Recent developments in the engineering of Rubisco activase for enhanced crop yield

Rubisco activase (RCA) catalyzes the release of inhibitory sugar phosphates from ribulose-1,6-biphosphate carboxylase/oxygenase (Rubisco) and can play an important role in biochemical limitations of photosynthesis under dynamic light and elevated temperatures. There is interest in increasing RCA activity to improve crop productivity, but a lack of understanding about the regulation of photosynthesis complicates engineering strategies. In this review, we discuss work relevant to improving RCA with a focus on advances in understanding the structural cause of RCA instability under heat stress and the regulatory interactions between RCA and components of photosynthesis. This reveals substantial variation in RCA thermostability that can be influenced by single amino acid substitutions, and that engineered variants can perform better in vitro and in vivo under heat stress. In addition, there are indications RCA activity is controlled by transcriptional, post-transcriptional, post-translational, and spatial regulation, which may be important for balancing between carbon fixation and light capture. Finally, we provide an overview of findings from recent field experiments and consider the requirements for commercial validation as part of efforts to increase crop yields in the face of global climate change.

Light acclimation interacts with thylakoid ion transport to govern the dynamics of photosynthesis in Arabidopsis

Understanding photosynthesis in natural, dynamic light environments requires knowledge of long-term acclimation, short-term responses, and their mechanistic interactions. To approach the latter, we systematically determined and characterized light-environmental effects on thylakoid ion transport-mediated short-term responses during light fluctuations. For this, Arabidopsis thaliana wild-type and mutants of the Cl^- channel VCCN1 and the K⁺ exchange antiporter KEA3 were grown under eight different light environments and characterized for photosynthesis-associated parameters and factors in steady state and during light fluctuations. For a detailed characterization of selected light conditions, we monitored ion flux dynamics at unprecedented high temporal resolution by a modified spectroscopy approach. Our analyses reveal that daily light intensity sculpts photosynthetic capacity as a main acclimatory driver with positive and negative effects on the function of KEA3 and VCCN1 during high-light phases, respectively. Fluctuations in light intensity boost the accumulation of the photoprotective pigment zeaxanthin (Zx). We show that KEA3 suppresses Zx accumulation during the day, which together with its direct proton transport activity accelerates photosynthetic transition to lower light intensities. In summary, both light-environment factors, intensity and variability, modulate the function of thylakoid ion transport in dynamic photosynthesis with distinct effects on lumen pH, Zx accumulation, photoprotection, and photosynthetic efficiency.

Elevated ozone decreases the activity of Rubisco in poplar but not its activation under fluctuating light

Increasing tropospheric ozone (O3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O3 on photosynthetic induction and its biochemical limitations in response to fluctuating light remain unclear, despite the rapid fluctuations of light intensity occurring under field conditions. In this study, two hybrid poplar clones with different O3 sensitivities were exposed to elevated O3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (Vcmax) and Rubisco content. In addition, elevated O3 significantly decreased the time constants for slow phases and weighting of the fast phase of the Vcmax induction in poplar clone '546' but not in clone '107'. However, elevated O3 did not affect the time, it took to reach a given percentage of full Vcmax activation or photosynthetic induction in either clone. Overall, photosynthetic induction was primarily limited by the activity of Rubisco rather than the regeneration of ribulose-1,5-biphosphate regardless of O3 concentration and exposure duration. The lack of O3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O3 on nonsteady-state photosynthesis in dynamic photosynthetic models.

