Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants

Abstract

Glucose 6 phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is well-known as the main regulatory enzyme of the oxidative pentose phosphate pathway (OPPP) in living organisms. Namely, in Planta, different G6PDH isoforms may occur, generally localized in cytosol and plastids/chloroplasts. These enzymes are differently regulated by distinct mechanisms, still far from being defined in detail. In the last decades, a pivotal function for plant G6PDHs during the assimilation of nitrogen, providing reductants for enzymes involved in nitrate reduction and ammonium assimilation, has been described. More recently, several studies have suggested a main role of G6PDH to counteract different stress conditions, among these salinity and drought, with the involvement of an ABA depending signal. In the last few years, this recognized vision has been greatly widened, due to studies clearly showing the non-conventional subcellular localization of the different G6PDHs, and the peculiar regulation of the different isoforms. The whole body of these considerations suggests a central question: how do the plant cells distribute the reductants coming from G6PDH and balance their equilibrium? This review explores the present knowledge about these mechanisms, in order to propose a scheme of distribution of reductants produced by G6PDH during nitrogen assimilation and stress.

Keywords: abiotic stress; nitrogen assimilation; oxidative pentose phosphate pathway; redox regulation.

Figure 1

Design of conventional (A) and alternative (B) oxidative pentose phosphate pathway (OPPP) according to [1]. Intermediates peculiar of the OPPP are highlighted in orange; intermediates common to glycolysis are highlighted in light blue; intermediates common to Calvin–Benson cycle are highlighted in light green. Enzymes are highlighted in yellow. CO₂ evolved is highlighted in red and NADPH reduced in blue. To avoid confusion, the inter-conversion of glyceraldehyde-3P and di-hydroxy-acetone-P by triose phosphate isomerase (TPI; EC 5.3.1.1) is omitted. List of enzymes abbreviations: G6PDH, Glucose-6-phosphate 1-dehydrogenase (EC 1.1.1.49); 6PGL, 6-Phosphogluconolactonase (EC 3.1.1.31); 6PGDH, 6-Phosphogluconate dehydrogenase (decarboxylating) (EC 1.1.1.44); R5P-ISO, Ribose-5-phosphate isomerase (EC 5.3.1.6); R5P-EPI, Ribulose-5-phosphate 3-epimerase (EC 5.1.3.1); TA, Transaldolase (EC 2.2.1.2); TK, Transketolase (EC 2.2.1.1); HPI, Hexose-6-phosphate isomerase (EC 5.3.1.9). Modified from [1].

Figure 2

Phylogenetic tree of G6PDH isoforms from various living organisms. Blue and violet highlights designate cytosolic isoforms (simply indicated as G6PDH in Bacteria). Green highlight indicates the chloroplastic P1 isoforms. Yellow indicates the catalytically inactive P0-isoforms. Orange for the plastidial P2-G6PDH isoforms. Red highlight is for Red Algae isoform. Other details in the text.

Figure 3

The fate of reductants derived from Glucose-6P dehydrogenase (G6PDH), and more generally Oxidative pentose phosphate pathway (OPPP), in plant cells. (A). In photosynthetic cells, cytosolic OPPP provides NADPH for basal metabolism in the light. Normally, photosynthesis is able to sustain the request of electrons for nitrogen metabolism. Chloroplastic P1-G6PDH results inhibited by NADPH and thus OPPP is inactive in the organelles. (B). In the leaves, under stress conditions, there is an increase of cytosolic OPPP in order to supply NADPH for stress response. An increased expression of cy-G6PDH occurs, together with expression and synthesis of P1- and P0-G6PDH: this causes the formation of heterodimers directed to peroxisomes; there is the activation of a specific machinery formed by oxidative section of OPPP to counteract the stress; or for different metabolic functions in specific plant organs. (C). In the dark, or in heterotrophic tissues—under physiological conditions—chloroplastic (or plastidial) OPPP are activated by P1-G6PDH or P2-G6PDH, respectively, providing reductants for nitrogen assimilation in the dark/heterotrophic conditions. As in leaves, cytosolic OPPP provides NADPH for basal metabolism. (D). In the dark, or in heterotrophic tissues, under stress there is an increase of both cytosolic OPPP (by cy-G6PDH), and chloroplastic/plastidial OPPPs (by P1-G6PDH or P2-G6PDH, respectively), in order to counteract the stress. It is possible to hypothesize (?) the formation of heterodimers directed to peroxisomes (dotted grey arrows), but this has not been proven yet. It must be underlined that P2-G6PDH is able to maintain a high rate even at NADPH/NADP⁺ ratios normally easily inhibiting P1-G6PDH, thus sustaining the stress response in root/heterotrophic tissues (e.g. enduring drought/salt stress conditions). Abbreviations: NTR, nitrate transporters; NR, Nitrate reductase; NiR, Nitrite reductase; GS, glutamine synthetase; GOGAT, glutamate synthase; Fd, Ferredoxin (red,reduced/ox, oxidated); R5P, ribose-5P; E4P, erythrose-4P; Glu, glutamate; Gln glutamine; 2-OG, 2 oxoglutarate; PTS, peroxisome targeting sequence.