# C4 carbon fixation

C$_4$ carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960s discovery by Marshall Davidson Hatch and Charles Roger Slack.
C$_4$ fixation is an addition to the ancestral and more common C$_3$ carbon fixation. The main carboxylating enzyme in C$_3$ photosynthesis is called RuBisCO, which catalyses two distinct reactions using either CO$_2$ (carboxylation) or oxygen (oxygenation) as a substrate. RuBisCO oxygenation gives rise to phosphoglycolate, which is toxic and requires the expenditure of energy to recycle through photorespiration. C$_4$ photosynthesis reduces photorespiration by concentrating CO$_2$ around RuBisCO.
To enable RuBisCO to work in a cellular environment where there is a lot of carbon dioxide and very little oxygen, C$_4$ leaves generally contain two partially isolated compartments called mesophyll cells and bundle-sheath cells. CO$_2$ is initially fixed in the mesophyll cells in a reaction catalysed by the enzyme PEP carboxylase in which the three-carbon phosphoenolpyruvate (PEP) reacts with CO$_2$ to form the four-carbon oxaloacetic acid (OAA). OAA can then be reduced to malate or transaminated to aspartate. These intermediates diffuse to the bundle sheath cells, where they are decarboxylated, creating a CO$_2$-rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate (PYR), together with about half of the phosphoglycerate (PGA) produced by RuBisCO, diffuses back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP). This exchange of metabolites is essential for C$_4$ photosynthesis to work.
Additional biochemical steps require more energy in the form of ATP to regenerate PEP, but concentrating CO$_2$ allows high rates of photosynthesis at higher temperatures. Higher CO$_2$ concentration overcomes the reduction of gas solubility with temperature (Henry's law). The CO$_2$ concentrating mechanism also maintains high gradients of CO$_2$ concentration across the stomatal pores. This means that C$_4$ plants have generally lower stomatal conductance, reduced water losses and have generally higher water-use efficiency. C$_4$ plants are also more efficient in using nitrogen, since PEP carboxylase is cheaper to make than RuBisCO. However, since the C$_3$ pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, typically at low temperatures and in the shade.

## Discovery

The first experiments indicating that some plants do not use C$_3$ carbon fixation but instead produce malate and aspartate in the first step of carbon fixation were done in the 1950s and early 1960s by Hugo Peter Kortschak and Yuri Karpilov. The C$_4$ pathway was elucidated by Marshall Davidson Hatch and Charles Roger Slack, in Australia, in 1966. While Hatch and Slack originally referred to the pathway as the "C$_4$ dicarboxylic acid pathway", it is sometimes called the Hatch–Slack pathway.

## Anatomy

C$_4$ plants often possess a characteristic leaf anatomy called kranz anatomy, from the German word for wreath. Their vascular bundles are surrounded by two rings of cells; the inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. The primary function of kranz anatomy is to provide a site in which CO$_2$ can be concentrated around RuBisCO, thereby avoiding photorespiration. Mesophyll and bundle sheath cells are connected through numerous cytoplasmic sleeves called plasmodesmata whose permeability at leaf level is called bundle sheath conductance. A layer of suberin is often deposed at the level of the middle lamella (tangential interface between mesophyll and bundle sheath) in order to reduce the apoplastic diffusion of CO$_2$ (called leakage). The carbon concentration mechanism in C$_4$ plants distinguishes their isotopic signature from other photosynthetic organisms.
Although most C$_4$ plants exhibit kranz anatomy, there are, however, a few species that operate a limited C$_4$ cycle without any distinct bundle sheath tissue. Suaeda aralocaspica, Bienertia cycloptera, Bienertia sinuspersici and Bienertia kavirense (all chenopods) are terrestrial plants that inhabit dry, salty depressions in the deserts of the Middle East. These plants have been shown to operate single-cell C$_4$ CO$_2$-concentrating mechanisms, which are unique among the known C$_4$ mechanisms. Although the cytology of both genera differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol are separated from decarboxylase enzymes and RuBisCO in the chloroplasts. A diffusive barrier is between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath-type area and a mesophyll-type area to be established within a single cell. Although this does allow a limited C$_4$ cycle to operate, it is relatively inefficient. Much leakage of CO$_2$ from around RuBisCO occurs.
There is also evidence of inducible C$_4$ photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which CO$_2$ leakage from around RuBisCO is minimised is currently uncertain.

