Comparative analysis of thylakoid protein complexes in the mesophyll and bundle sheath cells from C3 , C4 and C3 -C4 Paniceae grasses

The photosynthetic apparatus of the CAM plant Tillandsia flabellate and its response to water deficit

CAM plants are superior to C₃ plants in drought resistance because of their peculiar photosynthesis pathway and morphological features. While those aspects have been studied for decades, little is known about the photosynthetic machinery of CAM plants. Here, we used a combination of biochemical and biophysical methods to study the photosynthetic apparatus of Tillandsia flabellate, an obligatory CAM plant. Most of the Photosystems super- and sub-complexes have properties very similar to those of Arabidopsis, with the main difference that in Tillandsia PSI-LHCI complexes bind extra LHCI. Functional measurements show that the PSI/PSII ratio is rather low compared to other plants and that the antenna size of both PSI and PSII is small. Upon 30-day water deficiency, the composition of the photosystems does not change significantly, PSII efficiency remains high and no Photosystem II photoinhibition was detected despite a reduction of non-photochemical quenching (NPQ).

Leaf pigments and photosystems stoichiometry underpin photosynthetic efficiency of related C3 , C-C4 and C4 grasses under shade

The quantum yield of photosynthesis (QY, CO₂ fixed per light absorbed) depends on the efficiency of light absorption, the coupling between light absorption and electron transport, and the coupling between electron transport and carbon metabolism. QY is generally lower in C₃ relative to C₄ plants at warm temperatures and differs among the C₄ subtypes. We investigated the acclimation to shade of light absorption and electron transport in six representative grasses with C₃ , C₃ -C₄ and C₄ photosynthesis. Plants were grown under full (control) or 25% (shade) sunlight. We measured the in vivo activity and stoichiometry of PSI and PSII, leaf spectral properties and pigment contents, and photosynthetic enzyme activities. Under control growth-light conditions, C₄ species had higher CO₂ assimilation rates, which declined to a greater extent relative to the C₃ species. Whole leaf PSII/PSI ratios were highest in the C₃ species, while QY and cyclic electron flow (CEF) were highest in the C₄ , NADP-ME species. Shade significantly reduced leaf PSII/PSI, linear electron flow (LEF) and CEF of most species. Overall, shade reduced leaf absorptance, especially in the green region, as well as carotenoid and chlorophyll contents in C₄ more than non-C₄ species. The NAD-ME species underwent the greatest reduction in leaf absorptance and pigments under shade. In conclusion, shade compromised QY the least in the C₃ and the most in the C₄ -NAD-ME species. Different sensitivity to shade was associated with the ability to maintain leaf absorptance and pigments. This is important for maximising light absorption and minimising photoprotection under low light.

Progress and prospects of C4 trait engineering in plants

Incorporating C₄ photosynthetic traits into C₃ crops is a rational approach for sustaining future demands for crop productivity. Using classical plant breeding, engineering this complex trait is unlikely to achieve its target. Therefore, it is critical and timely to implement novel biotechnological crop improvement strategies to accomplish this goal. However, a fundamental understanding of C₃ , C₄ , and C₃ -C₄ intermediate metabolism is crucial for the targeted use of biotechnological tools. This review assesses recent progress towards engineering C₄ photosynthetic traits in C₃ crops. We also discuss lessons learned from successes and failures of recent genetic engineering attempts in C₃ crops, highlighting the pros and cons of using rice as a model plant for short-, medium- and long-term goals of genetic engineering. This review provides an integrated approach towards engineering improved photosynthetic efficiency in C₃ crops for sustaining food, fibre and fuel production around the globe.

Inorganic Nitrogen-Containing Aerosol Deposition Caused “Excessive Photosynthesis” of Herbs, Resulting in Increased Nitrogen Demand

