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

Miyaji T, Kuromori T, Takeuchi Y, Yamaji N, Yokosho K, Shimazawa A, Sugimoto E, Omote H, Ma JF, Shinozaki K, Moriyama Y - Nat Commun (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Advanced Science Research Center, Okayama University, Okayama 700-8530, Japan.

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.

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Schematic model of ascorbate transport in chloroplasts.Upon photostress, PHT4;4 gene expression is enhanced, and the PHT4;4 protein at the envelope membranes takes up ascorbate from mitochondria, which is transferred into the thylakoid through an as yet unknown transporter. PHT4;1 is a candidate ascorbate transporter at the thylakoid membrane.
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f7: Schematic model of ascorbate transport in chloroplasts.Upon photostress, PHT4;4 gene expression is enhanced, and the PHT4;4 protein at the envelope membranes takes up ascorbate from mitochondria, which is transferred into the thylakoid through an as yet unknown transporter. PHT4;1 is a candidate ascorbate transporter at the thylakoid membrane.

Mentions: It should be noted that AtPHT4;1 shows a high degree of identity with AtPHT4;4 (identity of 70% at the amino-acid sequence level). Previous studies indicated that both AtPHT4;1 and AtPHT4;4 gene expression are induced by light12. AtPHT4;4 is present in the envelope membrane of the chloroplast, as shown in Fig. 3c, while AtPHT4;1 is localized at the thylakoid membrane of the chloroplast1920. Thus, AtPHT4;1 is likely to also function as an ascorbate transporter at the thylakoid membrane. AtPHT4;4 transports ascorbate into the stroma of the chloroplast, and AtPHT4;1 transports ascorbate from the stroma to the thylakoids to control the dynamic state of ascorbate in the chloroplasts (Fig. 7). Further studies of the transport function and physiological role of PHT4;1 are currently in progress in our laboratory.


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

Miyaji T, Kuromori T, Takeuchi Y, Yamaji N, Yokosho K, Shimazawa A, Sugimoto E, Omote H, Ma JF, Shinozaki K, Moriyama Y - Nat Commun (2015)

Schematic model of ascorbate transport in chloroplasts.Upon photostress, PHT4;4 gene expression is enhanced, and the PHT4;4 protein at the envelope membranes takes up ascorbate from mitochondria, which is transferred into the thylakoid through an as yet unknown transporter. PHT4;1 is a candidate ascorbate transporter at the thylakoid membrane.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308718&req=5

f7: Schematic model of ascorbate transport in chloroplasts.Upon photostress, PHT4;4 gene expression is enhanced, and the PHT4;4 protein at the envelope membranes takes up ascorbate from mitochondria, which is transferred into the thylakoid through an as yet unknown transporter. PHT4;1 is a candidate ascorbate transporter at the thylakoid membrane.
Mentions: It should be noted that AtPHT4;1 shows a high degree of identity with AtPHT4;4 (identity of 70% at the amino-acid sequence level). Previous studies indicated that both AtPHT4;1 and AtPHT4;4 gene expression are induced by light12. AtPHT4;4 is present in the envelope membrane of the chloroplast, as shown in Fig. 3c, while AtPHT4;1 is localized at the thylakoid membrane of the chloroplast1920. Thus, AtPHT4;1 is likely to also function as an ascorbate transporter at the thylakoid membrane. AtPHT4;4 transports ascorbate into the stroma of the chloroplast, and AtPHT4;1 transports ascorbate from the stroma to the thylakoids to control the dynamic state of ascorbate in the chloroplasts (Fig. 7). Further studies of the transport function and physiological role of PHT4;1 are currently in progress in our laboratory.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Advanced Science Research Center, Okayama University, Okayama 700-8530, Japan.

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.

Show MeSH
Related in: MedlinePlus