa L.)

In this study, chitosan-based silicon nanoparticles (Chsi-NPs) are prepared that primarily consists of C (57.9%), O (31.3%), N (5.6%), and Si (3.5%) and are 10-180 nm in size. We then explore the effect on the foliage applied on rice planted on soil contaminated with 104 mg·kg^-1 arsenic (As); low (3 mg·L^-1)and high (15 mg·L^-1) doses of the foliar Chsi-NPs are administered during the rice grain filling stage. The results showed that the higher dose foliar Chsi-NPs treatment reduced the As concentration in the grain by 61.2% but increased As concentration in the leaves by 47.1% compared to the control treatment. The foliar spraying of the Chsi-NPs inhibited As transport to the grain by facilitating the attachment of As to the cell wall, with higher doses of the foliar Chsi-NPs treatment increased by 8.7%. The foliar spraying of Chsi-NPs increased the malondialdehyde levels by 18.4%, the catalase activity by 49.0%, and the glutathione activity by 99.0%. These results indicated that the foliar Chsi-NPs application was effective for alleviating As toxicity and accumulation in rice. This study provides a novel method for effectively alleviating As accumulation in rice.

A crucial role for a node-localized transporter, HvSPDT, in loading phosphorus into barley grains

Grains are the major sink of phosphorus (P) in cereal crops, accounting for 60-85% of total plant P, but the mechanisms underlying P loading into the grains are poorly understood. We functionally characterized a transporter gene required for the distribution of P to the grains in barley (Hordeum vulgare), HvSPDT (SULTR-like phosphorus distribution transporter). HvSPDT encoded a plasma membrane-localized Pi/H⁺ cotransporter. It was mainly expressed in the nodes at both the vegetative and reproductive stages. Furthermore, its expression was induced by inorganic phosphate (Pi) deficiency. In the nodes, HvSPDT was expressed in both the xylem and phloem region of enlarged and diffuse vascular bundles. Knockout of HvSPDT decreased the distribution of P to new leaves, but increased the distribution to old leaves at the vegetative growth stage under low P supply. However, knockout of HvSPDT did not alter the redistribution of P from old to young organs. At the reproductive stage, knockout of HvSPDT significantly decreased P allocation to the grains, resulting in a considerable reduction in grain yield, especially under P-limited conditions. Our results indicate that node-based HvSPDT plays a crucial role in loading P into barley grains through preferentially distributing P from the xylem and further to the phloem.

Characterization of Stem Nodes Associated with Carbon Partitioning in Maize in Response to Nitrogen Availability

Stem node has been found to be a hub for controlling mineral nutrient distribution in gramineous plants. However, the characteristics of stem nodes associated with whole-plant carbon partitioning in maize (Zea mays L.) and their responses to nitrogen (N) availability remains elusive. Maize plants were grown in greenhouse under low to high N supply. Plant growth, sugar accumulation, and sugar transporters in nodes and leaves, as well as the anatomical structure of nodes, were investigated at vegetative phase. When compared to N-sufficient plants, low-N availability stunted growth and resulted in 49–64% less sugars in leaves, which was attributed to low photosynthesis or the accelerated carbon export, as evidenced by more ¹³C detected further below leaf tips. Invariably higher sugar concentrations were found in the stem nodes, rather than in the leaves across N treatments, indicating a crucial role of nodes in facilitating whole-plant carbon partitioning. More and smaller vascular bundles and phloem were observed in stem nodes of N-deficient plants, while higher sugar levels were found in the bottom nodes than in the upper ones. Low-N availability upregulated the gene expressions of sugar transporters, which putatively function in nodes such as ZmSWEETs and ZmSUTs at the bottom stem, but suppressed them in the upper ones, showing a developmental impact on node function. Further, greater activity of sugar transporters in the bottom nodes was associated with less sugars in leaves. Overall, these results highlighted that stem nodes may play an important role in facilitating long-distance sugar transport in maize.

