Photorespiration in C3-C 4 intermediate species of Alternanthera and Parthenium: Reduced ammonia production and increased capacity of CO2 refixation in the light

Photosynthetic Linear Electron Flow Drives CO2 Assimilation in Maize Leaves

Photosynthetic organisms commonly develop the strategy to keep the reaction center chlorophyll of photosystem I, P700, oxidized for preventing the generation of reactive oxygen species in excess light conditions. In photosynthesis of C₄ plants, CO₂ concentration is kept at higher levels around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by the cooperation of the mesophyll and bundle sheath cells, which enables them to assimilate CO₂ at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C₄ plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C₄ subtype species) in comparison with mustard, a C₃ plant. Instead of the alternative electron sink due to photorespiration in the C₃ plant, photosynthetic linear electron flow was strongly suppressed between photosystems I and II, dependent on the difference of proton concentration across the thylakoid membrane (ΔpH) in response to the suppression of CO₂ assimilation in maize. Linear relationships among CO₂ assimilation rate, linear electron flow, P700 oxidation, ΔpH, and the oxidation rate of ferredoxin suggested that the increase of ΔpH for P700 oxidation was caused by the regulation of proton conductance of chloroplast ATP synthase but not by promoting cyclic electron flow. At the scale of intact leaves, the ratio of PSI to PSII was estimated almost 1:1 in both C₃ and C₄ plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C₃ plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C₃ and C₄ plants.

Parthenium weed (Parthenium hysterophorus L.) and climate change: the effect of CO2 concentration, temperature, and water deficit on growth and reproduction of two biotypes

Climate change will have a considerable impact upon the processes that moderate weed invasion, in particular to that of parthenium weed (Parthenium hysterophorus L.). This study evaluated the performance of two Australian biotypes of parthenium weed under a range of environmental conditions including soil moisture (100 and 50% of field capacity), atmospheric carbon dioxide (CO₂) concentration (390 and 550 ppm), and temperature (35/20 and 30/15 °C/day/night). Measurements were taken upon growth, reproductive output, seed biology (fill, viability and dormancy) and soil seed longevity. Parthenium weed growth and seed output were significantly increased under the elevated CO₂ concentration (550 ppm) and in the cooler (30/15 °C) and wetter (field capacity) conditions. However, elevated CO₂ concentration could not promote growth or seed output when the plants were grown under the warmer (35/20 °C) and wetter conditions. Warm temperatures accelerated the growth of parthenium weed, producing plants with greater height biomass but with a shorter life span. Warm temperatures also affected the reproductive output by promoting both seed production and fill, and promoting seed longevity. Dryer soil conditions (50% of field capacity) also promoted the reproductive output, but did not retain high seed fill or promote seed longevity. Therefore, the rising temperatures, the increased atmospheric CO₂ concentration and the longer periods of drought predicted under climate change scenarios are likely to substantially enhance the growth and reproductive output of these two Australian parthenium weed biotypes. This may facilitate the further invasion of this noxious weed in tropical and sub-tropical natural and agro-ecosystems.

Tuning of Redox Regulatory Mechanisms, Reactive Oxygen Species and Redox Homeostasis under Salinity Stress

Soil salinity is a crucial environmental constraint which limits biomass production at many sites on a global scale. Saline growth conditions cause osmotic and ionic imbalances, oxidative stress and perturb metabolism, e.g., the photosynthetic electron flow. The plant ability to tolerate salinity is determined by multiple biochemical and physiological mechanisms protecting cell functions, in particular by regulating proper water relations and maintaining ion homeostasis. Redox homeostasis is a fundamental cell property. Its regulation includes control of reactive oxygen species (ROS) generation, sensing deviation from and readjustment of the cellular redox state. All these redox related functions have been recognized as decisive factors in salinity acclimation and adaptation. This review focuses on the core response of plants to overcome the challenges of salinity stress through regulation of ROS generation and detoxification systems and to maintain redox homeostasis. Emphasis is given to the role of NADH oxidase (RBOH), alternative oxidase (AOX), the plastid terminal oxidase (PTOX) and the malate valve with the malate dehydrogenase isoforms under salt stress. Overwhelming evidence assigns an essential auxiliary function of ROS and redox homeostasis to salinity acclimation of plants.

On the mechanism and rate of spontaneous decomposition of amino acids

Spontaneous decarboxylation of amino acids is among the slowest known reactions; it is much less facile than the cleavage of amide bonds in polypeptides. Establishment of the kinetics and mechanisms for this fundamental reaction is important for gauging the proficiency of enzymes. In the present study, multiple mechanisms for glycine decomposition in water are explored using QM/MM Monte Carlo simulations and free energy perturbation theory. Simple CO(2) detachment emerges as the preferred pathway for decarboxylation; it is followed by water-assisted proton transfer to yield the products: CO(2) and methylamine. The computed free energy of activation of 45 kcal/mol, and the resulting rate-constant of 1 × 10(-21) s(-1), can be compared with an extrapolated experimental rate constant of ~2 × 10(-17) s(-1) at 25 °C. The half-life for the reaction is more than 1 billion years. Furthermore, examination of deamination finds simple NH(3)-detachment yielding α-lactone to be the favored route, though it is less facile than decarboxylation by 6 kcal/mol. Ab initio and DFT calculations with the CPCM hydration model were also carried out for the reactions; the computed free energies of activation for glycine decarboxylation agree with the QM/MM result, whereas deamination is predicted to be more favorable. QM/MM calculations were also performed for decarboxylation of alanine; the computed barrier is 2 kcal/mol higher than for glycine in qualitative accord with experiment.

Purification and properties of glycolate oxidase from plants with different photosynthetic pathways: Distinctness of C4 enzyme from that of a C 3 species and a C 3-C 4 intermediate

Glycolate oxidase (GO; EC 1.1.3.1) was purified from the leaves of three plant species:Amaranthus hypochondriacus L.(NAD-ME type C4 dicot),Pisum sativum L. (C3 species) andParthenium hysterophorus L. (C3-C4. intermediate). A flavin moiety was present in the enzyme from all the three species. The enzyme from the C4 plant had a low specific activity, exhibited lower KM for glycolate, and required a lower pH for maximal activity, compared to the C3 enzyme. The enzyme from the C4 species oxidized glyoxylate at <10% of the rate with glycolate, while the GO from the C3 plant oxidized glyoxylate at a rate of about 35 to 40% of that with glycolate. The sensitivity of GO from C4 plant to α-hydroxypyridinemethane sulfonate, 2-hydroxy-3-butynoate and other inhibitors was less than that of the enzyme from C3 source. The properties of GO fromParthenium hysterophorus, were similar to those of the enzyme fromPisum sativum. The characteristics of glycolate oxidase from leaves of a C4 plant,Amaranthus hypochondriacus are different from those of the C3 species or the C3-C4 intermediate.