C4 photosynthesis: 50 years of discovery and innovation

Habitat differentiation and environmental adaptability contribute to leaf size variations globally in C3 and C4 grasses

The grass family (Poaceae) dominates ~43 % of Earth's land area and contributes 33 % of terrestrial primary productivity that is critical to naturally regulating atmosphere CO2 concentration and global climate change. Currently grasses comprise ~11,780 species and ~50 % of them (~6000 species) utilize C4 photosynthetic pathway. Generally, grass species have smaller leaves under colder and drier environments, but it is unclear whether the primary drivers of leaf size differ between C3 and C4 grasses on a global scale. Here, we analyzed 34 environmental variables, such as latitude, elevation, mean annual temperature, mean annual precipitation, and solar radiation etc., through a comparatively comprehensive database of ~3.0 million occurrence records from 1380 C3 and 978 C4 grass species (2358 species in total). Results from this study confirm that C4 grasses have occupied habitats with lower latitudes and elevations, characterized by warmer, sunnier, drier and less fertile environmental conditions. Grass leaf size correlates positively with mean annual temperature and precipitation as expected. Our results also demonstrate that the mean temperature of the wettest quarter of the year is the primary control for C3 leaf size, whereas C4 leaf size is negatively correlated with the difference between summer and winter temperatures. For C4 grasses, phylogeny exerts a significant effect on leaf size but is less important than environmental factors. Our findings highlight the importance of evolutionarily contrasting variations in leaf size between C3 and C4 grasses for shaping their geographical distribution and habitat suitability at the global scale.

Regulatory NADH dehydrogenase-like complex optimizes C4 photosynthetic carbon flow and cellular redox in maize

C4 plants typically operate a CO2 concentration mechanism from mesophyll (M) cells into bundle sheath (BS) cells. NADH dehydrogenase-like (NDH) complex is enriched in the BS cells of many NADP-malic enzyme (ME) type C4 plants and is more abundant in C4 than in C3 plants, but to what extent it is involved in the CO2 concentration mechanism remains to be experimentally investigated. We created maize and rice mutants deficient in NDH function and then used a combination of transcriptomic, proteomic, and metabolomic approaches for comparative analysis. Considerable decreases in growth, photosynthetic activities, and levels of key photosynthetic proteins were observed in maize but not rice mutants. However, transcript abundance for many cyclic electron transport (CET) and Calvin-Benson cycle components, as well as BS-specific C4 enzymes, was increased in maize mutants. Metabolite analysis of the maize ndh mutants revealed an increased NADPH : NADP ratio, as well as malate, ribulose 1,5-bisphosphate (RuBP), fructose 1,6-bisphosphate (FBP), and photorespiration intermediates. We suggest that by optimizing NADPH and malate levels and adjusting NADP-ME activity, NDH functions to balance metabolic and redox states in the BS cells of maize (in addition to ATP supply), coordinating photosynthetic transcript abundance and protein content, thus directly regulating the carbon flow in the two-celled C4 system of maize.

Global distribution, climatic preferences and photosynthesis-related traits of C4 eudicots and how they differ from those of C4 grasses

C₄ is one of three known photosynthetic processes of carbon fixation in flowering plants. It evolved independently more than 61 times in multiple angiosperm lineages and consists of a series of anatomical and biochemical modifications to the ancestral C3 pathway increasing plant productivity under warm and light-rich conditions. The C4 lineages of eudicots belong to seven orders and 15 families, are phylogenetically less constrained than those of monocots and entail an enormous structural and ecological diversity. Eudicot C4 lineages likely evolved the C4 syndrome along different evolutionary paths. Therefore, a better understanding of this diversity is key to understanding the evolution of this complex trait as a whole. By compiling 1207 recognised C4 eudicots species described in the literature and presenting trait data among these species, we identify global centres of species richness and of high phylogenetic diversity. Furthermore, we discuss climatic preferences in the context of plant functional traits. We identify two hotspots of C4 eudicot diversity: arid regions of Mexico/Southern United States and Australia, which show a similarly high number of different C4 eudicot genera but differ in the number of C4 lineages that evolved in situ. Further eudicot C4 hotspots with many different families and genera are in South Africa, West Africa, Patagonia, Central Asia and the Mediterranean. In general, C4 eudicots are diverse in deserts and xeric shrublands, tropical and subtropical grasslands, savannas and shrublands. We found C4 eudicots to occur in areas with less annual precipitation than C4 grasses which can be explained by frequently associated adaptations to drought stress such as among others succulence and salt tolerance. The data indicate that C4 eudicot lineages utilising the NAD-ME decarboxylating enzyme grow in drier areas than those using the NADP-ME decarboxylating enzyme indicating biochemical restrictions of the later system in higher temperatures. We conclude that in most eudicot lineages, C4 evolved in ancestrally already drought-adapted clades and enabled these to further spread in these habitats and colonise even drier areas.