The rice phosphate transporter OsPHT1;7 plays a dual role in phosphorus redistribution and anther development

Inorganic phosphate (Pi) is the predominant form of phosphorus (P) readily accessible to plants, and Pi Transporter 1 (PHT1) genes are the major contributors to root Pi uptake. However, the mechanisms underlying the transport and recycling of Pi within plants, which are vital for optimizing P use efficiency, remain elusive. Here, we characterized a functionally unknown rice (Oryza sativa) PHT1 member barely expressed in roots, OsPHT1;7. Yeast complementation and Xenopus laevis oocyte assay demonstrated that OsPHT1;7 could mediate Pi transport. Reverse-transcription quantitative polymerase chain reaction and histochemical analyses showed that OsPHT1;7 was preferentially expressed in source leaves and nodes. A further fine-localization analysis by immunostaining showed that OsPHT1;7 expression was restricted in the vascular bundle (VB) sheath and phloem of source leaves as well as in the phloem of regular/diffuse- and enlarged-VBs of nodes. In accordance with this expression pattern, mutation of OsPHT1;7 led to increased and decreased P distribution in source (old leaves) and sink organs (new leaves/panicles), respectively, indicating that OsPHT1;7 is involved in P redistribution. Furthermore, OsPHT1;7 showed an overwhelmingly higher transcript abundance in anthers than other PHT1 members, and ospht1;7 mutants were impaired in P accumulation in anthers but not in pistils or husks. Moreover, the germination of pollen grains was significantly inhibited upon OsPHT1;7 mutation, leading to a >80% decrease in seed-setting rate and grain yield. Taken together, our results provide evidence that OsPHT1;7 is a crucial Pi transporter for Pi transport and recycling within rice plants, stimulating both vegetative and reproductive growth.

Zinc transport in rice: how to balance optimal plant requirements and human nutrition

Zinc (Zn) is an essential micronutrient for both plants and animals, while its deficiency in crops and humans is a global problem that affects both crop productivity and human health. Since plants and humans differ in their Zn requirements, it is crucial to balance plant nutrition and human nutrition for Zn. In this review, we focus on the transport system of Zn from soil to grain in rice (Oryza sativa), which is a major dietary source of Zn for people subsiding on rice-based diets. We describe transporters belonging to the different families that are involved in the uptake, vacuolar sequestration, root-to-shoot translocation, and distribution of Zn, and discuss their mechanisms of regulation. We give examples for enhancing Zn accumulation and bioavailability in rice grains through the manipulation of genes that are highly expressed in the nodes, where Zn is deposited at high concentrations. Finally, we provide our perspectives on breeding rice cultivars with both increased tolerance to Zn-deficiency stress and high Zn density in the grains.

Zinc regulation of iron uptake and translocation in rice (Oryza sativa L.): Implication from stable iron isotopes and transporter genes

Iron (Fe) is an essential nutrient for living organisms and Fe deficiency is a worldwide problem for the health of both rice and humans. Zinc (Zn) contamination in agricultural soils is frequently observed. Here, we studied Fe isotope compositions and transcript levels of Fe transporter genes in rice growing in nutrient solutions having a range of Zn concentrations. Our results show Zn stress reduces Fe uptake by rice and drives its δ⁵⁶Fe value to that of the nutrient solution. These observations can be explained by the weakened Fe(II) uptake through Strategy I but enhanced Fe(III) uptake through Strategy II due to the competition between Zn and Fe(II) combining with OsIRT1 (Fe(II) transporter) in root, which is supported by the downregulated expression of OsIRT1 and upregulated expression of OsYSL15 (Fe(III) transporter). Using a mass balance box model, we also show excess Zn reduces Fe(II) translocation in phloem and its remobilization from senescent leaf, indicating a competition of binding sites on nicotianamine between Zn and Fe(II). This study provides direct evidence that how Zn regulates Fe uptake and translocation in rice and is of practical significance to design strategies to treat Fe deficiency in rice grown in Zn-contaminated soils.

Toxic metals and metalloids: Uptake, transport, detoxification, phytoremediation, and crop improvement for safer food

Agricultural soils are under threat of toxic metal/metalloid contamination from anthropogenic activities, leading to excessive accumulation of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in food crops that poses significant risks to human health. Understanding how these toxic metals and their methylated species are taken up, translocated, and detoxified is prerequisite to developing strategies to limit their accumulation for safer food. Toxic metals are taken up and transported across different cellular compartments and plant tissues via various transporters for essential or beneficial nutrients, e.g. As by phosphate and silicon transporters, and Cd by manganese (Mn), zinc (Zn), and iron (Fe) transporters. These transport processes are subjected to interactions with nutrients and the regulation at the transcriptional and post-translational levels. Complexation with thiol-rich compounds, such as phytochelatins, and sequestration in the vacuoles are the common mechanisms for detoxification and for limiting their translocation. A number of genes involved in toxic metal uptake, transport, and detoxification have been identified, offering targets for genetic manipulation via gene editing or transgenic technologies. Natural variations in toxic metal accumulation exist within crop germplasm, and some of the quantitative trait loci underlying these variations have been cloned, paving the way for marker-assisted breeding of low metal accumulation crops. Using plants to extract and remove toxic metals from soil is also possible, but this phytoremediation approach requires metal hyperaccumulation for efficiency. Knowledge gaps and future research needs are also discussed.

