Computational Analysis of Alternative Photosynthetic Electron Flows Linked With Oxidative Stress

During photosynthesis, organisms respond to their energy demand and ensure the supply of energy and redox equivalents that sustain metabolism. Hence, the photosynthetic apparatus can, and in fact should, be treated as an integrated supply-demand system. Any imbalance in the energy produced and consumed can lead to adverse reactions, such as the production of reactive oxygen species (ROS). Reaction centres of both photosystems are known sites of ROS production. Here, we investigate in particular the central role of Photosystem I (PSI) in this tightly regulated system. Using a computational approach we have expanded a previously published mechanistic model of C3 photosynthesis by including ROS producing and scavenging reactions around PSI. These include two water to water reactions mediated by Plastid terminal oxidase (PTOX) and Mehler and the ascorbate-glutathione (ASC-GSH) cycle, as a main non-enzymatic antioxidant. We have used this model to predict flux distributions through alternative electron pathways under various environmental stress conditions by systematically varying light intensity and enzymatic activity of key reactions. In particular, we studied the link between ROS formation and activation of pathways around PSI as potential scavenging mechanisms. This work shines light on the role of alternative electron pathways in photosynthetic acclimation and investigates the effect of environmental perturbations on PSI activity in the context of metabolic productivity.

The diversification of downy mildew species was not driven by the loss of mycorrhizal associations or the evolution of C4 photosynthesis

There are approximately 700 obligate biotrophic species grouped into 20 genera (Oomycota, Peronosporaceae) that cause downy mildew diseases. Dick hypothesized in 2001 that diversification of downy mildew species was driven, in part, by host plant secondary metabolites. Dick further speculated that this was driven by the transition of host plants away from mycorrhizal associations or the evolution of C4 photosynthesis. Specifically, loss of mycorrhizal associations or the use of C4 photosynthesis would result in more free carbon that the plants could then use to produce more secondary metabolites. If true, then there should be more downy mildew species that infect hosts from plant lineages that lack mycorrhizal associations or use C4 photosynthesis. However, analysis of 677 downy mildew species for host plant mycorrhizal associations and host plant photosynthetic pathway type show that this is not what occurred. Seventy percent of downy mildew species parasitize hosts that form mycorrhizal associations and 94% of downy mildew species parasitize hosts that use C3 photosynthesis. From this, it is concluded that the diversification of downy mildew species was not driven by the loss of mycorrhizal associations or the evolution of C4 photosynthesis. However, 85% of downy mildew species that parasitize Poaceae (grasses) parasitize C4 hosts. Thus, it is possible that C4 photosynthesis plays a role in the diversification of these genera.

Pathways of Photosynthesis in Non-Leaf Tissues

Plants have leaves as specialised organs that capture light energy by photosynthesis. However, photosynthesis is also found in other plant organs. Photosynthesis may be found in the petiole, stems, flowers, fruits, and seeds. All photosynthesis can contribute to the capture of carbon and growth of the plant. The benefit to the plant of photosynthesis in these other tissues or organs may often be associated with the need to re-capture carbon especially in storage organs that have high respiration rates. Some plants that conduct C3 photosynthesis in the leaves have been reported to use C4 photosynthesis in petioles, stems, flowers, fruits, or seeds. These pathways of non-leaf photosynthesis may be especially important in supporting plant growth under stress and may be a key contributor to plant growth and survival. Pathways of photosynthesis have directionally evolved many times in different plant lineages in response to environmental selection and may also have differentiated in specific parts of the plant. This consideration may be useful in the breeding of crop plants with enhanced performance in response to climate change.

Thiol Redox Regulation of Plant β-Carbonic Anhydrase

β-carbonic anhydrases (βCA) accelerate the equilibrium formation between CO2 and carbonate. Two plant βCA isoforms are targeted to the chloroplast and represent abundant proteins in the range of >1% of chloroplast protein. While their function in gas exchange and photosynthesis is well-characterized in carbon concentrating mechanisms of cyanobacteria and plants with C4-photosynthesis, their function in plants with C3-photosynthesis is less clear. The presence of conserved and surface-exposed cysteinyl residues in the βCA-structure urged to the question whether βCA is subject to redox regulation. Activity measurements revealed reductive activation of βCA1, whereas oxidized βCA1 was inactive. Mutation of cysteinyl residues decreased βCA1 activity, in particular C280S, C167S, C230S, and C257S. High concentrations of dithiothreitol or low amounts of reduced thioredoxins (TRXs) activated oxidized βCA1. TRX-y1 and TRX-y2 most efficiently activated βCA1, followed by TRX-f1 and f2 and NADPH-dependent TRX reductase C (NTRC). High light irradiation did not enhance βCA activity in wildtype Arabidopsis, but surprisingly in βca1 knockout plants, indicating light-dependent regulation. The results assign a role of βCA within the thiol redox regulatory network of the chloroplast.

