The effect of increasing temperature on crop photosynthesis: from enzymes to ecosystems.

As global land surface temperature continues to rise and heatwave events increase in frequency, duration, and/or intensity, our key food and fuel cropping systems will likely face increased heat-related stress. A large volume of literature exists on exploring measured and modelled impacts of rising temperature on crop photosynthesis, from enzymatic responses within the leaf up to larger ecosystem-scale responses that reflect seasonal and interannual crop responses to heat. This review discusses (i) how crop photosynthesis changes with temperature at the enzymatic scale within the leaf; (ii) how stomata and plant transport systems are affected by temperature; (iii) what features make a plant susceptible or tolerant to elevated temperature and heat stress; and (iv) how these temperature and heat effects compound at the ecosystem scale to affect crop yields. Throughout the review, we identify current advancements and future research trajectories that are needed to make our cropping systems more resilient to rising temperature and heat stress, which are both projected to occur due to current global fossil fuel emissions.

Similar photosynthetic response to elevated carbon dioxide concentration in species with different phloem loading strategies.

Species have different strategies for loading sugars into the phloem, which vary in the route that sugars take to enter the phloem and the energetics of sugar accumulation. Species with passive phloem loading are hypothesized to have less flexibility in response to changes in some environmental conditions because sucrose export from mesophyll cells is dependent on fixed anatomical plasmodesmatal connections. Passive phloem loaders also have high mesophyll sugar content, and may be less likely to exhibit sugar-mediated down-regulation of photosynthetic capacity at elevated CO2 concentrations. To date, the effect of phloem loading strategy on the response of plant carbon metabolism to rising atmospheric CO2 concentrations is unclear, despite the widespread impacts of rising CO2 on plants. Over three field seasons, five species with apoplastic loading, passive loading, or polymer-trapping were grown at ambient and elevated CO2 concentration in free air concentration enrichment plots. Light-saturated rate of photosynthesis, photosynthetic capacity, leaf carbohydrate content, and anatomy were measured and compared among the species. All five species showed significant stimulation in midday photosynthetic CO2 uptake by elevated CO2 even though the two passive loading species showed significant down-regulation of maximum Rubisco carboxylation capacity at elevated CO2. There was a trend toward greater starch accumulation at elevated CO2 in all species, and was most pronounced in passive loaders. From this study, we cannot conclude that phloem loading strategy is a key determinant of plant response to elevated CO2, but compelling differences in response counter to our hypothesis were observed. A phylogenetically controlled experiment with more species may be needed to fully test the hypothesis.

Expression of Arabidopsis sugar transport protein STP13 differentially affects glucose transport activity and basal resistance to Botrytis cinerea.

Botrytis cinerea is the causing agent of the grey mold disease in more than 200 crop species. While signaling pathways leading to the basal resistance against this fungus are well described, the role of the import of sugars into host cells remains to be investigated. In Arabidopsis thaliana, apoplastic hexose retrieval is mediated by the activity of sugar transport proteins (STPs). Expression analysis of the 14 STP genes revealed that only STP13 was induced in leaves challenged with B. cinerea. STP13-modified plants were produced and assayed for their resistance to B. cinerea and glucose transport activity. We report that STP13-deficient plants exhibited an enhanced susceptibility and a reduced rate of glucose uptake. Conversely, plants with a high constitutive level of STP13 protein displayed an improved capacity to absorb glucose and an enhanced resistance phenotype. The correlation between STP13 transcripts, protein accumulation, glucose uptake rate and resistance level indicates that STP13 contributes to the basal resistance to B. cinerea by limiting symptom development and points out the importance of the host intracellular sugar uptake in this process. We postulate that STP13 would participate in the active resorption of hexoses to support the increased energy demand to trigger plant defense reactions and to deprive the fungus by changing sugar fluxes toward host cells.

Simultaneous and Independent Abaxial and Adaxial Gas Exchange Measurements.

Stomata can be distributed exclusively on the abaxial or adaxial leaf surface, but they are most commonly found on both leaf surfaces. Variations in stomatal arrangement, patterning, and the impact on photosynthesis can be measured using an infrared gas exchange system. However, when using standard gas exchange techniques, both surfaces are measured together and averaged to provide leaf-level values. Employing an innovative gas exchange apparatus with two infrared gas analyzers, separate gaseous flux from both leaf surfaces can be quantified simultaneously and independently. Here, we provide examples of typical measurements that can be performed using a "split chamber" gas exchange system.

Phloem function: a key to understanding and manipulating plant responses to rising atmospheric [CO2]?

Increasing atmospheric carbon dioxide concentration ([CO2]) directly stimulates photosynthesis and reduces stomatal conductance in C3 plants. Both of these physiological effects have the potential to alter phloem function at elevated [CO2]. Recent research has clearly established that photosynthetic capacity is correlated to vascular traits associated with phloem loading and water transport, but the effects of elevated [CO2] on these relationships are largely unexplored. Plants also employ different strategies for loading sucrose and other sugars into the phloem, and there is potential for species with different phloem loading strategies to respond differently to elevated [CO2]. Recent research manipulating sucrose transporters and other key enzymes with roles in phloem loading show promise for maximizing crop performance in an elevated [CO2] world.

The molecular dialogue between Arabidopsis thaliana and the necrotrophic fungus Botrytis cinerea leads to major changes in host carbon metabolism.

Photoassimilates play crucial roles during plant-pathogen interactions, as colonizing pathogens rely on the supply of sugars from hosts. The competition for sugar acquisition at the plant-pathogen interface involves different strategies from both partners which are critical for the outcome of the interaction. Here, we dissect individual mechanisms of sugar uptake during the interaction of Arabidopsis thaliana with the necrotrophic fungus Botrytis cinerea using millicell culture insert, that enables molecular communication without physical contact. We demonstrate that B. cinerea is able to actively absorb glucose and fructose with equal capacities. Challenged Arabidopsis cells compete for extracellular monosaccharides through transcriptional reprogramming of host sugar transporter genes and activation of a complex sugar uptake system which displays differential specificity and affinity for hexoses. We provide evidence that the molecular dialogue between Arabidopsis cells and B. cinerea triggers major changes in host metabolism, including apoplastic sucrose degradation and consumption of carbohydrates and oxygen, suggesting an enhanced activity of the glycolysis and the cellular respiration. We conclude that beside a role in sugar deprivation of the pathogen by competing for sugar availability in the apoplast, the enhanced uptake of hexoses also contributes to sustain the increased activity of respiratory metabolism to fuel plant defences.

Source-to-sink transport of sugar and regulation by environmental factors.

Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.