Zinc in plants: Integrating homeostasis and biofortification

Zinc plays many essential roles in life. As a strong Lewis acid that lacks redox activity under environmental and cellular conditions, the Zn²⁺ cation is central in determining protein structure and catalytic function of nearly 10% of most eukaryotic proteomes. While specific functions of zinc have been elucidated at a molecular level in a number of plant proteins, wider issues abound with respect to the acquisition and distribution of zinc by plants. An important challenge is to understand how plants balance between Zn supply in soil and their own nutritional requirement for zinc, particularly where edaphic factors lead to a lack of bioavailable zinc or, conversely, an excess of zinc that bears a major risk of phytotoxicity. Plants are the ultimate source of zinc in the human diet, and human Zn deficiency accounts for over 400 000 deaths annually. Here, we review the current understanding of zinc homeostasis in plants from the molecular and physiological perspectives. We provide an overview of approaches pursued so far in Zn biofortification of crops. Finally, we outline a "push-pull" model of zinc nutrition in plants as a simplifying concept. In summary, this review discusses avenues that can potentially deliver wider benefits for both plant and human Zn nutrition.

Rice functional genomics: decades' efforts and roads ahead

Rice (Oryza sativa L.) is one of the most important crops in the world. Since the completion of rice reference genome sequences, tremendous progress has been achieved in understanding the molecular mechanisms on various rice traits and dissecting the underlying regulatory networks. In this review, we summarize the research progress of rice biology over past decades, including omics, genome-wide association study, phytohormone action, nutrient use, biotic and abiotic responses, photoperiodic flowering, and reproductive development (fertility and sterility). For the roads ahead, cutting-edge technologies such as new genomics methods, high-throughput phenotyping platforms, precise genome-editing tools, environmental microbiome optimization, and synthetic methods will further extend our understanding of unsolved molecular biology questions in rice, and facilitate integrations of the knowledge for agricultural applications.

Metalloid transporters and their regulation in plants

Transport of metalloids including B, Si, and As is mediated by a combination of channels and efflux transporters in plants, which are strictly regulated in response to environmental changes.

Water Management Alters Cadmium Isotope Fractionation between Shoots and Nodes/Leaves in a Soil-Rice System

The drainage of rice soils increases Cd solubility and results in high Cd concentrations in rice grains. However, plant Cd uptake is limited by sorption to iron plaques, and Cd redistribution in the plant is regulated by the nodes. To better understand the interplay of Cd uptake and redistribution in rice under drained and flooded conditions, we determined stable Cd isotope ratios and the expression of genes coding transporters that can transport Cd into the plant cells in a pot experiment. In soil, both water management practices showed similar patterns of isotope variation: the soil solution was enriched in heavy isotopes, and the root Fe plaque was enriched in light isotopes. In rice, the leaves were heavier (Δ^114/110Cd_leaf-shoot = 0.17 to 0.96‰) and the nodes were moderately lighter (Δ^114/110Cd_node-shoot = -0.26 to 0.00‰) relative to the shoots under flooded conditions, indicating preferential retention of light isotopes in nodes and export of heavy isotopes toward leaves. This is generally reversed under drained conditions (Δ^114/110Cd_leaf-shoot = -0.25 to -0.04‰, Δ^114/110Cd_node-shoot = 0.10 to 0.19‰). The drained treatment resulted in significantly higher expression of OsHMA2 and OsLCT1 (phloem loading) but lower expression of OsHMA3 (vacuolar sequestration) in nodes and flag leaves relative to the flooded treatment. It appeared that OsHMA2 and OsLCT1 might preferentially transport isotopically heavier Cd, and the excess Cd was purposefully retranslocated via the phloem under drained conditions when the vacuoles could not retain more Cd. Cd in seeds was isotopically heavier than that in stems under both water management practices, indicating that heavy isotopes were preferentially transferred toward seeds via the phloem, leaving light isotopes retained in stems. These findings demonstrate that the Fe plaque preferentially adsorbs and occludes light Cd isotopes on the root surface, and distinct water management practices alter the gene expression of key transporters in the nodes, which corresponds to a change in isotope fractionation between shoots and nodes/leaves.

