Phosphate Translocators

In higher plants, photosynthesis is compartmentalized in the chloroplast, which is bounded by the envelope membranes that serve both as a barrier separating the chloroplast stroma from the cytoplasm and a bridge enabling rapid exchange of specific metabolites between the two (Figure 7.1) [45-47]. The outer envelope membrane is nonspecifically permeable to all molecules, both charged and uncharged. The impermeability of the inner envelope membrane to hydrophilic solutes such as Pi, phosphate esters, dicarboxylates, and glucose is overcome by translocators that catalyze specific transfer of metabolites across the envelope [46,47]. The energy-transducing thylakoid membranes, located within the chloroplasts, are distinct from the envelope membranes.

The mechanism by which external Pi influences photosynthesis has been attributed to the operation of the Pi translocator, an antiport located in the inner membrane of the chloroplast envelope that facilitates a rapid counterexchange of Pi, triose-P, and 3-phosphoglyceric acid (PGA) [39,46,47]. The major flow of metabolites across the chloroplast envelope is mediated by the Pi translocator, which enables the specific transport of Pi and phosphorylated compounds such that photosynthetically fixed carbon in the form of triose-P can be exported from the stroma to the cytosol in a one-to-one stoichiometric and obligatory exchange for Pi [48]. The Pi released during biosynthetic processes is shuttled back through the Pi translocator into the chloroplasts for the formation of ATP catalyzed by the thylakoid ATPase [49].

If triose-P is regarded as the end product of the PCR cycle (Figure 7.1), then one molecule of Pi must be made available for incorporation into triose-P for every three molecules of CO2 fixed. Some Pi will be released within the stroma as triose-P is utilized for starch synthesis, but starch synthesis is usually slower (by a factor of 3 to 4) than maximal CO2 fixation. Virtually all the remaining Pi must enter the chloro-plast in exchange for exported triose-P [46-48]. In the short term, a sudden decrease in the Pi concentration in the cytosol of photosynthetic mesophyll cells will have a direct effect on the triose-P and Pi exchange between the chloroplast and the cytosol, decreasing the availability of Pi in the chloroplast and thus decreasing the production of ATP needed in the turnover of the PCR cycle.

Triose phosphate/phosphate translocator (TPT) was the first phosphate transporter to be cloned from plants [50]. The activity of TPT is closely associated with photosynthetic carbon metabolism and the expression of the TPT gene is observed only in photosynthetic tissues [41]. Its importance in in vivo communication between chloroplast and cytosol was demonstrated in transgenic potato plants with reduced expression of the TPT at both RNA and protein levels due to antisense inhibition [51]. Four different groups of Pi transporters have been described so far in plastids and one among them is phosphoenolpyruvate/phosphate transporter, which transports Pi out of the chloroplast into cytosol under most physiological conditions [52].

Recently, Versaw and Harrison [53] described a low-affinity Pi transporter PHT2;1, H+/Pi symporter, located in the inner envelope of the chloroplast. The identification of the null mutant of Arabidopsis thaliana, pht2;1-1, revealed that the PHT2;1 transporter affects Pi allocation and modulates Pi-starvation responses including the expression of genes and the translocation of Pi within leaves [53]. The presence of several transporters indicates highly controlled transport of phosphate into and out of the chloro-plast.

The synthesis of sucrose from triose-P is believed to make the major contribution to the recycling of Pi (Figure 7.1). Sucrose synthesis releases Pi due to the action of a phosphatase and rapid export of sucrose from the cytoplasm will make Pi available as fast as the plant can synthesize triose-P; little or none will be available for storage within the stroma as starch. If the demand for sucrose by growing sinks is less however, excess triose-P would be stored as starch and the rate of photosynthesis possibly diminished.

Another important function of the Pi translocator is to link intra- and extrachloroplast pyridine nucleo-tide and adenylate systems through shuttles involving the exchange of DHAP and PGA. Photosynthetically produced ATP and NADPH are not directly available to the extrachloroplastic compartments due to the low permeability of the inner envelope membrane to these compounds in mature tissue. The Pi translo-cator provides an indirect shuttle system for transferring ATP and NADPH to the cytoplasm involving exchange of triose-P and PGA. This shuttle can operate in either direction depending on the redox potential of the pyridine nucleotides in the cytoplasm and stroma [46].

Gerhardt et al. [54] observed asymmetric distribution of DHAP and 3-PGA across the chloroplast envelope in spinach leaves and suggested that the Pi translocator may be kinetically limiting in vivo. The reduction of TPT activity in vivo by antisense repression of chloroplast TPT resembles the situation of chloroplasts performing photosynthesis under Pi limitation [39]. To examine more specifically the role of the Pi translocator in assimilate partitioning in photosynthetic tissues, Barnes et al. [55] transformed tobacco plants with sense and antisense constructs of a cDNA encoding the tobacco Pi translocator. Although the transformed plants showed a 15-fold variation in Pi translocator activity, the growth and development and the rate of photosynthesis showed no consistent differences between antisense and sense transformants. In contrast, the distribution of assimilate between starch and sugar had been altered with no change in the amount of sucrose in leaves, suggesting a homeostatic mechanism for maintaining sucrose concentrations in the leaves at the expense of glucose and fructose. However, in potato plants antisense repression of the triose-P translocator affected carbon partitioning as chloroplasts isolated from such plants showed reduced import of Pi, reduced rate of photosynthesis, and change in carbon partitioning into starch at the expense of sucrose and amino acids [56]. Published evidence indicates that TPT exerts a considerable control on the rate of both CO2 assimilation and sucrose biosynthesis [41].

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