Abstract
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.
Highlights
Ascorbate is an antioxidant and coenzyme for various metabolic reactions in vivo
Chloroplasts use ascorbate in these metabolic processes to eliminate active oxygen produced by electron transmission of PSII for the synthesis of NADPH in the stroma and as a coenzyme of violaxanthin de-epoxidase (VDE), which is involved in the release of photoenergy by heat dissipation in the xanthophyll cycle[4,6,7,8]
A series of studies performed in our laboratory as well as those reported by other groups indicated that the mammalian SLC17 transporter family consists of nine members, which were shown to be membrane potential (Dc)- and Cl À -dependent organic anion transporters: SLC17A1–2 act as urate exporters at the apical membranes of renal proximal tubules, SLC17A4 acts as a urate exporter at the apical membranes of intestinal ducts, SLC17A5 acts as a vesicular excitatory amino-acid transporter in synaptic vesicles, SLC17A6–8 act as vesicular glutamate transporters in synaptic vesicles, and SLC17A9 acts as a vesicular nucleotide transporter in synaptic vesicles and secretory granules[13,14,15]
Summary
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. (c) Na þ /Pi uptake by proteoliposomes containing purified AtPHT4 proteins at 2 min.
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