A single nucleotide substitution in the SPDT transporter gene reduced phytic acid and increased mineral bioavailability from Rice grain (Oryza sativa L.)

Phosphorus (P) flow in agricultural land depends on the P taken off from harvested product, its losses through runoff and fertilizer applied to balance the removed P. Phytic acid (PA), the major storage form of phosphorus (P) in cereal grains is a key anti-nutrient for human and non-ruminants leads to eutrophication of waterways. As the natural non-renewable P reserves are limited, enhancing P use efficiency is needed for field crops. SULTR-like phosphorus distribution transporter (SPDT) is a novel rice transporter transfer P to the grain. Any alteration in transporter gene reduce grain P with concomitant rise in the leaves. A low PA (3.0 g/kg) rice Khira was identified where a single nucleotide mutation in LOC_Os06g05160 gene encoding SPDT showed low P transportation to grain. An amino acid change was detected as Valine-330 to Alanine at the 3' end of fifth exon. Highest expression of SPDT was observed in node I of rice as compared to low PA genotype. The mutation in SPDT could significantly affect P and PA accumulation in the grains with increased mineral bioavailability. PRACTICAL APPLICATIONS: Excessive P application in crop leads to higher production cost as well as rapid depletion of limited rock phosphate. Alteration of P transporter function in the rice lower PA and total P accumulation in the grains with increased mineral bioavailability. The re-distributed P in the straw can be applied as manure to the rice field. Thus, less P will be removed from the field, result in the decreased requirement for P fertilizer.

Iron Biofortification in Rice: An Update on Quantitative Trait Loci and Candidate Genes

Rice is the most versatile model for cereals and also an economically relevant food crop; as a result, it is the most suitable species for molecular characterization of Fe homeostasis and biofortification. Recently there have been significant efforts to dissect genes and quantitative trait loci (QTL) associated with Fe translocation into rice grains; such information is highly useful for Fe biofortification of cereals but very limited in other species, such as maize (Zea mays) and wheat (Triticum aestivum). Given rice's centrality as a model for Poaceae species, we review the current knowledge on genes playing important roles in Fe transport, accumulation, and distribution in rice grains and QTLs that might explain the variability in Fe concentrations observed in different genotypes. More than 90 Fe QTLs have been identified over the 12 rice chromosomes. From these, 17 were recorded as stable, and 25 harbored Fe-related genes nearby or within the QTL. Among the candidate genes associated with Fe uptake, translocation, and loading into rice grains, we highlight the function of transporters from the YSL and ZIP families; transporters from metal-binding molecules, such as nicotianamine and deoxymugineic acid; vacuolar iron transporters; citrate efflux transporters; and others that were shown to play a role in steps leading to Fe delivery to seeds. Finally, we discuss the application of these QTLs and genes in genomics assisted breeding for fast-tracking Fe biofortification in rice and other cereals in the near future.

Role of a vacuolar iron transporter OsVIT2 in the distribution of iron to rice grains

Iron (Fe) from rice grains is an important source of dietary intake; however, the molecular mechanisms responsible for loading of Fe to the grains are poorly understood. We functionally characterized a vacuolar iron transporter gene, OsVIT2 in terms of expression pattern, cellular localization, and mutant phenotypes. OsVIT2 was expressed in the parenchyma cell bridges of nodes, in the mestome sheath of leaf sheath and aleurone of the caryopsis. Mutation of OsVIT2 resulted in decreased Fe distribution to the leaf sheath, nodes, and aleurone, but increased Fe to the leaf blade and grains. Furthermore, Fe was heavily deposited in the parenchyma cell bridges, mestome sheath and aleurone in the wild-type rice, but this accumulation was decreased in the knockout lines. Conversely, heavier deposition of Fe was observed in the embryo and endosperm of the grains of knockout lines compared with the wild-type rice, resulting in increased Fe accumulation in the polished rice without yield penalty. These results indicate that OsVIT2 is involved in the distribution of Fe to the grains through sequestering Fe into vacuoles in mestome sheath, nodes, and aleurone layer and that knockout of this gene provides a potential way for Fe biofortification without yield penalty.