From Soil Amendments to Controlling Autophagy: Supporting Plant Metabolism under Conditions of Water Shortage and Salinity

Crop resistance to environmental stress is a major issue. The globally increasing land degradation and desertification enhance the demand on management practices to balance both food and environmental objectives, including strategies that tighten nutrient cycles and maintain yields. Agriculture needs to provide, among other things, future additional ecosystem services, such as water quantity and quality, runoff control, soil fertility maintenance, carbon storage, climate regulation, and biodiversity. Numerous research projects have focused on the food–soil–climate nexus, and results were summarized in several reviews during the last decades. Based on this impressive piece of information, we have selected only a few aspects with the intention of studying plant–soil interactions and methods for optimization. In the short term, the use of soil amendments is currently attracting great interest to cover the current demand in agriculture. We will discuss the impact of biochar at water shortage, and plant growth promoting bacteria (PGPB) at improving nutrient supply to plants. In this review, our focus is on the interplay of both soil amendments on primary reactions of photosynthesis, plant growth conditions, and signaling during adaptation to environmental stress. Moreover, we aim at providing a general overview of how dehydration and salinity affect signaling in cells. With the use of the example of abscisic acid (ABA) and ethylene, we discuss the effects that can be observed when biochar and PGPB are used in the presence of stress. The stress response of plants is a multifactorial trait. Nevertheless, we will show that plants follow a general concept to adapt to unfavorable environmental conditions in the short and long term. However, plant species differ in the upper and lower regulatory limits of gene expression. Therefore, the presented data may help in the identification of traits for future breeding of stress-resistant crops. One target for breeding could be the removal and efficient recycling of damaged as well as needless compounds and structures. Furthermore, in this context, we will show that autophagy can be a useful goal of breeding measures, since the recycling of building blocks helps the cells to overcome a period of imbalanced substrate supply during stress adjustment.

Distinct C4 sub-types and C3 bundle sheath isolation in the Paniceae grasses

In C4 plants, the enzymatic machinery underpinning photosynthesis can vary, with, for example, three distinct C4 acid decarboxylases being used to release CO2 in the vicinity of RuBisCO. For decades, these decarboxylases have been used to classify C4 species into three biochemical sub-types. However, more recently, the notion that C4 species mix and match C4 acid decarboxylases has increased in popularity, and as a consequence, the validity of specific biochemical sub-types has been questioned. Using five species from the grass tribe Paniceae, we show that, although in some species transcripts and enzymes involved in multiple C4 acid decarboxylases accumulate, in others, transcript abundance and enzyme activity is almost entirely from one decarboxylase. In addition, the development of a bundle sheath isolation procedure for a close C3 species in the Paniceae enables the preliminary exploration of C4 sub-type evolution.

Angiosperms at the edge: Extremity, diversity, and phylogeny

A hallmark of flowering plants is their ability to invade some of the most extreme and dynamic habitats, including cold and dry biomes, to a far greater extent than other land plants. Recent work has provided insight to the phylogenetic distribution and evolutionary mechanisms which have enabled this success, yet needed is a synthesis of evolutionary perspectives with plant physiological traits, morphology, and genomic diversity. Linking these disparate components will not only lead to better understand the evolutionary parallelism and diversification of plants with these two strategies, but also to provide the framework needed for directing future research. We summarize the primary physiological and structural traits involved in response to cold- and drought stress, outline the phylogenetic distribution of these adaptations, and describe the recurring association of these changes with rapid diversification events that occurred in multiple lineages over the past 15 million years. Across these threefold facets of dry-cold correlation (traits, phylogeny, and time) we stress the contrast between (a) the amazing diversity of solutions flowering plants have developed in the face of extreme environments and (b) a broad correlation between cold and dry adaptations that in some cases may hint at deep common origins.

Identification and evolution of C4 photosynthetic pathway genes in plants

BACKGROUND

NADP-malic enzyme (NAPD-ME), and pyruvate orthophosphate dikinase (PPDK) are important enzymes that participate in C₄ photosynthesis. However, the evolutionary history and forces driving evolution of these genes in C₄ plants are not completely understood.

RESULTS

We identified 162 NADP-ME and 35 PPDK genes in 25 species and constructed respective phylogenetic trees. We classified NADP-ME genes into four branches, A1, A2, B1 and B2, whereas PPDK was classified into two branches in which monocots were in branch I and dicots were in branch II. Analyses of selective pressure on the NAPD-ME and PPDK gene families identified four positively selected sites, including 94H and 196H in the a5 branch of NADP-ME, and 95A and 559E in the e branch of PPDK at posterior probability thresholds of 95%. The positively selected sites were located in the helix and sheet regions. Quantitative RT-PCR (qRT-PCR) analyses revealed that expression levels of 6 NADP-ME and 2 PPDK genes from foxtail millet were up-regulated after exposure to light.

CONCLUSION

This study revealed that positively selected sites of NADP-ME and PPDK evolution in C₄ plants. It provides information on the classification and positive selection of plant NADP-ME and PPDK genes, and the results should be useful in further research on the evolutionary history of C₄ plants.

