Chloroplasts are metabolic powerhouses within leaf cells. They are not only the main site of carbon, nitrogen, and sulfur assimilation but also share metabolic pathways—such as the photorespiratory carbon cycle—with other plant cell compartments. A continuous exchange of metabolites and ions with the cytosol is necessary for playing these roles. In all vascular plants, plastids are surrounded by an envelope that restricts the nonspecific diffusion of polar molecules. Thus, a pair of concentric membranes modulate the exchange of metabolites between the stromal interior and the cytosol (Neuhaus and Wagner 2000). Although many pore-forming proteins that are substrate-specific were characterized in the outer membrane, the inner membrane is considered the main permeability barrier between the cytosol and the chloroplast. All transporters of the inner membrane are nuclear-encoded membrane proteins whose precursors are synthesized in the cytosol and post-translationally imported into the inner membrane.

Chloroplast transporters collectively grouped as phosphate translocators catalyze a strict 1:1 counter-exchange (antiport) of orthophosphate with triose phosphates, pentose phosphates, and hexose phosphates (Weber 2004; Weber et al. 2005). The Arabidopsis genome contains six genes that encode four classes of functional plastidial phosphate translocators (Web Table 8.11.A): single copies of the triose phosphate/phosphate translocator (TPT) and the phosphoenolpyruvate/phosphate translocators (PPT); and two copies each of the glucose 6-phosphate/phosphate translocators (GPT) and the xylulose 5-phosphate/phosphate translocator (XPT).

Translocator	MATDB entry	Compounds transported
TPT	TPT (At5g46110)	triose phosphates, 3-phosphoglycerate
PPT	PPT1 (At5g54800), PPT2 (At3g01550)	phosphoenolpyruvate, 2-phosphoglycerate
GPT	GPT1 (At5g33320), GPT2 (At1g61800)	glucose 6-phosphate, triose phosphates
XPT	(At5g17630)	xylulose 5-phosphate, triose phosphates

Web Table 8.11.A Plastidic Phosphate Translocators

The Triose Phosphate/Phosphate Translocator and the Allocation of Carbon in Plants

Triose phosphates, generated by the Calvin cycle at the expense of photosynthetically generated ATP and NADPH, flow to the cytosol across the chloroplast envelope. In turn, the orthophosphate released in the cytosol from biosynthetic processes is transported back into chloroplasts to replenish the ATP necessary for sustaining the assimilation of CO₂ and photosynthetic electron transport. Both the efflux of triose phosphates and the influx of orthophosphate are driven by a dimer composed of two identical subunits—the chloroplastic TPT. This membrane protein functions as an antiport system that exchanges phosphorylated—mainly three-, but also admits five- or six-carbon compounds—for orthophosphate. Hence, the chloroplast TPT exports the fixed carbon for the cytosolic synthesis of sucrose, the main product allocated to heterotrophic plant organs for further metabolism or conversion into storage products (e.g., starch, fructans). Therefore, the chloroplast TPT represents the diurnal path for carbon export.

Mutants defective in chloroplast TPT rely on different paths to transfer the photoassimilate from the chloroplast to the cytosol and, in so doing, compensate for the deficiency in TPT activity. Therefore, knockout mutations of this translocator increase the allocation of recently fixed carbon into transitory starch, followed by the degradation of the starch in the light and the subsequent export of neutral sugars to the cytosol. On the other hand, the simultaneous knockout of chloroplast TPT and starch synthesis drastically impairs plant growth.

The Phosphoenolpyruvate/Phosphate Translocator is Essential for the Chloroplast Metabolism

PPT in the inner chloroplast membrane also links the stromal metabolism of all plants with the surrounding cytosol. C₄-plants of the NADP-malic enzyme type (e.g., maize) (see Web Topic 8.9) under active photosynthesis produce pyruvate in bundle sheath cells. This three-carbon compound returns to mesophyll cells for the conversion to phosphoenolpyruvate by the chloroplast-localized pyruvate-phosphate dikinase (Web Figure 8.11.A). Subsequently, the phosphoenolpyruvate flows from the stroma to the cytosol for the primary carbon fixation via PEPCase. To ensure high rates of phosphoenolpyruvate efflux for the proper functioning of this cycle, the envelope of mesophyll chloroplasts bears the PPT, which concurrently drives the chloroplast uptake of orthophosphate to restore the lost phosphorus.