Molecular Interaction and Evolution of Jasmonate Signaling With Transport and Detoxification of Heavy Metals and Metalloids in Plants

An increase in environmental pollution resulting from toxic heavy metals and metalloids [e.g., cadmium (Cd), arsenic (As), and lead (Pb)] causes serious health risks to humans and animals. Mitigation strategies need to be developed to reduce the accumulation of the toxic elements in plant-derived foods. Natural and genetically-engineered plants with hyper-tolerant and hyper-accumulating capacity of toxic minerals are valuable for phytoremediation. However, the molecular mechanisms of detoxification and accumulation in plants have only been demonstrated in very few plant species such as Arabidopsis and rice. Here, we review the physiological and molecular aspects of jasmonic acid and the jasmonate derivatives (JAs) in response to toxic heavy metals and metalloids. Jasmonates have been identified in, limiting the accumulation and enhancing the tolerance to the toxic elements, by coordinating the ion transport system, the activity of antioxidant enzymes, and the chelating capacity in plants. We also propose the potential involvement of Ca2+ signaling in the stress-induced production of jasmonates. Comparative transcriptomics analyses using the public datasets reveal the key gene families involved in the JA-responsive routes. Furthermore, we show that JAs may function as a fundamental phytohormone that protects plants from heavy metals and metalloids as demonstrated by the evolutionary conservation and diversity of these gene families in a large number of species of the major green plant lineages. Using ATP-Binding Cassette G (ABCG) transporter subfamily of six representative green plant species, we propose that JA transporters in Subgroup 4 of ABCGs may also have roles in heavy metal detoxification. Our paper may provide guidance toward the selection and development of suitable plant and crop species that are tolerant to toxic heavy metals and metalloids.

Fine regulation system for distribution of boron to different tissues in rice

Boron (B) is essential for growth and development, with the B requirement differing depending on the particular organs and tissues, but the molecular mechanisms underlying the preferential distribution of B to different tissues are poorly understood. We investigated the role of a rice gene (OsBOR1) encoding a B efflux transporter in the distribution of B to different tissues under different B supplies. OsBOR1 was highly expressed in the nodes at all growth stages. The OsBOR1 protein shows polar localization at the distal side of bundle sheath cells in nodes and xylem parenchyma cells of elongating leaf sheath, but in the mature leaf sheath and blade at the proximal side of bundle sheath cells. Furthermore, the expression of OsBOR1 was not affected by external B fluctuations, but the OsBOR1 protein was gradually degraded in response to high B. Knockout of this gene altered B distribution, decreasing the distribution of B to new leaves and panicles but increasing B distribution to old leaves. These results indicate that OsBOR1 expressed in nodes and leaf sheath is involved in the preferential distribution of B to different tissues in rice. Furthermore, the OsBOR1 undergoes degradation in response to high B for fine regulation of B distribution to different tissues.

Ionomic variation in leaves of 819 plant species growing in the botanical garden of Hokkaido University, Japan

Ionomics is the measurement of total metal, metalloid, and nonmetal accumulation in living organisms. Plant ionomics has been applied to various types of research in the last decade. It has been reported that the ionome of a plant is strongly affected by its evolution and by environmental factors. In this study, we analyzed the concentration of 23 elements in leaves of 819 plant species (175 families) growing in the Botanic Garden of Hokkaido University, Japan. Relative variation estimated by the coefficient of variation in foliar concentrations of essential elements among various plant species tended to be low, whereas nickel concentration showed exceptionally large relative variation. By contrast, the relative variation in nonessential elements was high, particularly in sodium, aluminum, and arsenic. The higher relative variations in these element concentrations can be explained by the occurrence of plants that are hyperaccumulators for these elements. Differences in life forms such as herbaceous/woody species, deciduous/evergreen woody species and annual/perennial herbaceous species affected the concentration of several elements in the leaves. These differences were considered to be due to the combined factors including differences in lifespan, growth rate, and cell wall thickness of the leaves. Results of principal component analyses (based on concentration data of essential and nonessential elements in leaf samples) indicated phylogenetic influences on plant ionomes at the family level in Polypodiales, Pinales, Poales, and Ericales. Furthermore, when analyzing correlations among concentrations of all elements in each order and comparing among different orders, the results also suggested that Polypodiales, Pinales, and Poales each had a specific ion homeostasis network.