Does Fluctuating Light Affect Crop Yield? A Focus on the Dynamic Photosynthesis of Two Soybean Varieties

In natural environments, plants are exposed to variable light conditions, but photosynthesis has been mainly studied at steady state and this might overestimate carbon (C) uptake at the canopy scale. To better elucidate the role of light fluctuations on canopy photosynthesis, we investigated how the chlorophyll content, and therefore the different absorbance of light, would affect the quantum yield in fluctuating light conditions. For this purpose, we grew a commercial variety (Eiko) and a chlorophyll deficient mutant (MinnGold) either in fluctuating (F) or non-fluctuating (NF) light conditions with sinusoidal changes in irradiance. Two different light treatments were also applied: a low light treatment (LL; max 650 μmol m^-2 s^-1) and a high light treatment (HL; max 1,000 μmol m^-2 s^-1). Canopy gas exchanges were continuously measured throughout the experiment. We found no differences in C uptake in LL treatment, either under F or NF. Light fluctuations were instead detrimental for the chlorophyll deficient mutant in HL conditions only, while the green variety seemed to be well-adapted to them. Varieties adapted to fluctuating light might be identified to target the molecular mechanisms responsible for such adaptations.

Elevated CO2 Enhances Dynamic Photosynthesis in Rice and Wheat

Crops developed under elevated carbon dioxide (eCO₂) exhibit enhanced leaf photosynthesis under steady states. However, little is known about the effect of eCO₂ on dynamic photosynthesis and the relative contribution of the short-term (substrate) and long-term (acclimation) effects of eCO₂. We grew an Oryza sativa japonica cultivar and a Triticum aestivum cultivar under 400 μmol CO₂ mol^-1 air (ambient, A) and 600 μmol CO₂ mol^-1 air (elevated, E). Regardless of growth [CO₂], the photosynthetic responses to the sudden increase and decrease in light intensity were characterized under 400 (a) or 600 μmol CO₂ mol^-1 air (e). The Aa, Ae, Ea, and Ee treatments were employed to quantify the acclimation effect (Ae vs. Ee and Aa vs. Ea) and substrate effect (Aa vs. Ae and Ea vs. Ee). In comparison with the Aa treatment, both the steady-state photosynthetic rate (P _N) and induction state (IS) were higher under the Ae and Ee treatments but lower under the Ea treatment in both species. However, IS reached at the 60 sec after the increase in light intensity, the time required for photosynthetic induction, and induction efficiency under Ae and Ee treatment did not differ significantly from those under Aa treatment. The substrate effect increased the accumulative carbon gain (ACG) during photosynthetic induction by 45.5% in rice and by 39.3% in wheat, whereas the acclimation effect decreased the ACG by 18.3% in rice but increased it by 7.5% in wheat. Thus, eCO₂, either during growth or at measurement, enhances the dynamic photosynthetic carbon gain in both crop species. This indicates that photosynthetic carbon loss due to an induction limitation may be reduced in the future, under a high-CO₂ world.

Identification of quantitative trait loci for dynamic and steady-state photosynthetic traits in a barley mapping population

Enhancing the photosynthetic induction response to fluctuating light has been suggested as a key target for improvement in crop breeding programmes, with the potential to substantially increase whole-canopy carbon assimilation and contribute to crop yield potential. Rubisco activation may be the main physiological process that will allow us to achieve such a goal. In this study, we assessed the phenotype of Rubisco activation rate in a doubled haploid (DH) barley mapping population [131 lines from a Yerong/Franklin (Y/F) cross] after a switch from moderate to saturating light. Rates of Rubisco activation were found to be highly variable across the mapping population, with a median activation rate of 0.1 min^-1 in the slowest genotype and 0.74 min^-1 in the fastest genotype. A unique quantitative trait locus (QTL) for Rubisco activation rate was identified on chromosome 7H. This is the first report on the identification of a QTL for Rubisco activation rate in planta and the discovery opens the door to marker-assisted breeding to improve whole-canopy photosynthesis of barley. This also suggests that genetic factors other than the previously characterized Rubisco activase (RCA) isoforms on chromosome 4H control Rubisco activity. Further strength is given to this finding as this QTL co-localized with QTLs identified for steady-state photosynthesis and stomatal conductance. Several other distinct QTLs were identified for these steady-state traits, with a common overlapping QTL on chromosome 2H, and distinct QTLs for photosynthesis and stomatal conductance identified on chromosomes 4H and 5H, respectively. Future work should aim to validate these QTLs under field conditions so that they can be used to aid plant breeding efforts.