## Biochemistry

In C$_3$ plants, the first step in the light-independent reactions of photosynthesis is the fixation of CO$_2$ by the enzyme RuBisCO to form 3-phosphoglycerate. However, RuBisCo has a dual carboxylase and oxygenase activity. Oxygenation results in part of the substrate being oxidized rather than carboxylated, resulting in loss of substrate and consumption of energy, in what is known as photorespiration. Oxygenation and carboxylation are competitive, meaning that the rate of the reactions depends on the relative concentration of oxygen and CO$_2$.
In order to reduce the rate of photorespiration, C$_4$ plants increase the concentration of CO$_2$ around RuBisCO. To do so two partially isolated compartments differentiate within leaves, the mesophyll and the bundle sheath. Instead of direct fixation by RuBisCO, CO$_2$ is initially incorporated into a four-carbon organic acid (either malate or aspartate) in the mesophyll. The organic acids then diffuse through plasmodesmata into the bundle sheath cells. There, they are decarboxylated creating a CO$_2$-rich environment. The chloroplasts of the bundle sheath cells convert this CO$_2$ into carbohydrates by the conventional C$_3$ pathway.
There is large variability in the biochemical features of C4 assimilation, and it is generally grouped in three subtypes, differentiated by the main enzyme used for decarboxylation ( NADP-malic enzyme, NADP-ME; NAD-malic enzyme, NAD-ME; and PEP carboxykinase, PEPCK). Since PEPCK is often recruited atop NADP-ME or NAD-ME it was proposed to classify the biochemical variability in two subtypes. For instance, maize and sugarcane use a combination of NADP-ME and PEPCK, millet uses preferentially NAD-ME and Megathyrsus maximus, uses preferentially PEPCK.

### NADP-ME

The first step in the NADP-ME type C$_4$ pathway is the conversion of pyruvate (Pyr) to phosphoenolpyruvate (PEP), by the enzyme Pyruvate phosphate dikinase (PPDK). This reaction requires inorganic phosphate and ATP plus pyruvate, producing PEP, AMP, and inorganic pyrophosphate (PP$_i$). The next step is the carboxylation of PEP by the PEP carboxylase enzyme (PEPC) producing oxaloacetate. Both of these steps occur in the mesophyll cells:
    pyruvate + P$_i$ + ATP → PEP + AMP + PP$_i$
    PEP + CO$_2$ → oxaloacetate
PEPC has a low K$_M$ for HCO$^−$ $_3$ — and, hence, high affinity, and is not confounded by O$_2$ thus it will work even at low concentrations of CO$_2$.
The product is usually converted to malate (M), which diffuses to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated by the NADP-malic enzyme (NADP-ME) to produce CO$_2$ and pyruvate. The CO$_2$ is fixed by RuBisCo to produce phosphoglycerate (PGA) while the pyruvate is transported back to the mesophyll cell, together with about half of the phosphoglycerate (PGA). This PGA is chemically reduced in the mesophyll and diffuses back to the bundle sheath where it enters the conversion phase of the Calvin cycle. For each CO$_2$ molecule exported to the bundle sheath the malate shuttle transfers two electrons, and therefore reduces the demand of reducing power in the bundle sheath.

### NAD-ME

Here, the OAA produced by PEPC is transaminated by aspartate aminotransferase to aspartate (ASP) which is the metabolite diffusing to the bundle sheath. In the bundle sheath ASP is transaminated again to OAA and then undergoes a futile reduction and oxidative decarboxylation to release CO$_2$. The resulting Pyruvate is transaminated to alanine, diffusing to the mesophyll. Alanine is finally transaminated to pyruvate (PYR) which can be regenerated to PEP by PPDK in the mesophyll chloroplasts. This cycle bypasses the reaction of malate dehydrogenase in the mesophyll and therefore does not transfer reducing equivalents to the bundle sheath.

### PEPCK

In this variant the OAA produced by aspartate aminotransferase in the bundle sheath is decarboxylated to PEP by PEPCK. The fate of PEP is still debated. The simplest explanation is that PEP would diffuse back to the mesophyll to serve as a substrate for PEPC. Because PEPCK uses only one ATP molecule, the regeneration of PEP through PEPCK would theoretically increase photosynthetic efficiency of this subtype, however this has never been measured. An increase in relative expression of PEPCK has been observed under low light, and it has been proposed to play a role in facilitating balancing energy requirements between mesophyll and bundle sheath.

### Metabolite exchange

While in C$_3$ photosynthesis each chloroplast is capable of completing light reactions and dark reactions, C$_4$ chloroplasts differentiate in two populations, contained in the mesophyll and bundle sheath cells. The division of the photosynthetic work between two types of chloroplasts results inevitably in a prolific exchange of intermediates between them. The fluxes are large and can be up to ten times the rate of gross assimilation. The type of metabolite exchanged and the overall rate will depend on the subtype. To reduce product inhibition of photosynthetic enzymes (for instance PECP) concentration gradients need to be as low as possible. This requires increasing the conductance of metabolites between mesophyll and bundle sheath, but this would also increase the retro-diffusion of CO$_2$ out of the bundle sheath, resulting in an inherent and inevitable trade off in the optimisation of the CO$_2$ concentrating mechanism.

## Light harvesting and light reactions

To meet the NADPH and ATP demands in the mesophyll and bundle sheath, light needs to be harvested and shared between two distinct electron transfer chains. ATP may be produced in the bundle sheath mainly through cyclic electron flow around Photosystem I, or in the mesophyll mainly through linear electron flow depending on the light available in the bundle sheath or in the mesophyll. The relative requirement of ATP and NADPH in each type of cells will depend on the photosynthetic subtype. The apportioning of excitation energy between the two cell types will influence the availability of ATP and NADPH in the mesophyll and bundle sheath. For instance, green light is not strongly adsorbed by mesophyll cells and can preferentially excite bundle sheath cells, or vice versa for blue light. Because bundle sheaths are surrounded by mesophyll, light harvesting in the mesophyll will reduce the light available to reach bundle sheath cells. Also, the bundle sheath size limits the amount of light that can be harvested.

## Efficiency

Different formulations of efficiency are possible depending on which outputs and inputs are considered. For instance, average quantum efficiency is the ratio between gross assimilation and either absorbed or incident light intensity. Large variability of measured quantum efficiency is reported in the literature between plants grown in different conditions and classified in different subtypes but the underpinnings are still unclear. One of the components of quantum efficiency is the efficiency of dark reactions, biochemical efficiency, which is generally expressed in reciprocal terms as ATP cost of gross assimilation (ATP/GA).
In C$_3$ photosynthesis ATP/GA depends mainly on CO$_2$ and O$_2$ concentration at the carboxylating sites of RuBisCO. When CO$_2$ concentration is high and O$_2$ concentration is low photorespiration is suppressed and C$_3$ assimilation is fast and efficient, with ATP/GA approaching the theoretical minimum of 3.
In C$_4$ photosynthesis CO$_2$ concentration at the RuBisCO carboxylating sites is mainly the result of the operation of the CO$_2$ concentrating mechanisms, which cost circa an additional 2 ATP/GA but makes efficiency relatively insensitive of external CO$_2$ concentration in a broad range of conditions.
Biochemical efficiency depends mainly on the speed of CO$_2$ delivery to the bundle sheath, and will generally decrease under low light when PEP carboxylation rate decreases, lowering the ratio of CO$_2$/O$_2$ concentration at the carboxylating sites of RuBisCO. The key parameter defining how much efficiency will decrease under low light is bundle sheath conductance. Plants with higher bundle sheath conductance will be facilitated in the exchange of metabolites between the mesophyll and bundle sheath and will be capable of high rates of assimilation under high light. However, they will also have high rates of CO$_2$ retro-diffusion from the bundle sheath (called leakage) which will increase photorespiration and decrease biochemical efficiency under dim light. This represents an inherent and inevitable trade off in the operation of C$_4$ photosynthesis. C$_4$ plants have an outstanding capacity to attune bundle sheath conductance. Interestingly, bundle sheath conductance is downregulated in plants grown under low light and in plants grown under high light subsequently transferred to low light as it occurs in crop canopies where older leaves are shaded by new growth.

## Evolution and advantages

C$_4$ plants have a competitive advantage over plants possessing the more common C$_3$ carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO$_2$ limitation. When grown in the same environment, at 30 °C, C$_3$ grasses lose approximately 833 molecules of water per CO$_2$ molecule that is fixed, whereas C$_4$ grasses lose only 277. This increased water use efficiency of C$_4$ grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.
C$_4$ carbon fixation has evolved in at least 62 independent occasions in 19 different families of plants, making it a prime example of convergent evolution. This convergence may have been facilitated by the fact that many potential evolutionary pathways to a C$_4$ phenotype exist, many of which involve initial evolutionary steps not directly related to photosynthesis. C$_4$ plants arose around 35 million years ago during the Oligocene (precisely when is difficult to determine) and were becoming ecologically significant in the early Miocene around 21 million years ago. C$_4$ metabolism in grasses originated when their habitat migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the C$_3$ pathway. Drought was not necessary for its innovation; rather, the increased parsimony in water use was a byproduct of the pathway and allowed C$_4$ plants to more readily colonize arid environments.
Today, C$_4$ plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this scarcity, they account for about 23% of terrestrial carbon fixation. Increasing the proportion of C$_4$ plants on earth could assist biosequestration of CO$_2$ and represent an important climate change avoidance strategy. Present-day C$_4$ plants are concentrated in the tropics and subtropics (below latitudes of 45 degrees) where the high air temperature increases rates of photorespiration in C$_3$ plants.

## Plants that use C$_4$ carbon fixation

About 8,100 plant species use C$_4$ carbon fixation, which represents about 3% of all terrestrial species of plants. All these 8,100 species are angiosperms. C$_4$ carbon fixation is more common in monocots compared with dicots, with 40% of monocots using the C$_4$ pathway, compared with only 4.5% of dicots. Despite this, only three families of monocots use C$_4$ carbon fixation compared to 15 dicot families. Of the monocot clades containing C$_4$ plants, the grass (Poaceae) species use the C$_4$ photosynthetic pathway most. 46% of grasses are C$_4$ and together account for 61% of C$_4$ species. C$_4$ has arisen independently in the grass family some twenty or more times, in various subfamilies, tribes, and genera, including the Andropogoneae tribe which contains the food crops maize, sugar cane, and sorghum. Various kinds of millet are also C$_4$. Of the dicot clades containing C$_4$ species, the order Caryophyllales contains the most species. Of the families in the Caryophyllales, the Chenopodiaceae use C$_4$ carbon fixation the most, with 550 out of 1,400 species using it. About 250 of the 1,000 species of the related Amaranthaceae also use C$_4$.
Members of the sedge family Cyperaceae, and members of numerous families of eudicots – including Asteraceae (the daisy family), Brassicaceae (the cabbage family), and Euphorbiaceae (the spurge family) – also use C$_4$.
No large trees (above 15 m in height) use C$_4$, however a number of small trees or shrubs smaller than 10 m exist which do: six species of Euphorbiaceae all native to Hawaii and two species of Amaranthaceae growing in deserts of the Middle-East and Asia.

## Converting C$_3$ plants to C$_4$

Given the advantages of C$_4$, a group of scientists from institutions around the world are working on the C$_4$ Rice Project to produce a strain of rice, naturally a C$_3$ plant, that uses the C$_4$ pathway by studying the C$_4$ plants maize and Brachypodium. As rice is the world's most important human food—it is the staple food for more than half the planet—having rice that is more efficient at converting sunlight into grain could have significant global benefits towards improving food security. The team claims C$_4$ rice could produce up to 50% more grain—and be able to do it with less water and nutrients.
The researchers have already identified genes needed for C$_4$ photosynthesis in rice and are now looking towards developing a prototype C$_4$ rice plant. In 2012, the Government of the United Kingdom along with the Bill & Melinda Gates Foundation provided US$14 million over three years towards the C$_4$ Rice Project at the International Rice Research Institute. In 2019, the Bill & Melinda Gates Foundation granted another US$15 million to the Oxford-University-led C4 Rice Project. The goal of the 5-year project is to have experimental field plots up and running in Taiwan by 2024.
C$_2$ photosynthesis, an intermediate step between C$_3$ and Kranz C$_4$, may be preferred over C$_4$ for rice conversion. The simpler system is less optimized for high light and high temperature conditions than C$_4$, but has the advantage of requiring fewer steps of genetic engineering and performing better than C$_3$ under all temperatures and light levels. In 2021, the UK Government provided £1.2 million on studying C$_2$ engineering.