A transporter for delivering zinc to the developing tiller bud and panicle in rice

Tiller number is one of the most important agronomic traits that determine rice (Oryza sativa) yield. Active growth of tiller bud (TB) requires high amount of mineral nutrients; however, the mechanism underlying the distribution of mineral nutrients to TB with low transpiration is unknown. Here, we found that the distribution of Zn to TB is mediated by OsZIP4, one of the ZIP (ZRT, IRT-like protein) family members. The expression of OsZIP4 was highly detected in TB and nodes, and was induced by Zn deficiency. Immunostaining analysis revealed that OsZIP4 was mainly expressed in phloem of diffuse vascular bundles in the nodes and the axillary meristem. The mutation of OsZIP4 did not affect the total Zn uptake, but altered Zn distribution; less Zn was delivered to TB and new leaf, but more Zn was retained in the basal stems at the vegetative growth stage. Bioimaging analysis showed that the mutant aberrantly accumulated Zn in enlarged and transit vascular bundles of the basal node, whereas in wild-type high accumulation of Zn was observed in the meristem part. At the reproductive stage, mutation of OsZIP4 resulted in delayed panicle development, which is associated with decreased Zn distribution to the panicles. Collectively, OsZIP4 is involved in transporting Zn to the phloem of diffuse vascular bundles in the nodes for subsequent distribution to TBs and other developing tissues. It also plays a role in transporting Zn to meristem cells in the TBs.

Leaf-derived ABA regulates rice seed development via a transporter-mediated and temperature-sensitive mechanism

Long-distance transport of the phytohormone abscisic acid (ABA) has been studied for ~50 years, yet its mechanistic basis and biological significance remain very poorly understood. Here, we show that leaf-derived ABA controls rice seed development in a temperature-dependent manner and is regulated by defective grain-filling 1 (DG1), a multidrug and toxic compound extrusion transporter that effluxes ABA at nodes and rachilla. Specifically, ABA is biosynthesized in both WT and dg1 leaves, but only WT caryopses accumulate leaf-derived ABA. Our demonstration that leaf-derived ABA activates starch synthesis genes explains the incompletely filled and floury seed phenotypes in dg1 Both the DG1-mediated long-distance ABA transport efficiency and grain-filling phenotypes are temperature sensitive. Moreover, we extended these mechanistic insights to other cereals by observing similar grain-filling defects in a maize DG1 ortholog mutant. Our study demonstrates that rice uses a leaf-to-caryopsis ABA transport-based mechanism to ensure normal seed development in response to variable temperatures.

Two metallothionein genes highly expressed in rice nodes are involved in distribution of Zn to the grain

A rice node is a hub for distribution of mineral elements; however, most genes highly expressed in the node have not been functionally characterized. Transcriptomic analysis of a rice node revealed that two metallothionein genes, OsMT2b and OsMT2c, were highly expressed in the node I. We functionally characterized these genes in terms of gene expression pattern, cellular and subcellular localization, phenotypic analysis of the single and double knockout mutants and metal-binding ability. Both OsMT2b and OsMT2c were mainly and constitutively expressed in the phloem region of enlarged and diffuse vascular bundles in the nodes and of the anther. Knockout of either OsMT2b or OsMT2c increased zinc (Zn) accumulation in the nodes, but decreased Zn distribution to the panicle, resulting in decreased grain yield. A double mutant, osmt2bmt2c, showed further negative effects on the Zn distribution and grain yield. By contrast, knockout of OsMT2b had a small effect on copper (Cu) accumulation. Both OsMT2b and OsMT2c showed binding ability with Zn, whereas only OsMT2b showed binding ability with Cu in yeast. Our results suggest that both OsMT2b and OsMT2c play an important role mainly in the distribution of Zn to grain through chelation and subsequent transport of Zn in the phloem in rice.

Recent Advances in Carbon and Nitrogen Metabolism in C3 Plants

C and N are the most important essential elements constituting organic compounds in plants. The shoots and roots depend on each other by exchanging C and N through the xylem and phloem transport systems. Complex mechanisms regulate C and N metabolism to optimize plant growth, agricultural crop production, and maintenance of the agroecosystem. In this paper, we cover the recent advances in understanding C and N metabolism, regulation, and transport in plants, as well as their underlying molecular mechanisms. Special emphasis is given to the mechanisms of starch metabolism in plastids and the changes in responses to environmental stress that were previously overlooked, since these changes provide an essential store of C that fuels plant metabolism and growth. We present general insights into the system biology approaches that have expanded our understanding of core biological questions related to C and N metabolism. Finally, this review synthesizes recent advances in our understanding of the trade-off concept that links C and N status to the plant's response to microorganisms.

Phylogeny and gene expression of the complete NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY in Triticum aestivum

NPF genes encode membrane transporters involved in the transport of a large variety of substrates including nitrate and peptides. The NPF gene family has been described for many plants, but the whole NPF gene family for wheat has not been completely identified. The release of the wheat reference genome has enabled the identification of the entire wheat NPF gene family. A systematic analysis of the whole wheat NPF gene family was performed, including responses of specific gene expression to development and nitrogen supply. A total of 331 NPF genes (113 homoeologous groups) have been identified in wheat. The chromosomal location of the NPF genes is unevenly distributed, with predominant occurrence in the long arms of the chromosomes. The phylogenetic analysis indicated that wheat NPF genes are closely clustered with Arabidopsis, Brachypodium, and rice orthologues, and subdivided into eight subfamilies. The expression profiles of wheat NPF genes were examined using RNA-seq data, and a subset of 44 NPF genes (homoeologous groups) with contrasting expression responses to nitrogen and/or development in different tissues were identified. The systematic identification of gene composition, chromosomal locations, evolutionary relationships, and expression profiles contributes to a better understanding of the roles of the wheat NPF genes and lays the foundation for further functional analysis in wheat.

Genetic Manipulation for Improved Nutritional Quality in Rice

Food with higher nutritional value is always desired for human health. Rice is the prime staple food in more than thirty developing countries, providing at least 20% of dietary protein, 3% of dietary fat and other essential nutrients. Several factors influence the nutrient content of rice which includes agricultural practices, post-harvest processing, cultivar type as well as manipulations followed by selection through breeding and genetic means. In addition to mutation breeding, genetic engineering approach also contributed significantly for the generation of nutrition added varieties of rice in the last decade or so. In the present review, we summarize the research update on improving the nutritional characteristics of rice by using genetic engineering and mutation breeding approach. We also compare the conventional breeding techniques of rice with modern molecular breeding techniques toward the generation of nutritionally improved rice variety as compared to other cereals in areas of micronutrients and availability of essential nutrients such as folate and iron. In addition to biofortification, our focus will be on the efforts to generate low phytate in seeds, increase in essential fatty acids or addition of vitamins (as in golden rice) all leading to the achievements in rice nutrition science. The superiority of biotechnology over conventional breeding being already established, it is essential to ascertain that there are no serious negative agronomic consequences for consumers with any difference in grain size or color or texture, when a nutritionally improved variety of rice is generated through genetic engineering technology.

Node-Localized Transporters of Phosphorus Essential for Seed Development in Rice

About 60–85% of total phosphorus (P) in cereal crops is finally allocated to seeds, where it is required for seed development, germination and early growth. However, little is known about the molecular mechanisms underlying P allocation to seeds. Here, we found that two members (OsPHO1;1 and OsPHO1;2) of the PHO1 gene family are involved in the distribution of P to seeds in rice. Both OsPHO1;1 and OsPHO1;2 were localized to the plasma membrane and showed influx transport activities for inorganic phosphate. At the reproductive stage, both OsPHO1;1 and OsPHO1;2 showed higher expression in node I, the uppermost node connecting to the panicle. OsPHO1;1 was mainly localized at the phloem region of diffuse vascular bundles (DVBs) of node I, while OsPHO1;2 was expressed in the xylem parenchyma cells of the enlarged vascular bundles (EVBs). In addition, they were also expressed in the ovular vascular trace, the outer layer of the inner integument (OsPHO1;1) and in the nucellar epidermis (OsPHO1;2) of caryopses. Knockout of OsPHO1;2, as well as OsPHO1;1 to a lesser extent, decreased the distribution of P to the seed, resulting in decreased seed size and delayed germination. Taken together, OsPHO1;2 expressed in node I is responsible for the unloading of P from the xylem of EVBs, while OsPHO1;1 is involved in reloading P into the phloem of DVBs for subsequent allocation of P to seeds. Furthermore, OsPHO1;1 and OsPHO1;2 expression in the caryopsis is important for delivering P from the maternal tissues to the filial tissues for seed development.