During photosynthetic induction, biochemical and stomatal limitations differ between Brassica crops

Interventions to increase crop radiation use efficiency rely on understanding of how biochemical and stomatal limitations affect photosynthesis. When leaves transition from shade to high light, slow increases in maximum Rubisco carboxylation rate and stomatal conductance limit net CO₂ assimilation for several minutes. However, as stomata open intercellular [CO₂ ] increases, so electron transport rate could also become limiting. Photosynthetic limitations were evaluated in three important Brassica crops: Brassica rapa, Brassica oleracea and Brassica napus. Measurements of induction after a period of shade showed that net CO₂ assimilation by B. rapa and B. napus saturated by 10 min. A new method of analyzing limitations to induction by varying intercellular [CO₂ ] showed this was due to co-limitation by Rubisco and electron transport. By contrast, in B. oleracea persistent Rubisco limitation meant that CO₂ assimilation was still recovering 15 min after induction. Correspondingly, B. oleracea had the lowest Rubisco total activity. The methodology developed, and its application here, shows a means to identify the basis of variation in photosynthetic efficiency in fluctuating light, which could be exploited in breeding and bioengineering to improve crop productivity.

Concurrent Increases in Leaf Temperature With Light Accelerate Photosynthetic Induction in Tropical Tree Seedlings

Leaf temperature changes with incident light intensity, but it is unclear how the concurrent changes influence leaf photosynthesis. We examined the time courses of CO₂ gas exchanges and chlorophyll fluorescence of seedling leaves in four tropical tree species in response to lightflecks under three different temperature conditions. The three conditions were two constant temperatures at 30°C (T ₃₀) and 40°C (T ₄₀), and a simulated gradually changing temperature from 30 to 40°C (T _dyn). The time required to reach 50% of the full photosynthetic induction under T ₄₀ was similar to, or even larger than, that under T ₃₀. However, the induction of assimilation rate (A) and electron transport rate of photosystem II (ETR II) and Rubisco activation process were generally accelerated under T _dyn compared to those at either T ₃₀ or T ₄₀. The acceleration in photosynthetic induction under T _dyn was significantly greater in the shade-tolerant species than in the shade-intolerant species. A modified photosynthetic limitation analysis indicated that the acceleration was likely to be mainly due to ETR II at the early stage of photosynthetic induction. The study suggests that concurrent increases in leaf temperature with light may increase leaf carbon gain under highly fluctuating light in tropical tree seedlings, particularly in shade-tolerant species.

Variation in photosynthetic induction between rice accessions and its potential for improving productivity

Photosynthetic induction describes the transient increase in leaf CO₂ uptake with an increase in light. During induction, efficiency is lower than at steady state. Under field conditions of fluctuating light, this lower efficiency during induction may cost > 20% of potential crop assimilation. Accelerating induction would boost photosynthetic and resource-use efficiencies. Variation between rice accessions and potential for accelerating induction was analysed by gas exchange. Induction during shade to sun transitions of 14 accessions representing five subpopulations from the 3000 Rice Genome Project Panel (3K RGP) was analysed. Differences of 109% occurred in the CO₂ fixed during the first 300 s of induction, 117% in the half-time to completion of induction, and 65% in intrinsic water-use efficiency during induction, between the highest and lowest performing accessions. Induction in three accessions with contrasting responses (AUS 278, NCS 771 A and IR64-21) was compared for a range of [CO₂ ] to analyse limitations. This showed in vivo capacity for carboxylation at Rubisco (V_c,max ), and not stomata, as the primary limitation to induction, with significant differences between accessions. Variation in nonsteady-state efficiency greatly exceeded that at steady state, suggesting a new and more promising opportunity for selection of greater crop photosynthetic efficiency in this key food crop.

