Sean E. Weise

In the future, plants may encounter increased light and elevated CO₂ levels. How consequent alterations in photosynthetic rates will impact fluxes in photosynthetic carbon metabolism remains uncertain. Respiration in light (R_L) is pivotal in plant carbon balance and a key parameter in photosynthesis models. Understanding the dynamics of photosynthetic metabolism and R_L under varying environmental conditions is essential for optimizing plant growth and agricultural productivity. However, measuring R_L under high light and high CO₂ (HLHC) conditions poses challenges using traditional gas exchange methods. In this study, we employed isotopically nonstationary metabolic flux analysis (INST-MFA) to estimate R_L and investigate photosynthetic carbon flux, unveiling nuanced adjustments in Camelina sativa under HLHC. Despite numerous flux alterations in HLHC, R_L remained stable. HLHC affects several factors influencing R_L, such as starch and sucrose partitioning, v_o/v_c ratio, triose phosphate partitioning, and hexose kinase activity. Analysis of A/C_i curve operational points reveals that HLHC’s major changes primarily stem from CO₂ suppressing photorespiration. Integration of these fluxes into a simplified model predicts changes in CBC labeling under HLHC. This study extends our prior discovery that incomplete CBC labeling is due to unlabeled carbon reimported during R_L, offering insights into manipulating labeling through adjustments in photosynthetic rates.

Daylength, a seasonal and latitudinal variable, exerts a substantial impact on plant growth. However, the relationship between daylength and growth is nonproportional, suggesting the existence of adaptive mechanisms. Thus, our study aimed to comprehensively investigate the adaptive strategies employed by plants in response to daylength variation. We grew false flax (Camelina sativa) plants, a model oilseed crop, under long-day (LD) and short-day (SD) conditions and used growth measurements, gas exchange measurements, and isotopic labeling techniques, including ¹³C, ¹⁴C, and ²H₂O, to determine responses to different daylengths. Our findings revealed that daylength influences various growth parameters, photosynthetic physiology, carbon partitioning, metabolic fluxes, and metabolite levels. SD plants employed diverse mechanisms to compensate for reduced CO₂ fixation in the shorter photoperiod. These mechanisms included enhanced photosynthetic rates and reduced respiration in the light (R_L), leading to increased shoot investment. Additionally, SD plants exhibited reduced rates of the glucose 6-phosphate (G6P) shunt and greater partitioning of sugars into starch, thereby sustaining carbon availability during the longer night. Isotopic labeling results further demonstrated substantial alterations in the partitioning of amino acids and TCA cycle intermediates between rapidly and slowly turning over pools. Overall, the results point to multiple developmental, physiological, and metabolic ways in which plants adapt to different daylengths to maintain growth.

The oxygenation of ribulose 1,5-bisphosphate by Rubisco is the first step in photorespiration and reduces the efficiency of photosynthesis in C₃ plants. Our recent data indicate that mutants in photorespiration have increased rates of photosynthetic cyclic electron flow around photosystem I. We investigated mutant lines lacking peroxisomal hydroxypyruvate reductase to determine if there are connections between 2-phosphoglycolate accumulation and cyclic electron flow in Arabidopsis (Arabidopsis thaliana). We found that 2-phosphoglycolate is a competitive inhibitor of triose phosphate isomerase, an enzyme in the Calvin-Benson cycle that converts glyceraldehyde 3-phosphate to dihydroxyacetone phosphate. This block in metabolism could be overcome if glyceraldehyde 3-phosphate is exported to the cytosol, where cytosolic triose phosphate isomerase could convert it to dihydroxyacetone phosphate. We found evidence that carbon is reimported as glucose-6-phosphate, forming a cytosolic bypass around the block of stromal triose phosphate isomerase. However, this also stimulates a glucose-6-phosphate shunt, which consumes ATP, which can be compensated by higher rates of cyclic electron flow.

It is just over 60 years since a cycle for the regeneration of the CO₂-acceptor used in photosynthesis was proposed. In this opinion paper, we revisit the origins of the Calvin–Benson cycle that occurred at the time that the hexose monophosphate shunt, now called the pentose phosphate pathway, was being worked out. Eventually the pentose phosphate pathway was separated into two branches, an oxidative branch and a non-oxidative branch. It is generally thought that the Calvin–Benson cycle is the reverse of the non-oxidative branch of the pentose phosphate pathway but we describe crucial differences and also propose that some carbon routinely passes through the oxidative branch of the pentose phosphate pathway. This creates a futile cycle but may help to stabilize photosynthesis. If it occurs it could explain a number of enigmas including the lack of complete labelling of the Calvin–Benson cycle intermediates when carbon isotopes are fed to photosynthesizing leaves.