Nonphotosynthetic plastids are important sites for the biosynthesis of starch, fatty acids, and amino acids. The uptake and subsequent use of cytosolic ATP to fuel these and other anabolic processes would lead to the accumulation of inorganic phosphate (Pi) if not balanced by a Pi export activity. However, the identity of the transporter(s) responsible for Pi export is unclear. The plastid-localized Pi transporter PHT4;2 of Arabidopsis (Arabidopsis thaliana) is expressed in multiple sink organs but is nearly restricted to roots during vegetative growth. We identified and used pht4;2 null mutants to confirm that PHT4; 2 contributes to Pi transport in isolated root plastids. Starch accumulation was limited in pht4; 2 roots, which is consistent with the inhibition of starch synthesis by excess Pi as a result of a defect in Pi export. Reduced starch accumulation in leaves and altered expression patterns for starch synthesis genes and other plastid transporter genes suggest metabolic adaptation to the defect in roots. Moreover, pht4; 2 rosettes, but not roots, were significantly larger than those of the wild type, with 40% greater leaf area and twice the biomass when plants were grown with a short (8-h) photoperiod. Increased cell proliferation accounted for the larger leaf size and biomass, as no changes were detected in mature cell size, specific leaf area, or relative photosynthetic electron transport activity. These data suggest novel signaling between roots and leaves that contributes to the regulation of leaf size.

The aim of this study is to identify and functionally characterizesolute transporters from the chloroplast thylakoid membrane ofArabidopsis thaliana. As compared to chloroplast envelope transporters,much less information is available for transport processesacross the thylakoid membrane, which is mostly studied as thesite of light-driven photosynthetic reactions coupled to ATP synthesis.Although there are many reported examples of transportactivities, only a few thylakoid transporters have been identifiedat the gene level. Using bioinformatics analyses, we have predictedthe existence of approx. fifteen thylakoid transporters. Forexperimental validation, we have carried out immuno-localizationstudies used peptide-specific antibodies, functional analyses inheterologous system and validation using knockout mutants. Wehave recently identified one ATP/ADP carrier (Thuswaldneret al. JBC 2007) and one Na(+)-dependent phosphate transporter(Ruiz Pavon et al. JBC 2008). They are proposed to participatein the nucleotide metabolism in the thylakoid lumen(Spetea et al. PNAS 20004) as well as to balance the transthylakodproton electrochemical gradient storage. Based on phenotypicanalyses of knockout mutants, we will present novel dataabout the key physiological role of the two transporters duringthe high-light-induced repair of photosystem II complex in thethylakoid membrane. Subsequently, we will make a survey on theoutlook of thylakoid activities awaiting identification of responsibleproteins. Such knowledge is necessary to understand the thylakoidnetwork of transporters, and its role in photosynthesisand adaptation to environmental stress.

The anion transporter 1 (ANTR1) from Arabidopsis thaliana, homologous to the mammalian SLC17 family, has recently been localized to the chloroplast thylakoid membrane. When expressed heterologously in Escherichia coli, ANTR1 mediates a Na⁺-dependent active transport of inorganic phosphate (Pi). The aim of this study was to identify amino acids involved in substrate binding/translocation by ANTR1 and in the Na⁺-dependence of its activity. A threedimensional structural model of ANTR1 was constructed using the crystal structure of glycerol-3-phosphate/phosphate antiporter (GlpT) from E.coli as a template. Based on this model and multiple sequence alignments, five highly conserved residues in plant ANTRs and mammalian SLC17 homologues have been selected for site-directed mutagenesis, namely Arg-120, Ser-124 and Arg-201 inside the putative translocation pathway, Arg-228 and Asp-382 exposed at the cytosolic surface of the protein. The activities of the wild type and mutant proteins have been analyzed using expression in E. coli and radioactive transport assays, and compared with bacterial cells carrying an empty plasmid. Based on P_i- and Na⁺-dependent kinetics, we propose that Arg-120, Arg-201 and Arg-228 are involved in binding and translocation of the substrate, Ser-124 functions as a periplasmic gate for Na⁺ ions, and finally Asp-382 participates in the turnover of the transporter via ionic interaction with either Arg-228 or Na⁺ ions. We also propose that the corresponding residues may have a similar function in other plant and mammalian SLC17 homologous transporters.

