Journal of Experimental Botany, Vol. 65, No. 13, pp. 3323–3325, 2014
doi:10.1093/jxb/eru262 Preface
Photosynthetic efficiency and carbon concentration in terrestrial plants: the C4 and CAM
solutions
As the provider of biological energy and reduced carbon, photosynthesis provides a foundation upon which the biosphere
and human society depend. Our food supply, our fibre, and much of our energy are derived from photosynthetic products.
Photosynthesis gave rise to most of the energy used by industrialized societies, mainly in the bank of hydrocarbons that have
accumulated in the Earth’s crust. As this fossil store of energy is depleted or, alternatively, if humans limit fossil fuel use to
forestall global warming, people will face the challenge of replacing this ancient energy supply with renewable sources. In the
form of biofuels, photosynthesis promises to be a major contributor to the new energy mix of a post-fossil-fuel world; however, this possibility raises the spectre of famine should resources be diverted from food production. Even without biofuels,
food security will be a major concern for the coming decades since population and economic growth threaten to overwhelm
existing supplies, while climate change, water depletion, land diversion to cities, and soil erosion threaten crop yields. To meet
these challenges, agricultural production has to increase; however, in a world where much of the arable land is already in use,
growing more on existing land and transforming non-arable into arable land are the principal strategies to enhance agricultural productivity. Increasing the output and efficiency of photosynthesis contributes to both objectives and thus has to be
viewed as a strategic goal for future global security.
The vast majority of land plants that directly contribute to the human food and energy supply use the C3 photosynthetic
pathway which, in warm conditions, is inefficient in terms of water, nitrogen, and radiation use compared with the C4 photosynthetic pathway. C4 photosynthesis is common amongst forage grasses of lower latitudes but is present in only a few major
C4 row crops. Of these, maize leads the world in terms of grain production while sugar cane is the current number one bioenergy crop. Because of its superior performance, C4 photosynthesis is considered to supercharge plant productivity in warm
environments and, consistently, peak yields of C4 plants exceed those of similar C3 crops by 50% or more. Hence, exploiting
C4 photosynthesis to a greater degree is one way to meet future agricultural needs and has led to efforts to engineer the C4
pathway into our leading C3 crops, such as rice. Crassulacean Acid Metabolism (CAM) represents the third major photosynthetic pathway in land plants. CAM is relatively unexploited for agriculture with only a few significant crops—notably pineapple and a few agave species, such as the tequila agave. CAM species exhibit the highest water-use efficiencies in plants which
enable them to do well in water-limited environments, such as semi-arid deserts, and as epiphytes. CAM photosynthesis is not
a supercharger but, instead, can increase the efficiency of water use to such a degree that crop production based on CAM
photosynthesis could bring marginal, drought-prone lands into steady production. In recognition of this, the US Department
of Energy has funded an effort to engineer CAM into the C3 tree poplar, which is currently a major source of wood fibre and
a promising biofuel crop.
The potential of C4 and CAM photosynthesis to enhance food and energy security has led to marked increases in research
on these photosynthetic systems in recent years. Leading efforts include the projects to engineer C4 and CAM photosynthesis into terrestrial C3 plants and research that has clarified when, where and how the C4 and CAM pathways evolved. Both
CAM and C4 evolved from C3 ancestors. By learning how these photosynthetic pathways evolved, humans may acquire new
approaches to guide the engineering strategies and to identify new ways to improve production in existing CAM and C4 species. Reduction in atmospheric CO2 and drying of the Earth’s climate in recent geological time are proposed to have promoted
C4 and CAM evolution which raises the possibility that elevated levels of CO2 in the near future may reverse the conditions
that have favoured these alternatives to C3 photosynthesis. As a result, there has been much research to evaluate the response
of C3, C4, and CAM plants to projected global change. The hope is, however, that global change may be minimized, in part
by adopting efficient bioenergy species, many of which would use the supercharged C4 and super-efficient CAM pathways.
The large bioenergy effort initiated in the mid-2000s includes many projects that seek to improve existing C4 bioenergy crops,
as well as developing new C4 and CAM bioenergy plants from wild species. If biofuels are successful, these efforts may bring
more species into domestication than at any time since the early days of agriculture.
© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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3324 | Preface
In recognition of the renewed emphasis on C4 and CAM photosynthesis, the editors of the Journal of Experimental Botany
have commissioned this special issue addressing recent advances in CAM and C4 plant biology. Thirteen reviews and 24
original reports address topics ranging from CAM and C4 evolution, genomics, physiological responses to the environment,
and impacts of past and future global change. While leading research laboratories in CAM and C4 plant biology are well represented, this special issue also highlights the work of many young researchers who will carry the CAM and C4 banners for
decades to come. This broad mix of papers reflects the increasing integration of disciplines that is becoming the hallmark of
the biological sciences in the 21st century.