Photosynthetic Acclimation to Fluctuating Irradiance in Plants

Unlike the short-term responses of photosynthesis to fluctuating irradiance, the long-term response (i.e., acclimation) at the chloroplast, leaf, and plant level has received less attention so far. The ability of plants to acclimate to irradiance fluctuations and the speed at which this acclimation occurs are potential limitations to plant growth under field conditions, and therefore this process deserves closer study. In the first section of this review, we look at the sources of natural irradiance fluctuations, their effects on short-term photosynthesis, and the interaction of these effects with circadian rhythms. This is followed by an overview of the mechanisms that are involved in acclimation to fluctuating (or changes of) irradiance. We highlight the chain of events leading to acclimation: retrograde signaling, systemic acquired acclimation (SAA), gene transcription, and changes in protein abundance. We also review how fluctuating irradiance is applied in experiments and highlight the fact that they are significantly slower than natural fluctuations in the field, although the technology to achieve realistic fluctuations exists. Finally, we review published data on the effects of growing plants under fluctuating irradiance on different plant traits, across studies, spatial scales, and species. We show that, when plants are grown under fluctuating irradiance, the chlorophyll a/b ratio and plant biomass decrease, specific leaf area increases, and photosynthetic capacity as well as root/shoot ratio are, on average, unaffected.

High Stomatal Conductance in the Tomato Flacca Mutant Allows for Faster Photosynthetic Induction

Due to their slow movement and closure upon shade, partially closed stomata can be a substantial limitation to photosynthesis in variable light intensities. The abscisic acid deficient flacca mutant in tomato (Solanum lycopersicum) displays very high stomatal conductance (g_s ). We aimed to determine to what extent this substantially increased g_s affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in flacca and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO₂, maximum quantum efficiency of photosystem II photochemistry (F_v/F_m ), as well as mesophyll conductance to CO₂ diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO₂ permeability. When leaves adapted to shade (50 µmol m^-2 s^-1) at 400 µbar CO₂ partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m^-2 s^-1), photosynthetic induction was faster in flacca compared to wildtype leaves, and this was attributable to high initial g_s in flacca (~0.6 mol m^-2 s^-1): in flacca, the times to reach 50 (t₅₀ ) and 90% (t₉₀ ) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced g_s and slowed down photosynthetic induction in the wildtype, while no change was observed in flacca; under low humidity, t₅₀ was 63% and t₉₀ was 36% of wildtype levels in flacca. Photosynthetic induction in low CO₂ partial pressure (200 µbar) increased g_s in the wildtype (but not in flacca), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher g_s in flacca were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO₂ partial pressure, wildtype g_s is a substantial limitation to the rate of photosynthetic induction, which flacca overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.

Short-term effects of high CO2 accelerate photosynthetic induction in Populus koreana × trichocarpa with always-open stomata regardless of phenotypic changes in high CO2 growth conditions

Long-term high CO2 exposure accelerates photosynthetic induction response due to rapid light increase. However, it is unclear whether the acceleration is caused by acclimation of photosynthetic components (long-term CO2 effect) and/or by the sufficient substrate under high CO2 at the measurement (short-term CO2 effect). Populus koreana × trichocarpa cv. Peace has wide-open stomata almost not responding to changes of photon flux density. Using this species, we examined the long- and short-term CO2 effects on photosynthetic induction by focusing on biochemical components. We grew the plants under [CO2] of 380, 700 and 1020 μmol CO2 mol-1 air and measured the photosynthetic induction response under [CO2] of 380 and 1020 μmol CO2 mol-1 air. Despite significant reduction in Rubisco content and light-saturated photosynthetic rate in the leaves from the high growth CO2, the photosynthetic induction time was similar in leaves from different growth CO2 plants when measurement [CO2] was the same. The induction, however, was significantly fast at the higher than at the lower measurement [CO2], regardless of growth CO2 of the plants. These results demonstrate that the acceleration of apparent photosynthetic induction under high CO2 environment was mainly contributed by a short-term CO2 effect rather than by a long-term acclimation effect when stomatal limitation is not the major factor.

