Martha Ludwig
Florian A. Busch
Roxana Khoshravesh
Sarah Covshoff- 1School of Molecular Sciences, University of Western Australia, Crawley, WA, Australia
- 2School of Biosciences and Birmingham Institute of Forest Research, University of Birmingham, Birmingham, United Kingdom
- 3Research School of Biology, Australian National University, Canberra, ACT, Australia
- 4Department of Biology, University of New Mexico, Albuquerque, MN, United States
- 5Independent Researcher, Las Vegas, NV, United States
Editorial on the Research Topic
Understanding C4 Evolution and Function
C4 photosynthesis is a remarkable example of convergent evolution, having independently evolved at least 62 times over the last 60 million years (Sage et al., 2011). In C4 species, Rubisco operates close to its maximal carboxylation rate through suppression of the oxygenation reaction. This activity is accomplished via the establishment of a molecular CO2 pump that delivers carbon in the form of C4 acid intermediates to a spatially sequestered Rubisco. This carbon pump can be set up using a diverse array of complex biochemical and morphological modifications relative to the ancestral C3 photosynthetic state.
The large number of independent origins of a C4 syndrome suggests that evolution from ancestral C3 photosynthesis to a derived C4 type is flexible at the molecular level and relatively easy in genetic terms (Gowik et al., 2004; Williams et al., 2013; Heckmann, 2016). With a large pool of biodiversity to exploit, such as in Southwest Asia, reviewed here by Rudov et al., natural variation in diverse phylogenetic lineages can be used to better understand the molecular changes enabling evolution of a functional C4 syndrome. The papers presented in this Research Topic make use of this biodiversity to expand our knowledge of C4 evolution and function.
Despite C4 photosynthesis being highly convergent, little work has been done to understand which C4 traits have arisen through convergence and could be considered essential for a C4 syndrome. Here, Khoshravesh et al. use gas exchange, leaf ultrastructure and biochemistry and carbon isotope ratios to characterize the carbon assimilation pathways used by species in the eudicot family Nyctaginaceae, and in the case of the C4 members, to determine the subtype of C4 photosynthesis. Combining these data with those from other C4 clades, they compiled a hierarchical list of convergent and divergent traits.
Gene duplication has been proposed as one of the early steps in the recruitment of genes during evolution of a C4 pathway (Monson, 2003). Tronconi et al. describe a complex evolutionary history responsible for present-day C4-associated NAD-malic enzyme (NAD-ME) in the Brassicales that involves ancestral gene duplication followed by degeneration, complementation subfunctionalization, and neofunctionalization. Gene duplication and co-option also appear to be responsible for the evolution of the C4-associated PEP transporter, PPT1. Lyu et al. identify differences in coding and non-coding regions between C3 and C4 orthologs of PPT1 associated with increased expression of the transporter in C4 mesophyll cells (MC). They find that gene duplication and neo-functionalization led to recruitment of a PPT1 paralog found in roots to a role in C4 function.
Most C4 species operate a carbon pump with the help of Kranz anatomy, wherein MC surround highly specialized bundle sheath cells (BSC) that are concentrically arranged around the vasculature (Sage et al., 2014). In a small number of species, special organellar arrangements within a single cell are used to achieve high CO2 concentrations around Rubisco (Sharpe and Offermann, 2014). In work on Suaeda aralocaspica, a single-cell C4 species, Cao et al. identify paralogs encoding the C4-associated phosphoenolpyruvate carboxylase (PEPC), which catalyzes the first step in the C4 pathway, a housekeeping isoform, and a bacterial-type PEPC.
Given the apparent flexibility of gene recruitment during evolution of C4 syndromes, identification of regulatory components controlling the spatial expression of C4-associated enzymes is important for understanding C4 function. Here, Afamefule and Raines use C3 and C4 grasses to screen upstream regions of genes encoding four enzymes in the Calvin-Benson-Bassham (CBB) cycle for conserved nucleotide sequences that might enable cell-preferential expression. They identify cis-regulatory elements putatively involved in BSC-enriched expression of genes encoding CBB enzymes as well as candidate transcription factors potentially binding to those sites. In addition, Górska et al. identify three trans-acting factors that bind the upstream region of the C4-associated PEPC homolog of maize. Characterization of these factors highlights the complexity of cell-preferential expression in a C4 leaf and the role of repression in establishing some C4-type expression patterns.