While the issue covers a broad range of subjects, certain topics are emphasized. The specialized Kranz anatomy of C4
photosynthesis is examined from developmental (Fouracre et al.; Hibberd et al.; Koteyeva et al.), evolutionary (Sage et al.;
Lundgren et al.; Kadereit et al.), and genomic (Tausta et al.) perspectives. Unravelling the genetic control of Kranz anatomy
is considered to be one of the significant hurdles in engineering the C4 pathway into C3 plants and it is likely that a mix of
perspectives, as presented here, will be needed to develop a comprehensive understanding of the Kranz enigma. The functional
significance of anatomical variation in CAM is also examined in the genus Clusia, which is becoming the leading model for
CAM evolution (Barrera Zambrano et al.). In combination with the papers on Kranz anatomy, this work presents a nice contrast of how anatomy influences function in CAM versus C4 systems.
Phylogenetics has become an important tool in the study of CAM and C4 evolution, for example, by identifying dozens of
independent origins in CAM and C4. The ability of phylogenetics to resolve evolutionary trajectories is variable, depending
upon the extent of taxon sampling. Using simulated data sets, Hancock and Edwards evaluate the power of phylogenetics to
infer the ordered acquisition of traits during photosynthetic evolution. The power improves with the number of taxa sampled
but even with large data sets, there is sufficient uncertainty such that the phylogenetic data should be coupled to other, complementary data to evaluate the evolutionary hypothesis. Kadereit et al. provide such a coupled approach, using a phylogenetic
analysis and anatomical data sets to evaluate whether there are multiple C4 origins in the Camphorosmeae (Chenopodiaceae).
Photosynthetic pathway enzymes also provide valuable complementary data sets to evaluate independent origins of C4 and
CAM. The evolution of PEP carboxylase (PEPCase) in C4 and CAM species is addressed in three separate studies: one CAM
(Silvera et al.), one C4 (Rosnow et al.), and a third on Portulaca species that engage in both C4 and CAM (Christin et al.). In
Portulaca species with both CAM and C4 metabolism, Christin et al. demonstrate that a different isoform of the PEPCase
gene is co-opted for the C4 pathway than for the CAM pathway, a situation enabled by extensive gene duplication. Studies of
CAM and C4 evolution are also facilitated by examining species with intermediate traits between CAM and C4. Two studies of
C3–C4 intermediate species are presented here. Keerberg et al. present new data on the efficiency of CO2 concentration in C3–
C4 intermediates which is valuable because there is much talk, but limited data, on the effectiveness of the CO2-concentrating
mechanism in these plants. In the second study involving C3–C4 intermediates, Way et al. evaluate the evolution of stomatal
control in C4 photosynthesis, which is integral to the higher WUE of the C4 system. Bellasio et al. also provide an interesting
new method for non-destructively assessing photorespiration in C3, C4, and plants of evolutionary intermediate status, such as
intermediates or mutants, trangenics or hybrids that have lost or gained some aspect of the C4 pathway.
Synthetic biology is represented by numerous papers. In addition to addressing the development of Kranz anatomy and
suberin biosynthesis in C4 plants (Fouracre et al.; Mertz and Brutnell), synthetic biology is used to evaluate CAM photosynthesis and its potential to develop new CAM crops or, more radically, improve C3 crops by engineering them to express CAM
(DePaoli et al.). Whole genome studies will aide synthetic biological approaches to crop improvement. It is therefore appropriate that one of the first CAM species sequenced is pineapple which is the leading CAM food crop in the world. Zhang et al.
discuss this genome sequence and how it can expand our understanding of CAM biology. Systems modelling approaches are
also re-evaluating long-held dogmas regarding the nature of the three decarboxylation types in C4 photosynthesis. Instead of
three subtypes of C4 photosynthesis (NADP-ME, NAD-ME, and PEP carboxykinase), there may be two principal subtypes
(NADP-ME and NAD-ME), with PEP carboxykinase acting in a supplementary role (Brautigam et al.; Wang et al.).
One of the major topics in C4 plant biology is the degree to which energetic efficiency is compromised by leakage of previously fixed CO2 from bundle sheath cells. New methodologies, some of which are discussed here (von Caemmerer, Ghannoum
et al.), enable insights into isotopic discrimination by Rubisco from C3 and C4 plants (von Caemmerer, Tazoe et al.) and the
leakiness of the C4 bundle sheath, particularly at low light intensities (Kromdijk et al.; Bellasio and Griffiths). Leakiness has
significance for C4 crop improvement in general and bioenergy production in particular, since it reduces the realized efficiency
of converting light energy into biological energy that humans can exploit. In a related paper, Sharwood et al. examine the
interactions of low light and salinity on the efficiency of C4 photosynthesis which is significant as many C4 species occur in
salinized environments. In many areas of the world, crop lands have fallen out of production due to salinization from past
irrigation efforts. These areas may be amenable to restoration by saline-tolerant C4 species. Two studies address future climate
change impacts on CAM and C4 species: Williams et al. examine climate change impacts on columnar cacti, the iconic symbols of the American Southwest, while Polley et al. review potential impacts on C4 grassland productivity. The impacts of past
global change are also addressed by Pinto et al. who evaluate the photosynthetic responses of C3 and C4 grasses to reduced
atmospheric CO2.
The ability of many facultative CAM species to switch between C3 and CAM modes has led to CAM being a model for
understanding the mechanisms of environmental induction of complex traits in plants. C4 photosynthesis, by contrast, is
Preface | 3325
largely constitutive, with only a handful of aquatic plants exhibiting an ability to induce the C4 pathway. Winter and Holtum
review how CAM induction in facultative CAM plants is a valuable tool for understanding CAM function, and in an original report, Winter et al. examine switching between C3 and CAM modes in Agave angustifolia, a wild relative of cultivated
agaves such as Agave tequiliana. CAM photosynthesis is also an important model for understanding circadian regulation in
plants, given the need for close synchronization of numerous metabolic and physiological processes during day/night cycles.
As described by Ceusters et al., light quality is critical to proper CAM function by setting the Circadian clock; in particular,
both blue and red light are needed because each alone cannot synchronize all of the various components of CAM metabolism
during a diurnal cycle.
While facultative CAM is a model for induction of complex traits within individuals, C4 photosynthesis is becoming a
model for the evolution of complex traits, given it has independently arisen over 65 times, as noted by Sage et al. While numerous authors address the potential of certain structural traits in C3 ancestors to facilitate the evolution of C4 photosynthesis
(Kadereit et al., Lundgren et al.; Sage et al.), Li et al. examine whether C3 Arabidopsis plants also up-regulate C4 metabolism
when grown in low CO2 atmospheres. They hypothesize that induction of a moderate C4 cycle in C3 plants at low CO2 may
have enhanced carbon assimilation and thus facilitated selection for a stronger C4 pathway. To identify the genetic changes
during C4 evolution, hybridization of related C3 and C4 species has been pursued numerous times in the nearly 50 years since
the discovery of the C4 pathway. Studies of C3 x C4 hybrids have been problematic, however, due to chromosome mismatching
and hybrid sterility, such that isogenic and inbred lines could not be produced. This constraint may be relaxed in the genomics era, as sequencing of hybrid lines and their parents may provide the ability to link traits with the controlling genes. In a
return to one of the first systems developed to study the genetics of C4 photosynthesis, Oakley et al. re-established the hybrids
between C3 and C4 Atriplex species that were first developed by Malcolm Nobs and Olle Bjorkman in 1969, just three years
after the first publication of the C4 pathway. Oakley et al. demonstrate the relative ease of making hybrid F1 and F2 lines
between C3 and C4 Atriplex plants, indicating that this system could be a widely exploitable tool for the rapid discovery of C4
genes in the genomics era.
The development of C4 and CAM bioenergy crops is evaluated from genomic, agronomic, and physiological perspectives.
Maize is currently the leading North American bioenergy crop but it is widely criticized as having marginal returns on the
energy invested. As a result, there is an intensive search for alternatives that can replace maize in the North American energy
market. Sorghum is a promising annual crop that can store sugar in stems in a similar manner as sugar cane and hence might
improve on the ethanol yields obtainable from grain alone, as discussed by Mullet et al. Perennial C4 crops such as Miscanthus
are widely touted as promising second generation bioenergy crops, due to high biomass production and relatively low costs;
however, much of the marginal land available for bioenergy production occurs at higher latitudes and drier regions where C4
species are not currently optimal. Two papers in the issue address cold tolerance in Miscanthus (Friesen et al.; Long et al.)
while Davis et al. review the potential for CAM biofuel species to bring semi-arid regions into production.
With the tools of the new biology, the coming years will be one of rapid progress in our understanding of C4 and CAM photosynthesis. This understanding will provide new insights into how humanity can exploit C4 and CAM plants to address the
major challenges of the 21st century. An important lesson that comes from the papers in this special issue is that research into
phylogenetically diverse, seemingly esoteric systems, provides insights into fundamental processes, thereby allowing people to
develop novel ways to exploit the potential of the C4 and CAM systems to improve photosynthetic performance. By bringing
these papers together in one issue, our hope is that a new era in C4 and CAM research will be promoted, for the benefit of all
humankind.
Acknowledgements
I thank Howard Griffiths, one of the regular editors at the Journal of Experimental Botany, who assisted in the editing of
numerous papers in this special issue. I also thank Mary Traynor and the staff at the Journal of Experimental Botany for their
support and editorial assistance which made the assembly of this issue possible.
Rowan F. Sage,
University of Toronto