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This review offers an overview of the current state of our knowledge concerning the role of fructose-1,6-bisphosphatase (FBPase) in sugar partitioning and biosynthesis, through the analysis of genetically manipulated plants. The existence of two well-characterized isoforms is a consequence of the subcellular compartmentalization of photosynthetic eukaryotes, conditioning their respective regulatory mechanisms and their influence over plant metabolism and photosynthesis. Both isoforms are important, as has been deduced from previous work with different plant species, although there is still much to be done in order to gain a definitive vision of this issue. Despite that, alteration of the FBPase content follows a general pattern, there are some differences that could be considered species-specific. Modifications lead to profound changes in the carbohydrate content and carbon allocation, raising questions as to whether flux of some sugars or sugar precursors from one side to the other of the chloroplast envelope occurs to rebalance carbohydrate metabolism or whether other compensatory, though not fully efficient, enzymatic activities come into play. Due to the pleiotropic nature of modifying the core carbon metabolism, an answer to the above questions would require an exhaustive study involving diverse aspects of plant physiology.

The sessile nature of plants forces them to face an ever-changing environment instead of escape from hostile conditions as animals do. In order to overcome this survival challenge, a fine monitoring and controlling of the status of the photosynthetic electron transport chain and the general metabolism is vital for these organisms. Frequently, evolutionary plant adaptation has consisted in the appearance of multigenic families, comprising an array of enzymes, structural components, or sensing, and signaling elements, in numerous occasions with highly conserved primary sequences that sometimes make it difficult to discern between redundancy and specificity among the members of a same family. However, all this gene diversity is aimed to sort environment-derived plant signals to efficiently channel the external incoming information inducing a right physiological answer. Oxygenic photosynthesis is a powerful source of reactive oxygen species (ROS), molecules with a dual oxidative/signaling nature. In response to ROS, one of the most frequent posttranslational modifications occurring in redox signaling proteins is the formation of disulfide bridges (from Cys oxidation). This review is focused on the role of plastid thioredoxins (pTRXs), proteins containing two Cys in their active site and largely known as part of the plant redox-signaling network. Several pTRXs types have been described so far, namely, TRX f, m, x, y, and z. In recent years, improvements in proteomic techniques and the study of loss-of-function mutants have enabled us to grasp the importance ofTRXs for the plastid physiology. We will analyze the specific signaling function of each TRX type and discuss about the emerging role in non-photosynthetic plastids of these redox switchers. [ABSTRACT FROM AUTHOR]

Fructose- 1,6-bisphosphatase (FBPase; EC 3.1.3.11) binds its putative physiological activator thioredoxin f (Trx f) at pH 7.9, the pH in the stroma of the illuminated chloroplast. Since Trx m, described as specific in NADP+-malate dehydrogenase (NADPMDH) activation, appears in pea (Pisum sativum L.) also to be functional in FBPase modulation, we have here analyzed the effect of pH and the redox status of the chloroplast stroma in the pea FBPase binding of homologous Trx f and m. Both pea Trx were strongly bound by purified FBPase when they were preincubated at pH 7.9 with 2.5 mM dithiothreitol (DTT), but not when the reductant was omitted. As occurs with Trx f, the Trx m/FBPase ratio of the complex was 4, but this was only observed with a Trx m/FBPase concentration ratio > 10 in the preincubation mixture. The FBPase-Trx m binding disappeared in the presence of 100 mM NaCl, even with 2.5 mM DTT at pH 7.9, with a concomitant appearance of different aggregation states of the FBPase subunit. A similar FBPase-Trx m complex was detected in the stromal solution when pea chloroplasts were lysed at pH 7.9 in the presence of DTT. No interaction was observed between NADP-MDH and Trx f or m, either in the presence or in the absence of DTT Pea FBPase showed sigmoidal activation kinetics with pea Trx m, and an S0.5 of 133 nM versus 6.6 nM with pea Trx f. About 10-fold higher concentration of the former than that of the latter was required for obtaining maximum activity; however, the Vmax with Trx f was only 2-fold higher than that with Trx m. We conclude that pea FBPase binds and is activated by the homologous Trx m, even though to a lesser extent than with Trx f. We also deduce that in the light the conditions in the chloroplast stroma are optimal for forming an FBPase-Trx complex. [ABSTRACT FROM AUTHOR]