Ecohydrology of CAM plants: environmental co-benefits for agroecosystems

<p>In spite of their potential as low input crops in arid and semi-arid regions, limitations in CAM (Crassulacean Acid Metabolism) modeling have made it difficult to compare the productivity and water use of CAM crops with their C3 and C4 counterparts on a consistent basis. Using the Photo3 model, which adds a malic acid storage and circadian rhythm to the widely accepted Farquhar et al. model for C3 photosynthesis, we analyze the coupling of CAM carbon assimilation and transpiration with the soil-plant-atmosphere continuum at an hourly timescale to explore the potential of prickly pear and agave in several semiarid locations. Model results are compared with experimental data and contrasted with results generated using the widely used and empirically based Environmental Productivity Index introduced by Park Nobel in 1980. Using model results for CAM as well as C3 and C4 species, we explore how agricultural productivity in these locations may be optimized by diversifying planting strategies among the three photosynthetic pathways. This work aims to assist in better understanding the potential of CAM for food and biofuel production in arid and semiarid regions of the world accounting also for environmental co-benefits related to enhanced carbon sequestration and reduced water demand in semi-arid, marginal lands.</p><p> </p>

Evolution of C4 photosynthesis predicted by constraint-based modelling

Constraint-based modelling (CBM) is a powerful tool for the analysis of evolutionary trajectories. Evolution, especially evolution in the distant past, is not easily accessible to laboratory experimentation. Modelling can provide a window into evolutionary processes by allowing the examination of selective pressures which lead to particular optimal solutions in the model. To study the evolution of C4 photosynthesis from a ground state of C3 photosynthesis, we initially construct a C3 model. After duplication into two cells to reflect typical C4 leaf architecture, we allow the model to predict the optimal metabolic solution under various conditions. The model thus identifies resource limitation in conjunction with high photorespiratory flux as a selective pressure relevant to the evolution of C4. It also predicts that light availability and distribution play a role in guiding the evolutionary choice of possible decarboxylation enzymes. The data shows evolutionary CBM in eukaryotes predicts molecular evolution with precision.

Halophyte Common Ice Plants: A Future Solution to Arable Land Salinization

The problems associated with the salinization of soils and water bodies and the increasing competition for scarce freshwater resources are increasing. Current attempts to adapt to these conditions through sustainable agriculture involves searching for new highly salt-tolerant crops, and wild species that have potential as saline crops are particularly suitable. The common ice plant (Mesembryanthemum crystallinum L.) is an edible halophyte member of the Aizoaceae family, which switches from C3 photosynthesis to crassulacean acid metabolism (CAM) when exposed to salinity or water stress. The aim of this review was to examine the potential of using the ice plant in both the wild and as a crop, and to describe its ecology and morphology, environmental and agronomic requirements, and physiology. The antioxidant properties and mineral composition of the ice plant are also beneficial to human health and have been extensively examined.

Evolution of C4 photosynthesis predicted by constraint-based modelling

Constraint-based modelling (CBM) is a powerful tool for the analysis of evolutionary trajectories. Evolution, especially evolution in the distant past, is not easily accessible to laboratory experimentation. Modelling can provide a window into evolutionary processes by allowing the examination of selective pressures which lead to particular optimal solutions in the model. To study the evolution of C4 photosynthesis from a ground state of C3 photosynthesis, we initially construct a C3 model. After duplication into two cells to reflect typical C4 leaf architecture, we allow the model to predict the optimal metabolic solution under various conditions. The model thus identifies resource limitation in conjunction with high photorespiratory flux as a selective pressure relevant to the evolution of C4. It also predicts that light availability and distribution play a role in guiding the evolutionary choice of possible decarboxylation enzymes. The data shows evolutionary CBM in eukaryotes predicts molecular evolution with precision.

The Evolution of CAM Photosynthesis in Australian Calandrinia Reveals Lability in C3+CAM Phenotypes and a Possible Constraint to the Evolution of Strong CAM

Australian Calandrinia has radiated across the Australian continent during the last 30 Ma, and today inhabits most Australian ecosystems. Given its biogeographic range and reports of facultative Crassulacean acid metabolism (CAM) photosynthesis in multiple species, we hypothesized (1) that CAM would be widespread across Australian Calandrinia and that species, especially those that live in arid regions, would engage in strong CAM, and (2) that Australian Calandrinia would be an important lineage for informing on the CAM evolutionary trajectory. We cultivated 22 Australian Calandrinia species for a drought experiment. Using physiological measurements and δ13C values we characterized photosynthetic mode across these species, mapped the resulting character states onto a phylogeny, and characterized the climatic envelopes of species in their native ranges. Most species primarily utilize C3 photosynthesis, with CAM operating secondarily, often upregulated following drought. Several phylogenetically nested species are C3, indicating evolutionary losses of CAM. No strong CAM was detected in any of the species. Results highlight the limitations of δ13C surveys in detecting C3+CAM phenotypes, and the evolutionary lability of C3+CAM phenotypes. We propose a model of CAM evolution that allows for lability and reversibility among C3+CAM phenotypes and C3 and suggest that an annual life-cycle may preclude the evolution of strong CAM.