The  thylakoid   anion  transporter 1  (ANTR1)   from  Arabidopsisthaliana,  has been characterized as a Na-dependent Pi transporter when expressed in E. coli (1), but  no data  is yet available  for the protein  structure  and  amino  acids involved in transport of Pi. In this  study  a  three-dimensional structural  model  of  ANTR1  was constructed in silico using the crystal structure  of glycerol-3- phosphate/phosphate antiporter from E. coli as a template.  Based on Multiple  Sequence Alignments (MSAs) with other plant  ANT- Rs  and  mammalian   SLC17  homologues,   five  highly  conserved amino  acids involved in Pi transport have been identified,  namely Arg-120, Ser-124 and Arg-201 inside the putative translocation pathway,  Arg-228  and  Asp-382  exposed  at  the  cytoplasmic  sur- face of the protein.  The activity of the protein  as a Na-dependent Pi transporter in the wild type and mutants  was analyzed  by het- erologous  expression  and  uptake   of  radioactive   Pi  into  E.  coli cells. Substitution of the three Arg (120, 201 and 228) for Glu residues  and  of Asp-382 for  an  Asn residue  resulted  in an  inac- tive ANTR1  transporter. All other  mutants  had sufficient activity to  allow  measurement   of  kinetic  parameters, attesting   that  the mutated  proteins  were functional.  Based on  our  results,  we pro- pose that Arg-201 is a critical residue for substrate  binding and translocation, whereas Ser-124 may function  as periplasmic  gate- way for  Na+   ions.  Residue  Arg-120  plays  an  important role  in Pi  binding  and  associated   conformational  changes,  and  finally that Arg-228 and Asp-382 only weakly participate  in interactions allowing conformational changes to occur at the cytoplasmic  sur-face of the transporter.

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The chloroplast thylakoid ATP/ADP carrier (TAAC) belongs to the mitochondrial carrier superfamily and supplies the thylakoid lumen with stromal ATP in exchange for ADP. Here, we investigate the physiological consequences of TAAC depletion in Arabidopsis (Arabidopsis thaliana). We show that the deficiency of TAAC in two T-DNA insertion lines does not modify the chloroplast ultrastructure, the relative amounts of photosynthetic proteins, the pigment composition, and the photosynthetic activity. Under growth light conditions, the mutants initially displayed similar shoot weight, but lower when reaching full development, and were less tolerant to high light conditions in comparison with the wild type. These observations prompted us to study in more detail the effects of TAAC depletion on photoinhibition and photoprotection of the photosystem II (PSII) complex. The steady-state phosphorylation levels of PSII proteins were not affected, but the degradation of the reaction center II D1 protein was blocked, and decreased amounts of CP43-less PSII monomers were detected in the mutants. Besides this, the mutant leaves displayed a transiently higher nonphotochemical quenching of chlorophyll fluorescence than the wild-type leaves, especially at low light. This may be attributed to the accumulation in the absence of TAAC of a higher electrochemical H+ gradient in the first minutes of illumination, which more efficiently activates photoprotective xanthophyll cycle-dependent and independent mechanisms. Based on these results, we propose that TAAC plays a critical role in the disassembly steps during PSII repair and in addition may balance the trans-thylakoid electrochemical H+ gradient storage.