Elevated CO2 increases photosynthesis in fluctuating irradiance regardless of photosynthetic induction state

Leaves are often exposed to fluctuating irradiance, which limits assimilation. Elevated CO2 enhances dynamic photosynthesis (i.e. photosynthesis in fluctuating irradiance) beyond its effects on steady-state photosynthesis rates. Studying the role of CO2 in dynamic photosynthesis is important for understanding plant responses to changing atmospheric CO2 partial pressures. The rise of photosynthesis after a step-wise increase to 1000 μmol m-2 s-1, the loss of photosynthetic induction after irradiance decreases, and rates of photosynthesis during sinusoidal changes in irradiance were studied in tomato (Solanum lycopersicum L.) leaves, using three CO2 partial pressures (200, 400, and 800 µbar). Initial irradiance was set to 0, 50, 100, and 200 μmol m-2 s-1 to vary the initial induction state. Most responses at 200 µbar were not different from those at 400 µbar. In contrast, CO2 at 800 µbar increased the relative carbon gain by 12% after an increase in irradiance, decreased the loss of photosynthetic induction by 14%, and increased dynamic photosynthesis during sine waves by 17%, compared with 400 µbar. These effects were additional to steady-state effects of elevated CO2 on photosynthesis. The enhancement of dynamic photosynthesis rates by elevated CO2 may therefore additionally increase photosynthesis in a future, CO2-enriched climate.

Nitrogen Can Alleviate the Inhibition of Photosynthesis Caused by High Temperature Stress under Both Steady-State and Flecked Irradiance

Nitrogen is one of the most important elements for plants and is closely related to photosynthesis. High temperature stress significantly inhibits photosynthesis under both steady-state and flecked irradiance. However, it is not known whether nitrogen can affect the decrease in photosynthesis caused by high temperature, especially under flecked irradiance. In the present study, a pot experiment was conducted under two nitrogen (N) supplies with rice plants, and the steady-state and dynamic photosynthesis rates were measured under 28 and 40°C. High temperature significantly increased leaf hydraulic conductance (Kleaf) under high N supply (HN) but not under low N supply (LN). The increased Kleaf maintained a constant leaf water potential (Ψleaf) and steady-state stomatal conductance (gs,sat) under HN, while the Ψleaf and gs,sat significantly decreased under high temperature in LN conditions. This resulted in a more severe decrease in steady-state photosynthesis (Asat) under high temperature in the LN conditions. After shifting from low to high light, high temperature significantly delayed the recovery of photosynthesis, which resulted in more carbon loss under flecked irradiance. These effects were obtained under HN to a lesser extent than under LN supply. Therefore, it is concluded that nitrogen can alleviate the inhibition of photosynthesis caused by high temperature stress under both steady-state and flecked irradiance.

High CO2 concentration increases relative leaf carbon gain under dynamic light in Dipterocarpus sublamellatus seedlings in a tropical rain forest, Malaysia

Understory plants in tropical forests often experience a low-light environment combined with high CO2 concentration. We hypothesized that the high CO2 concentration may compensate for leaf carbon loss caused by the low light, through increasing light-use efficiency of both steady-state and dynamic photosynthetic properties. To test the hypothesis, we examined CO2 gas exchange in response to an artificial lightfleck in Dipterocarpus sublamellatus Foxw. seedlings under contrasting CO2 conditions: 350 and 700 μmol CO2 mol(-1) air in a tropical rain forest, Pasoh, Malaysia. Total photosynthetic carbon gain from the lightfleck was about double when subjected to the high CO2 when compared with the low CO2 concentration. The increase of light-use efficiency in dynamic photosynthesis contributed 7% of the increased carbon gain, most of which was due to reduction of photosynthetic induction to light increase under the high CO2. The light compensation point of photosynthesis decreased by 58% and the apparent quantum yield increased by 26% at the high CO2 compared with those at the low CO2. The study suggests that high CO2 increases photosynthetic light-use efficiency under both steady-state and fluctuating light conditions, which should be considered in assessing the leaf carbon gain of understory plants in low-light environments.