Web Figure 8.11.A   Mobilization of phosphoenolpyruvate in mesophyll chloroplasts of C₄ plants of the NAD-malic enzyme type. The pyruvate produced by the malic enzyme (ME) in bundle sheath cells flows to mesophyll cells for conversion to phosphoenolpyruvate (PEP) by the pyruvate-phosphate dikinase (PPDK) localized at the chloroplast stroma. The efflux of PEP to the cytosol through the phosphoenolpyruvate/phosphate translocator (PPT) restores the acceptor of HCO_3- required by the phosphoenolpyruvate carboxylase (PEPCase) for the primary carbon fixation. CA: carbonic anhydrase; NADP-MD: NADP-dependent malate dehydrogenase.

On the other hand, CAM plants of the malic enzyme-type decarboxylate during the day the malate accumulated at night. Again, two successive processes account for the production of phosphoenolpyruvate, the primary acceptor of CO₂. First, the pyruvate formed in decarboxylation of the malate enters into the chloroplast stroma for its transformation to phosphoenolpyruvate catalyzed by the pyruvate-phosphate dikinase. Second, the phosphoenolpyruvate leaves the chloroplast in exchange for orthophosphate via the PPT.

Knowledge on the function of the PPT in C₃ plants or nongreen tissues of C₄ plants is scarce. Apparently, the uptake—rather than the export—of phosphoenolpyruvate is the primary function of PPT in these tissues. Chloroplasts and most nongreen plastids are devoid of phosphoglyceromutase and enolase and, consequently, are unable to convert 3-phosphoglycerate to phosphoenolpyruvate. However, phosphoenolpyruvate combines with erythrose 4-phosphate, an intermediate of the pentose-phosphate pathway, for the synthesis of aromatic compounds through the shikimic acid pathway (Web Figure 8.11.B). Thus, the provision of cytosolic phosphoenolpyruvate to the stroma via the PPT would be essential for the biosynthesis of flavonoids, lignins, alkaloids.

Web Figure 8.11.B   Mobilization of phosphoenolpyruvate in chloroplasts of C₃ plants. Under active photosynthesis, TPT furnish the cytosol with carbon skeletons for the biosynthesis of phosphoenolpyruvate. Upon the release of orthophosphate, the uptake of the cytosolic phosphoenolpyruvate, which combines in the stroma with erythrose 4-phosphate, triggers the formation of 3-deoxy-D-arabino-heptulosonic acid-7-phosphate (DHAP). DHAP is a scaffold for the biosynthesis of shikimic acid-5-phosphate, which in turn starts the pathway leading to the aromatic amino acids tyrosine and phenylalanine, the precursors of flavonoids, lignins, and alkaloids.

The Glucose 6-Phosphate/Phosphate Translocator Functions in Nongreen Tissues and is Essential for Plant Development

The genome of Arabidopsis harbors two paralogous GPT genes, AtGPT1 and AtGPT2, that, apparently, function in plastids of nongreen tissues for the import of glucose 6-phosphate. Two transgenic lines, bearing T-DNA insertions in the GPT1 gene, exhibited not only impaired pollen and ovule development but also were lethal in the homozygous state. At variance, the disruption of the GPT2 genes have no effect on growth and development. On the basis of drastic gametogenesis defects, it appears that the import of glucose 6-phosphate into nongreen plastids by the translocator AtGPT1 is crucial for the maturation of the pollen and the development of the female gametophyte.

The Xylulose 5-Phosphate/Phosphate Translocator

The XPT constitutes the fourth subfamily of phosphate translocators that exhibits substrate specificity similar to the GPT but lacks the capacity to transport glucose 6-phosphate. In Arabidopsis, the lack of cytosolic transketolase and transaldolase halts the cytosolic oxidative pentose phosphate pathway at the stage of pentose phosphates (Kruger and von Schaewen 2003). In this context, the putative function of the XPT would be to furnish the stromal pentose phosphate pathway with cytosolic five-carbon skeletons in the form of xylulose 5-phosphate, especially under conditions that require its removal from the cytosol.