Plants utilize sunlight to drive photosynthetic energy conversion in the chloroplast thylakoid membrane. Here are located four major photosynthetic complexes, about which we have great knowledge in terms of structure and function. However, much less we know about auxiliary proteins, such as transporters, ensuring an optimum function and turnover of these complexes. The most prominent thylakoid transporter is the proton-translocating ATP-synthase. Recently, four additional transporters have been identified in the thylakoid membrane of Arabidopsis thaliana, namely one copper-transporting P-ATPase, one chloride channel, one phosphate transporter, and one ATP/ADP carrier. Here, we review the current knowledge on the function and physiological role of these transporters during photosynthesis and light stress in plants. Subsequently, we make a survey on the outlook of thylakoid activities awaiting identification of responsible proteins. Such knowledge is necessary to understand the thylakoid network of transporters, and to design strategies for bioengineering crop plants in the future.

Genome sequence of Arabidopsis thaliana (Arabidopsis) revealed two psbO genes (At5g66570 and At3g50820) which encode two distinct PsbO isoforms: PsbO1 and PsbO2, respectively. To get insights into the function of the PsbO1 and PsbO2 isoforms in Arabidopsis we have performed systematic and comprehensive investigations of the whole photosynthetic electron transfer chain in the T-DNA insertion mutant lines, psbO1 and psbo2. The absence of the PsbO1 isoform and presence of only the PsbO2 isoform in the psbo1 mutant results in (i) malfunction of both the donor and acceptor sides of Photosystem (PS) 11 and (ii) high sensitivity of PSII centers to photodamage, thus implying the importance of the PsbO1 isoform for proper structure and function of PSII. The presence of only the PsbO2 isoform in the PSII centers has consequences not only to the function of PSII but also to the PSI/PSII ratio in thylakoids. These results in modification of the whole electron transfer chain with higher rate of cyclic electron transfer around PSI, faster induction of NPQ and a larger size of the PQ-pool compared to WT, being in line with apparently increased chlororespiration in the psbo1 mutant plants. The presence of only the PsbO1 isoform in the psbo2 mutant did not induce any significant differences in the performance of PSII under standard growth conditions as compared to WT. Nevertheless, under high light illumination, it seems that the presence of also the PsbO2 isoform becomes favourable for efficient repair of the PSII complex.

In this study, the putative anion transporter 1 (ANTR1) from Arabidopsis thaliana was shown to be localized to the chloroplast thylakoid membrane by Western blotting with two different peptide-specific antibodies. ANTR1 is homologous to the type I of mammalian Na⁺-dependent inorganic phosphate (P_i) transporters. The function of ANTR1 as a Na⁺-dependent P_i transporter was demonstrated by heterologous expression and uptake of radioactive P_i into Escherichia coli cells. The expression of ANTR1 conferred increased growth rates to the transformed cells and stimulated P_i uptake in a pH- and Na⁺-dependent manner as compared with the control cells. Among various tested effectors, P_i was the preferred substrate. Although it competed with the uptake of P_i, glutamate was not transported by ANTR1 into E. coli. In relation to its function as a P_i transporter, several physiological roles for ANTR1 in the thylakoid membrane are proposed, such as export of P_i produced during nucleotide metabolism in the thylakoid lumen back to the chloroplast stroma and balance of the trans-thylakoid H⁺ electrochemical gradient storage.

Crucial for the optimal function of the oxygen-evolving complex (OEC) is the PsbO subunit of the photosystem II (PSII) complex. Previously we reported the ability of PsbO in spinach to bind and hydrolyze GTP. GTP stimulates the dissociation of PsbO from PSII following illumination and induces the degradation of the D1 protein. We have predicted four plant-specific binding motifs for GTP, which are not conserved in the sequences of the cyanobacteria or green algae PsbO proteins. We have proposed a location of the GTP-binding site inside the β-barrel exposed to the lumenal side. Arabidopsis thaliana has two PsbO isoforms encoded by two different genes: psbO1 and psbO2. Here we have measured and compared the GTPase activities of PSII membranes isolated from Arabidopsis knockouts mutants containing T-DNA insertions in one or the other of the psbO genes. The specific GTPase activity of PsbO2 is three fold higher than that of PsbO1. Furthermore, PsbO2 is more efficiently released than PsbO1 from PSII following light treatment. We conclude that PsbO2 is a better GTPase than Psb.