The roles of malate and aspartate in C4 photosynthetic metabolism of Flaveria bidentis (L.)

Abstract

Abstract. In C4 grasses belonging to the NADP-malic enzyme-type subgroup, malate is considered to be the predominant C4 acid metabolized during C4 photosynthesis, and the bundle sheath cell chloroplasts contain very little photosystem-II (PSII) activity. The present studies showed that Flaveria bidentis (L.), an NADP-malic enzyme-type C4 dicotyledon, had substantial PSII activity in bundle sheath cells and that malate and aspartate apparently contributed about equally to the transfer of CO2 to bundle sheath cells. Preparations of bundle sheath cells and chloroplasts isolated from these cells evolved O2 at rates between 1.5 and 2 gmol'min -1.mg 1 chlorophyll (Chl) in the light in response to adding either 3-phosphoglycerate plus HCO~-or aspartate plus 2-oxoglutarate. Rates of more than 2 ~tmol 02' min -1" mgChl were recorded for cells provided with both sets of these substrates. With bundle sheath cell preparations the maximum rates of light-dependent CO2 fixation and malate decarboxylation to pyruvate recorded were about 1.7 ~tmol -min-1. mg-1 Chl. Compared with NADPmalic enzyme-type grass species, F. bidends bundle sheath cells contained much higher activities of NADP-malate dehydrogenase and of aspartate and alanine aminotransferases. Time-course and pulse-chase studies following the kinetics of radiolabelling of the C-4 carboxyl of C4 acids from t4CO2 indicated that the photosynthetically active pool of malate was about twice the size of the aspartate pool. However, there was strong evidence for a rapid flux of carbon through both these pools. Possible routes of aspartate metabolism and the relationship between this metabolism and PSII activity in bundle sheath cells are considered.Abbreviations: DHAP = dihydroxyacetone phosphate; NADP-ME(-type) = NADP-malic enzyme (type); NADP-MDH = NADPmalate dehydrogenase; OAA = oxaloacetic acid; 2-OG = 2-oxoglutarate; PEP = phosphoenolpyruvate; PGA = 3-phosphoglycerate; Pi = orthophosphate; Ru5P = ribulose 5-phosphate Correspondence to: M.D. Hatch; FAX:61 (6)2465000; Tel.: 61 (6) 2465264

The Operation of Two Decarboxylases, Transamination, and Partitioning of C4 Metabolic Processes between Mesophyll and Bundle Sheath Cells Allows Light Capture To Be Balanced for the Maize C4 Pathway

“…This shift between Aspand MAL-mediated CCM indicates the importance of maintaining both pathways in NADP-ME subtype C 4 plants. The predicted T varied in response to environmental conditions from a minimum PEP carboxylation rate (V P ) of 0.35 to the entirety of the CO 2 delivered to BS, in line with the observation that Asp can support physiological rates of photosynthesis (Rathnam, 1978;Chapman and Hatch, 1981;Meister et al, 1996;Pick et al, 2011). Under white light, the model predicted a 33% T/V P , which is in line with radiolabeling and biochemical observations (Downton, 1970;Hatch, 1971;Chapman and Hatch, 1981).…”

“…Recent developments in C 4 research have highlighted the complexity of C 4 metabolism in terms of extensive overlapping of BS and M functions Majeran et al, 2010), the presence of transamination and two distinct decarboxylating pathways (Meister et al, 1996;Wingler et al, 1999;Pick et al, 2011), and plasticity in MAL metabolism (Eprintsev et al, 2011, and refs. therein).…”

NADP-Malate Dehydrogenase in the C4 Plant Flaveria bidentis (Cosense Suppression of Activity in Mesophyll and Bundle-Sheath Cells and Consequences for Photosynthesis)

“…About 5 g of deribbed plant material was blended with 70 mL of ice-cold buffer C (0.3 M sorbitol, 20 mM Hepes [K+], pH 7.7, 2 mM Na,EDTA, 2 mM isoascorbate, 2 mM Na,HPO,, and 1 mM PMSF) in a mixer (Omnimixer, Sorvall), as described by Meister et al (1996), to prepare bundle-sheath cell strands for O,-exchange measurements. A filtrate enriched in mesophyll cell contents was obtained from a portion of the leaf extract after the first blend (10 s at 60% of line voltage) by filtration through an 80-pm nylon net.…”

“…Analysis of these plants has allowed us to assess the role of this enzyme in regulating the rate of C, photosynthesis. F. bidentis differs from maize in that it has significant activities of NADP-MDH in the bundle-sheath cell chloroplast (Meister et al, 1996). Therefore, we have also considered the significance of this form of the enzyme in the regulation of C, photosynthesis in this plant.…”

Understanding metabolite transport and metabolism in C4 plants through RNA-seq

“…OAA is reduced in the chloroplasts after counter-exchange with malate by dicarboxylate translocator 1 (DiT1) across the plastid envelope [45]. Alternatively, the OAA is converted to aspartate (Asp) in chloroplasts by plastidic aspartate transaminase (AspAT) [12,46] after import by DiT2 [36] in exchange for aspartate. Both enzymes and transporters are moderately upregulated [7]; in maize, AspAT, MDH, and DiT1 are mesophyll specific while DiT2 is bundle sheath specific ( Table 1).…”