The amount of atmospheric nitrogen-containing aerosols has increased dramatically due to the globally rising levels of nitrogen from fertilization and atmospheric deposition. Although the balance of carbon and nitrogen in plants is a crucial component of physiological and biochemical indexes and plays a key role in adaptive regulation, our understanding of how nitrogen-containing aerosols affect this remains limited; in particular, regarding the associated mechanisms. Using a fumigation particle generator, we generated ammonium nitrate solution (in four concentrations of 0, 15, 30, 60 kg N hm⁻² year⁻¹) into droplets, in 90% of which the diameters were less than 2.5 μm, in the range of 0.35–4 μm, and fumigated Iris germanica L. and Portulaca grandiflora Hook. for 30 days in April and August. We found that the weight percentage of nitrogen in the upper epidermis, mesophyll tissue, and bulk of leaves decreased significantly with the N addition rate, which caused a decrease of carbon:nitrogen ratio, due to the enhanced net photosynthetic rate. Compared with Portulaca grandiflora Hook., Iris germanica L. responded more significantly to the disturbance of N addition, resulting in a decrease in the weight percentage of nitrogen in the roots, due to a lower nitrogen use efficiency. In addition, the superoxide dismutase activity of the two plants was inhibited with a higher concentration of nitrogen sol; a reduction of superoxide dismutase activity in plants means that the resistance of plants to various environmental stresses is reduced, and this decrease in superoxide dismutase activity may be related to ROS signaling. The results suggest that inorganic nitrogen-containing aerosols caused excessive stress to plants, especially for Iris germanica L.

Upregulation of bundle sheath electron transport capacity under limiting light in C4 Setaria viridis

C₄ photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO₂ concentration at the site of CO₂ fixation. C₄ plants benefit from high irradiance but their efficiency decreases under shade, causing a loss of productivity in crop canopies. We investigated shade acclimation responses of Setaria viridis, a model monocot of NADP-dependent malic enzyme subtype, focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO₂ assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b₆ f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C₄ plants and will support the development of strategies for crop improvement, including the engineering of C₄ photosynthesis into C₃ plants.

Different physiological responses of C3 and C4 plants to nanomaterials

Several studies have previously reported that nanomaterial uptake and toxicity in plants are species dependent. However, the differences between photosynthetic pathways, C3 and C4, following nanomaterial exposure are poorly understood. In the current work, wheat and rice, two C3 pathway species are compared to amaranth and maize, which utilize the C4 photosynthetic mechanism. These plants were cultured in soils which were spiked with CuO, Ag, TiO₂, MWCNT, and FLG nanomaterials. Overall, the C4 plant exhibited higher resilience to NM stress than C3 plants. In particular, significant differences were observed in chlorophyll contents with rice returning a 40.9-54.2% decrease compared to 3.5-15.1% for maize. Fv/Fm levels were significantly reduced by up to 51% in rice whereas no significant reductions were observed in amaranth and maize. Furthermore, NM uptake in the C3 species was greater than that in C4 plants, a trend that was also seen in metal concentration. TEM results showed that CuO NPs altered the chloroplast thylakoid structure in rice leaves and a large number of CuO NPs were observed in the vascular sheath cells. In contrast, there were no significant changes in the chloroplasts in the vascular sheath and no significant CuO NPs were found in maize leaves. This study was the first to systematically characterize the effect of metal and carbon-based nanomaterials in soil on C3 and C4 plants, providing a new perspective for understanding the impact of nanomaterials on plants.

Installation of C4 photosynthetic pathway enzymes in rice using a single construct

Introduction of a C₄ photosynthetic mechanism into C₃ crops offers an opportunity to improve photosynthetic efficiency, biomass and yield in addition to potentially improving nitrogen and water use efficiency. To create a two-cell metabolic prototype for an NADP-malic enzyme type C₄ rice, we transformed Oryza sativa spp. japonica cultivar Kitaake with a single construct containing the coding regions of carbonic anhydrase, phosphoenolpyruvate (PEP) carboxylase, NADP-malate dehydrogenase, pyruvate orthophosphate dikinase and NADP-malic enzyme from Zea mays, driven by cell-preferential promoters. Gene expression, protein accumulation and enzyme activity were confirmed for all five transgenes, and intercellular localization of proteins was analysed. ¹³ CO₂ labelling demonstrated a 10-fold increase in flux though PEP carboxylase, exceeding the increase in measured in vitro enzyme activity, and estimated to be about 2% of the maize photosynthetic flux. Flux from malate via pyruvate to PEP remained low, commensurate with the low NADP-malic enzyme activity observed in the transgenic lines. Physiological perturbations were minor and RNA sequencing revealed no substantive effects of transgene expression on other endogenous rice transcripts associated with photosynthesis. These results provide promise that, with enhanced levels of the C₄ proteins introduced thus far, a functional C₄ pathway is achievable in rice.