Of course, evolution is ongoing. As suggested by the results of Moody et al. in a study on PEPCs from older and younger C4 lineages, optimization of the enzyme continues after a C4 syndrome is realized. Similar comparative studies of other enzymes in the C4 acid cycle may also contribute to our understanding of how a C4 syndrome evolves at the molecular level.
A better understanding of the molecular events underpinning evolution of a C4 syndrome could enable a C3 plant to be engineered for C4 features. This is highly desirable because C4 crops have higher yields and increased nitrogen and water use efficiency relative to C3 crops. Replicating the C4 process in C3 crops such as rice would therefore help feed a growing world population. Support for introducing a C4 pathway into rice is provided by Lin et al. Genes encoding four of the major enzymes in the maize NADP-ME-type C4 pathway, PEPC, NADP-malate dehydrogenase (NADP-MDH), NADP-ME and pyruvate phosphate dikinase (PPDK), were inserted into the rice genome. Subsequent measurements with 13CO2 demonstrate that production of 13C-labeled malate was high in the transformants, suggesting that a partial C4 pathway is functioning in these plants.
Studies on C4 physiology and metabolism are also important to improve breeding programs of C4 crops. In particular, light harvesting and nutrient availability and uptake are key determinants for crop productivity. Collison et al. explore relationships between leaf age and light availability with the phenomenon of shade maladaptation exhibited by the NADP-ME-type C4 crops maize, sorghum and sugarcane. Leaf age had little influence on the quantum yield of CO2 assimilation. Instead, optimization of the leaf light environment mitigates the negative effects on productivity associated with this maladaptive response. These results can inform breeding strategies related to canopy structure and agricultural practices such as planting densities to increase crop yield.
Jobe et al. highlight the need to consider the nutritional value of C4 crops in addition to yield. They review nutrient assimilation pathways in C4 plants and how they differ from C3 plants as well as discuss gaps in our knowledge of how nutrient uptake and levels are controlled in C4 plants. They also consider the effects of increasing atmospheric CO2 on C3 and C4 crop micronutrient assimilation and content in light of micronutrient-related malnutrition (i.e., hidden hunger). Such considerations are important for producing future C4 crops that will effectively address global food needs.
In summary, this collection of articles expands our understanding of C4 evolution and function. This new knowledge will inform future work in evolutionary biology, C4 metabolism, and crop improvement strategies.
Author Contributions
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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References
Gowik, U., Burscheidt, J., Akyildiz, M., Schlue, U., Koczor, M., Streubel, M., et al. (2004). cis-Regulatory elements for mesophyll-specific gene expression in the C4 plant Flaveria trinervia, the promoter of the C4 phosphoenolpyruvate carboxylase gene. Plant Cell. 16, 1077–1090. doi: 10.1105/tpc.019729
Heckmann, D. (2016). C4 photosynthesis evolution: the conditional Mt. Fuji. Curr Opin Plant Biol. 31, 149–154. doi: 10.1016/j.pbi.2016.04.008
Monson, R. (2003). Gene duplication, neofunctionalization, and the evolution of C4 photosynthesis. Int. J. Plant Sci. 164, S43–S54. doi: 10.1086/368400
Sage, R. F., Christin, P. A., and Edwards, E. J. (2011). The C4 plant lineages of planet Earth. J. Exp. Bot. 62, 3155–3169. doi: 10.1093/jxb/err048
Sage, R. F., Khoshravesh, R., and Sage, T. L. (2014). From proto-Kranz to C4 Kranz: building the bridge to C4 photosynthesis. J. Exp. Bot. 65, 3341–3356. doi: 10.1093/jxb/eru180
Sharpe, R. M., and Offermann, S. (2014). One decade after the discovery of single-cell C4 species in terrestrial plants: what did we learn about the minimal requirements of C4 photosynthesis? Photosynth. Res. 119, 169–180. doi: 10.1007/s11120-013-9810-9
Keywords: C4 photosynthesis, convergent evolution, comparative biology, biodiversity, regulatory mechanisms
Citation: Ludwig M, Busch FA, Khoshravesh R and Covshoff S (2021) Editorial: Understanding C4 Evolution and Function. Front. Plant Sci. 12:774818. doi: 10.3389/fpls.2021.774818
Received: 13 September 2021; Accepted: 29 September 2021;
Published: 22 October 2021.
Edited and reviewed by: Carl J. Rothfels, University of California, Berkeley, United States
Copyright © 2021 Ludwig, Busch, Khoshravesh and Covshoff. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Sarah Covshoff, c2FyYWhjb3ZzaG9mZiYjeDAwMDQwO2dtYWlsLmNvbQ==