AtPHT4;4 is a chloroplast-localized ascorbate transporter in Arabidopsis

Ascorbate is an antioxidant and coenzyme for various metabolic reactions in vivo. In plant chloroplasts, high ascorbate levels are required to overcome photoinhibition caused by strong light. However, ascorbate is synthesized in the mitochondria and the molecular mechanisms underlying ascorbate transport into chloroplasts are unknown. Here we show that AtPHT4;4, a member of the phosphate transporter 4 family of Arabidopsis thaliana, functions as an ascorbate transporter. In vitro analysis shows that proteoliposomes containing the purified AtPHT4;4 protein exhibit membrane potential- and Cl-dependent ascorbate uptake. The AtPHT4;4 protein is abundantly expressed in the chloroplast envelope membrane. Knockout of AtPHT4;4 results in decreased levels of the reduced form of ascorbate in the leaves and the heat dissipation process of excessive energy during photosynthesis is compromised. Taken together, these observations indicate that the AtPHT4;4 protein is an ascorbate transporter at the chloroplast envelope membrane, which may be required for tolerance to strong light stress

LjMATE1: a citrate transporter responsible for iron supply to the nodule infection zone of Lotus japonicus.

Symbiotic nitrogen fixation by intracellular rhizobia within legume root nodules requires the exchange of nutrients between host plant cells and their resident bacteria. While exchanged molecules imply nitrogen compounds, carbohydrates and also various minerals, knowledge of the molecular basis of plant transporters that mediate those metabolite exchanges is still limited. In this study, we have shown that a multidrug and toxic compound extrusion (MATE) protein, LjMATE1, is specifically induced during nodule formation, which nearly paralleled nodule maturation, in a model legume Lotus japonicus. Reporter gene experiments indicated that the expression of LjMATE1 was restricted to the infection zone of nodules. To characterize the transport function of LjMATE1, we conducted a biochemical analysis using a heterologous expression system, Xenopus oocytes, and found that LjMATE1 is a specific transporter for citrate. The physiological role of LjMATE1 was analyzed after generation of L. japonicus RNA interference (RNAi) lines. One RNAi knock-down line revealed limited growth under nitrogen-deficient conditions with inoculation of rhizobia compared with the controls (the wild type and an RNAi line in which LjMATE1 was not suppressed). It was noteworthy that Fe localization was clearly altered in nodule tissues of the knock-down line. These results strongly suggest that LjMATE1 is a nodule-specific transporter that assists the translocation of Fe from the root to nodules by providing citrate

OsFRDL1 Is a Citrate Transporter Required for Efficient Translocation of Iron in Rice1[OA]

Multidrug and toxic compound extrusion (MATE) transporters represent a large family in plants, but their functions are poorly understood. Here, we report the function of a rice (Oryza sativa) MATE gene (Os03g0216700, OsFRDL1), the closest homolog of barley (Hordeum vulgare) HvAACT1 (aluminum [Al]-activated citrate transporter 1), in terms of metal stress (iron [Fe] deficiency and Al toxicity). This gene was mainly expressed in the roots and the expression level was not affected by either Fe deficiency or Al toxicity. Knockout of this gene resulted in leaf chlorosis, lower leaf Fe concentration, higher accumulation of zinc and manganese concentration in the leaves, and precipitation of Fe in the root's stele. The concentration of citrate and ferric iron in the xylem sap was lower in the knockout line compared to the wild-type rice. Heterologous expression of OsFRDL1 in Xenopus oocytes showed transport activity for citrate. Immunostaining showed that OsFRDL1 was localized at the pericycle cells of the roots. On the other hand, there was no difference in the Al-induced secretion of citrate from the roots between the knockout line and the wild-type rice. Taken together, our results indicate that OsFRDL1 is a citrate transporter localized at the pericycle cells, which is necessary for efficient translocation of Fe to the shoot as a Fe-citrate complex

Identification of a novel QTL for shoot Cd accumulation in rice

Low cadmium (Cd) accumulation in brown rice (Oryza sativa) is an important trait for food safety. An effective way to reduce Cd accumulation in the grain is to control Cd transfer from the roots to the shoots. In the present study, we characterized high-Cd accumulating accession (Nepal 555, indica) and performed QTL analysis for shoot Cd accumulation in a F2 population derived from a cross with a low-Cd accumulating accession (Shwe War, japonica). Physiological study showed that Nepal 555 possesses higher ability for Cd translocation from the roots to shoots, while there was no difference in Cd uptake from external solution between two accessions. Two major QTLs were detected on chromosome 11 and 6. These QTLs explained 47% and 12%, respectively, of phyenotypic variations of Cd accumulation. Furthermore, an interaction between the QTLs on the chromosome 6 and 11 was found

Identification of a novel QTL for shoot Cd accumulation in rice

Low cadmium (Cd) accumulation in brown rice (Oryza sativa) is an important trait for food safety. An effective way to reduce Cd accumulation in the grain is to control Cd transfer from the roots to the shoots. In the present study, we characterized high-Cd accumulating accession (Nepal 555, indica) and performed QTL analysis for shoot Cd accumulation in a F2 population derived from a cross with a low-Cd accumulating accession (Shwe War, japonica). Physiological study showed that Nepal 555 possesses higher ability for Cd translocation from the roots to shoots, while there was no difference in Cd uptake from external solution between two accessions. Two major QTLs were detected on chromosome 11 and 6. These QTLs explained 47% and 12%, respectively, of phyenotypic variations of Cd accumulation. Furthermore, an interaction between the QTLs on the chromosome 6 and 11 was found

A tonoplast‐localized magnesium transporter is crucial for stomatal opening in Arabidopsis under high Mg2+ conditions

Plant stomata play an important role in CO2 uptake for photosynthesis and transpiration, but the mechanisms underlying stomatal opening and closing under changing environmental conditions are still not completely understood. Through large-scale genetic screening, we isolated an Arabidopsis mutant (closed stomata2 (cst2)) that is defective in stomatal opening. We cloned the causal gene (MGR1/CST2) and functionally characterized this gene. The mutant phenotype was caused by a mutation in a gene encoding an unknown protein with similarities to the human magnesium (Mg2+) efflux transporter ACDP/CNNM. MGR1/CST2 was localized to the tonoplast and showed transport activity for Mg2+. This protein was constitutively and highly expressed in guard cells. Knockout of this gene resulted in stomatal closing, decreased photosynthesis and growth retardation, especially under high Mg2+ conditions, while overexpression of this gene increased stomatal opening and tolerance to high Mg2+ concentrations. Furthermore, guard cell-specific expression of MGR1/CST2 in the mutant partially restored its stomatal opening. Our results indicate that MGR1/CST2 expression in the leaf guard cells plays an important role in maintaining cytosolic Mg2+ concentrations through sequestering Mg2+ into vacuoles, which is required for stomatal opening, especially under high Mg2+ conditions