Transcriptome analysis of a novel maize bsd C4 mutant using RNA-seq

The C4 plants like maize are at an advantage because they exhibit higher carbon conversion efficiency than C3 during photosynthesis. Using the evaluation of photosynthetic phenotypes and subcellular structure, and high-quality transcriptome analysis for four types of leaves from a bundle sheath defective maize mutant (bsd) and 501 wild line, the key target genes, important transcription factors, and specific pathways were obtained, which may regulate the C-concentrating mechanisms and antioxidant protection of the photosynthetic system, gibberellin signaling, ribosome editing, glycolysis, and chlorophyll biosynthesis. Based on these target genes, a novel network with photosynthetic transformation efficiency with oxidative decarboxylation and ribosome regulation was filtered innovatively by Cytoscape, which adds to our understanding for high-efficiency C-fixation and its genetic improvement in C3 and C4 plants.

The transcriptomic responses of C4 grasses to subambient CO2 and low light are largely species specific and only refined by photosynthetic subtype

Three subtypes of C₄ photosynthesis exist (NADP-ME, NAD-ME and PEPCK), each known to be beneficial under specific environmental conditions. However, the influence of photosynthetic subtype on transcriptomic plasticity, as well as the genes underpinning this variability, remain largely unknown. Here, we comprehensively investigate the responses of six C₄ grass species, spanning all three C₄ subtypes, to two controlled environmental stresses: low light (200 µmol m^-2  sec^-1 ) and glacial CO₂ (subambient; 180 ppm). We identify a susceptibility within NADP-ME species to glacial CO₂ . Notably, although glacial CO₂ phenotypes could be tied to C₄ subtype, biochemical and transcriptomic responses to glacial CO₂ were largely species specific. Nevertheless, we were able to identify subtype specific subsets of significantly differentially expressed transcripts which link resource acquisition and allocation to NADP-ME species susceptibility to glacial CO₂ . Here, low light phenotypes were comparable across species with no clear subtype response, while again, transcriptomic responses to low light were largely species specific. However, numerous functional similarities were noted within the transcriptomic responses to low light, suggesting these responses are functionally relatively conserved. Additionally, PEPCK species exhibited heightened regulation of transcripts related to metabolism in response to both stresses, likely tied to their C₄ metabolic pathway. These results highlight the influence that both species and subtype can have on plant responses to abiotic stress, building on our mechanistic understanding of acclimation within C₄ grasses and highlighting avenues for future crop improvements.

On the road to C4 rice: advances and perspectives

The international C4 rice consortium aims to introduce into rice a high capacity photosynthetic mechanism, the C4 pathway, to increase yield. The C4 pathway is characterised by a complex combination of biochemical and anatomical specialisation that ensures high CO2 partial pressure at RuBisCO sites in bundle sheath (BS) cells. Here we report an update of the progress of the C4 rice project. Since its inception in 2008 there has been an exponential growth in synthetic biology and molecular tools. Golden Gate cloning and synthetic promoter systems have facilitated gene building block approaches allowing multiple enzymes and metabolite transporters to be assembled and expressed from single gene constructs. Photosynthetic functionalisation of the BS in rice remains an important step and there has been some success overexpressing transcription factors in the cytokinin signalling network which influence chloroplast volume. The C4 rice project has rejuvenated the research interest in C4 photosynthesis. Comparative anatomical studies now point to critical features essential for the design. So far little attention has been paid to the energetics. C4 photosynthesis has a greater ATP requirement, which is met by increased cyclic electron transport in BS cells. We hypothesise that changes in energy statues may drive this increased capacity for cyclic electron flow without the need for further modification. Although increasing vein density will ultimately be necessary for high efficiency C4 rice, our modelling shows that small amounts of C4 photosynthesis introduced around existing veins could already provide benefits of increased photosynthesis on the road to C4 rice.

Cyclic electron flow and light partitioning between the two photosystems in leaves of plants with different functional types

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (fI) and PSII (fII), which are only known for a few model species such as spinach. No measures of fI are available for C4 grasses under different irradiances. We developed a new method to estimate (1) fII in vivo by concurrently measuring linear electron flux through both photosystems [Formula: see text] in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1-[Formula: see text]. For a C3 grass, fI was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C4 species belonging to NADP-ME and NAD-ME subtypes had fI of 0.6 and PCK subtype had 0.5 under control. All shade-grown C4 species had fI of 0.6 except for NADP-ME grass which had 0.7. It was also observed that fI ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C4 grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C4 NADP-ME species. Our study reveals a range of CEF and fI values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling.