Metabolite control of Sorghum C4 phosphoenolpyruvate carboxylase catalytic activity and phosphorylation state

Kinetic analyses were performed on the nonphosphorylated and in vitro phosphorylated forms of recombinant Sorghum C4 phospho enolpyruvate carboxylase (C4 PEPC). The native enzyme was purified by immunoaffinity chromatography and its integrity demonstrated by Western blot analyses using anti N- and C-terminus antibodies. At suboptimal pH (7.1 to 7.3) and PEP concentration (2.5 mM), phosphorylation, positive metabolite effectors e.g., glucose-6-phosphate, glycine and dihydroxyacetone-phosphate, or an increase in pH strongly activated the enzyme and lowered the inhibitory effect of L-malate. C4 PEPC phosphorylation strengthened the effect of the positive effectors thereby decreasing further the enzyme's sensitivity to this inhibitor. L-malate also decreased the phosphorylation rate of C4 PEPC, a process antagonized by positive metabolite effectors. This was shown both in vitro, in a reconstituted phosphorylation assay containing the catalytic subunit of a cAMP-dependent protein kinase or the Sorghum leaf PEPC-PK and in situ, during induction of C4 PEPC phosphorylation in mesophyll cell protoplasts.     

The temperature acclimation potential of tropical bryophytes

Bryophyte biomass and diversity in tropical moist forests decrease dramatically from higher altitudes towards the lowlands. High respiratory carbon losses at high temperatures may partly explain this pattern, if montane species are unable to acclimatise their metabolic rates to lowland temperatures. We transplanted ten bryophyte species from two altitudes (1200 and 500 m a.s.l.) to lower (warmer) altitudes (500 m and sea level) in Panama. We studied short‐term temperature acclimation of CO₂ exchange for 2.5 months, and survival and growth for 21 months following transplantation. Short‐term acclimation did not occur, and on a longer time scale mortality was highest and growth lowest in the transplanted samples. A few transplanted samples of most species, however, survived the whole experiment and finished with growth rates similar to controls. This recovery of growth rate suggests temperature acclimation, in spite of no measurable metabolic changes in smaller random samples. This acclimation even compensated for shorter periods of CO₂ uptake due to more rapid drying. Nevertheless, these species are not abundant in lowland forests, perhaps due to dispersal or establishment limitation. The apparent heterogeneity of the acclimation potential within species may allow populations to adapt locally and avoid being forced uphill under climatic warming.     

Intraspecific competition for host resources in a parasite

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Upregulation of bundle sheath electron transport capacity under limiting light in C4 Setaria viridis

C₄ photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO₂ concentration at the site of CO₂ fixation. C₄ plants benefit from high irradiance but their efficiency decreases under shade, causing a loss of productivity in crop canopies. We investigated shade acclimation responses of Setaria viridis, a model monocot of NADP-dependent malic enzyme subtype, focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO₂ assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b₆f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII – light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C₄ plants and will support the development of strategies for crop improvement, including the engineering of C₄ photosynthesis into C₃ plants.     

The cooperativity phenomena in a pigment-protein complex of light-harvesting antenna revealed by picosecond absorbance difference spectroscopy

A model of the cooperative changes in optical properties of light-harvesting bacteriochlorophyll molecules of complex B890 in response to the absorption of light quanta is proposed. According to the model, each antenna chromophore may persist in either of two optically non-excited states, R and T. The occurrence of at least one excitation per complex causes all optically non-excited chromophores of the complex to be converted from state R to state T. The theory is shown to be in good agreement with experimental ‘light curves’ (ΔAvs intensity of picosecond excitation pulse) for the ‘minor’ and ‘major’ signals of light-harvesting bacteriochlorophylls of complex B890 from Chromatium minutissimum.     

The phase behavior of lipids in photosynthetic membranes

The phase behavior of the main classes of polar lipids found in the photosynthetic membranes of higher plants and algae is reviewed and compared to that of binary lipid mixtures and total lipid extracts of such membranes. Particular interest is paid to the way in which factors such as temperature and acyl chain saturation influence the phase behavior of these lipids and the implications this has in terms of the ability of photosynthetic membranes to resist environmental stress.     

Structural and functional analysis of the reducing side of photosystem I

Structural analysis of the reducing side of photosystem I (PSI) has been carried out using chemical cross-linking and monospecific antibodies. Incubation of PSI isolated from barley (Hordeum vulgare L.) with the hydrophilic cross-linking agent N-ethyl-3-[3-(dimethylamino) propyl]-carbodiimide leads to cross-linking of the PSI-D subunit with the PSI-E and PSI-H subunits. In the presence of ferredoxin, cross-linking results in the formation of cross-linked products composed of PSI-D, PSI-E and ferredoxin and in a block in steady state NADP⁺ photoreduction. No cross-linking of ferredoxin occurs at elevated ionic strength or using heat-denatured ferredoxin. Cross-linking of ferredoxin does not inhibit electron transfer from plastocyanin to methyl viologen. Steady state NADP⁺ photoreduction was analyzed in PSI or thyla-koids incubated with antibodies against individual PSI subunits. Incubation with antibodies against PSI-C, -H, -I, or -L had no effect on PSI activity, whereas antibodies against PSI-D or PSI-E had similar effects and caused a large decrease in activity. The results provide evidence that the PSI-D and PSI-E subunits are localized on the reducing side of PSI, forming a barrier between PSI-C and the stroma as well as a docking site for ferredoxin. The PSI-H subunit has an exposed, stromal domain but this does not appear to contribute to the ferredoxin docking.