Engineering Improved Photosynthesis in the Era of Synthetic Biology

Much attention has been given to the enhancement of photosynthesis as a strategy for the optimization of crop productivity. As traditional plant breeding is most likely reaching a plateau, there is a timely need to accelerate improvements in photosynthetic efficiency by means of novel tools and biotechnological solutions. The emerging field of synthetic biology offers the potential for building completely novel pathways in predictable directions and, thus, addresses the global requirements for higher yields expected to occur in the 21st century. Here, we discuss recent advances and current challenges of engineering improved photosynthesis in the era of synthetic biology toward optimized utilization of solar energy and carbon sources to optimize the production of food, fiber, and fuel.

SMRT sequencing of a full-length transcriptome reveals transcript variants involved in C18 unsaturated fatty acid biosynthesis and metabolism pathways at chilling temperature in Pennisetum giganteum

Background

Pennisetum giganteum, an abundant, fast-growing perennial C₄ grass that belongs to the genus Pennisetum, family Poaceae, has been developed as a source of biomass for mushroom cultivation and production, as a source of forage for cattle and sheep, and as a tool to remedy soil erosion. However, having a chilling-sensitive nature, P. giganteum seedlings need to be protected while overwintering in most temperate climate regions.

Results

To elucidate the cold stress responses of P. giganteum, we carried out comprehensive full-length transcriptomes from leaf and root tissues under room temperature (RT) and chilling temperature (CT) using PacBio Iso-Seq long reads. We identified 196,124 and 140,766 full-length consensus transcripts in the RT and CT samples, respectively. We then systematically performed functional annotation, transcription factor identification, long non-coding RNAs (lncRNAs) prediction, and simple sequence repeat (SSR) analysis of those full-length transcriptomes. Isoform analysis revealed that alternative splicing events may be induced by cold stress in P. giganteum, and transcript variants may be involved in C18 unsaturated fatty acid biosynthesis and metabolism pathways at chilling temperature in P. giganteum. Furthermore, the fatty acid composition determination and gene expression level analysis supported that C18 unsaturated fatty acid biosynthesis and metabolism pathways may play roles during cold stress in P. giganteum.

Conclusions

We provide the first comprehensive full-length transcriptomic resource for the abundant and fast-growing perennial grass Pennisetum giganteum. Our results provide a useful transcriptomic resource for exploring the biological pathways involved in the cold stress responses of P. giganteum.

Exploring natural variation of photosynthesis in a site-specific manner: evolution, progress, and prospects

Site-specific changes of photosynthesis, a relatively new concept, can be used to improve the productivity of critical food crops to mitigate the foreseen food crisis. Global food security is threatened by an increasing population and the effects of climate change. Large yield improvements were achieved in major cereal crops between the 1950s and 1980s through the Green Revolution. However, we are currently experiencing a significant decline in yield progress. Of the many approaches to improved cereal yields, exploitation of the mode of photosynthesis has been intensely studied. Even though the C₄ pathway is considered the most efficient, mainly because of the carbon concentrating mechanisms around the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, which minimize photorespiration, much is still unknown about the specific gene regulation of this mode of photosynthesis. Most of the critical cereal crops, including wheat and rice, are categorized as C₃ plants based on the photosynthesis of major photosynthetic organs. However, recent findings raise the possibility of different modes of photosynthesis occurring at different sites in the same plant and/or in plants grown in different habitats. That is, it seems possible that efficient photosynthetic traits may be expressed in specific organs, even though the major photosynthetic pathway is C₃. Knowledge of site-specific differences in photosynthesis, coupled with site-specific regulation of gene expression, may therefore hold a potential to enhance the yields of economically important C₃ crops.

Photosynthesis capacity diversified by leaf structural and physiological regulation between upland and lowland switchgrass in different growth stages

Understanding and enhancing switchgrass (Panicum virgatum L.) photosynthesis will help to improve yield and quality for bio-industrial applications on cellulosic biofuel production. In the present study, leaf anatomical traits and physiological characteristics related to photosynthetic capacity of both lowland and upland switchgrass were recorded from four varieties across the vegetative, elongation and reproductive growth stages. Compared with the upland varieties, the lowland switchgrass showed 37-59, 22-64 and 27-73% higher performance on height, stem and leaf over all three growth stages. Leaf anatomical traits indicated that the leaves of lowland varieties provided more space for carbon assimilation and transportation caused by enhanced cell proliferation with more bundles sheath cells and larger contact areas between the bundle sheath and mesophyll cells (CAMB), which lead to the 32-72% higher photosynthetic capacity found in the lowland varieties during vegetative and elongation growth. However, photosynthetic capacity became 22-51% higher in the upland varieties during the reproductive stage, which is attributed to more photosynthetic pigment. In conclusion, lowland varieties gain a photosynthetic advantage with enhanced bundle sheath cell proliferation, while the upland varieties preserved more photosynthetic pigments. Our study provides new insights for improving the yield in crops by enhancing photosynthesis with anatomical and physiological strategies.