C4 carbon fixation

C4 plants use a pathway to first vorzufixieren carbon dioxide for photosynthesis spatially and then build up as C3 plants in the Calvin cycle to carbohydrates. The name derives from the first C4 fixation product, which is formed by the assimilation of carbon dioxide. While this is a carbon compound with three carbon atoms in C3 - plants is found in C4 plants a compound having four carbon atoms.

The carbon dioxide assimilation and the Calvin cycle take place in C4 plants spatially separated. Through the application of energy carbon dioxide is thereby actively enriched, leading to a higher rate of photosynthesis - leads - especially from lack of water and the resulting narrowing of the stomata. Therefore, C4 plants are superior to C3 plants under arid conditions. By actively enrichment photorespiration occurs much less frequently. Typical C4 plants are especially grasses, including popular crops such as corn, sugar cane and sorghum, but also other types, such as amaranth.

Plants with Crassulacean acid metabolism process similar to C4 plants, among them pre-fixation and the Calvin cycle, however, are separated in time.

• 6.1 Optional C4 photosynthesis

• 7.1 Water Requirements
• 7.2 nitrogen requirements
• 7.3 Energy Requirements

Occurrence of C4 photosynthesis in the plant kingdom

Approximately 5% of angiosperms operate C4 photosynthesis. Known C4 plants are amaranth, millet, maize, sugar cane and miscanthus. Most are among the grasses, followed by sedges. But even with a number of dicotyledons there this pathway, particularly in the fox tail plants and other carnation -like, with spurge and occasionally at wind plants and composite flowers. Since the C4 photosynthesis is 50 times arose independently in 10 different families from each other, one speaks of a polyphyletic trait.

The majority of all C4 grasses growing in small regions of the 30th parallel. C4 plants are rare to find in cold regions, for example in the boreal zone between the 50th and 65th degree of latitude and high altitude. An exception is the treeless tundra of the alpine zone, where it is dry. In Tibet the C4 grass orinus thoroldii was discovered in 5200 meters altitude. Generally they come in polar and subpolar regions not available ( beyond the 65 ° latitude ).

There are some cold- tolerant C4 plants, the frost and winter temperatures ( - 20 ° C) can be, for example, C4 grasses in the Andes. Even in temperate areas such as the coast of New Zealand, the Atlantic coasts of Canada and Great Britain or some swamps grow C4 plants.

In the last thirty years, the spread of C4 plants can also be observed on warm, sunny locations in Central Europe. Mostly it is a millet -like grasses and pigweed species. Their propagation is at least so far not considered a threat to native flora.

Evolution

The first occurrence of C4 plants is still the subject of research. For the dating of various techniques such as DNA analysis ( phylogenetic studies ), geochemical signals (eg, the isotopic ratio of 12C and 13C), Fossil and micro- fossils (pollen, phytoliths ) are used. A proliferation of C4 plants, and thus an extension of the dominant C4 plants ecosystems, took place before 2-8 million years ago at the end of the Miocene and the beginning of the Pliocene. As a reason, various factors, such as climatic changes, fire or the co-evolution of grazing animals apply.

There is increasing evidence that the origin of C4 photosynthesis is about 25 million years before our time. Falling temperatures and carbon dioxide concentrations characterize this period. Whether already a C4 photosynthesis has taken place much earlier, can not be clearly concluded by today's investigative techniques. For example, isotope values ​​can be interpreted from marine sediments in Africa today so that first C4 plants existed before 90 million years. An even earlier possible date would be the transition from the Late Carboniferous to Early Permian about 300 million years, since the prevailing ratio of oxygen to carbon dioxide would have caused a selection pressure in favor of C4 plants.

Discovery

The first studies on a C4 plant were conducted by Hugo Kortschak. In 1950 he identified on a sugarcane research institute in Hawaii as the first CO2 fixation product L- malate and L- aspartate. These are C4 compounds and therefore stood in contradiction to the findings of Melvin Calvin, Andrew Benson and James Bassham. This showed that the first metabolic product of CO2 fixation in the dark reaction, a C3 compound, 3 -phosphoglycerate is. The results Kortschaks were not published until ten years later. Also, the Russian Yuri Karpilov discovered on corn, that the first fixation product a C4 compound is also coined the name of the photosynthetic type.

Only the Australian researchers Marshall Davidson Hatch and Charles Roger Slack were able to determine with those results and our own studies the biochemistry of the pathway. The function and importance of which was published at the end of the 1970s. As a result, the C4 metabolism is referred to by its discoverers as the Hatch- Slack pathway or Hatch - Slack cycle.

Anatomy

The reactions of the C4 pathway extend mostly over two adjacent cell types, which give the C4 plants, a typical anatomy. Here, CO2 is first in mesophyll cells (A, see figure ) prefixed to a C4 compound. Mesophyll cells do not contain RuBisCO. The CO2 in the form of bound from that C4 compound is transported via plasmodesmata in bundle sheath cells (B, see figure ) and released there. These can run the Calvin cycle due to their enzyme equipment. The resulting in the release of compound C3 is transported back to the mesophyll cells. Since the bundle sheath cells are surrounded by mesophyll cells in a ring, one also speaks of a wreath anatomy. For this typical anatomy of C4 plants the Wurzelgen SCARECROW seems to be responsible, this show experiments on maize plants.

The CO2 fixation ( in mesophyll cells ) and the actual carbon assimilation in the Calvin cycle ( in bundle sheath cells) are spatially separated.

Biochemistry

This process represents a CO2 pump that starts with the fixation of CO2 in the form of bicarbonate (HCO3-). The formation of CO2 from HCO3- is catalyzed by carbonic anhydrase.

Reactions in the mesophyll

A phosphoenolpyruvate carboxylase catalyzes the condensation of a molecule phosphoenolpyruvate with HCO3 -, so that oxaloacetate is formed. This is converted into L- malate ( " Malatbildner " ), in some C4 plants in L- aspartate.

In this form it is transported through the plasmodesmata into the bundle sheath cells.

Reactions in the bundle sheath cell

Referring now to the carbon dioxide is again released and fed to the Calvin cycle, is different in C4 plants.

The cell walls of the bundle sheath cells are usually suberinisiert, whereby the released CO2 can not diffuse out of the cell. It accumulates in the cell, so that a high concentration of CO2 for the RuBisCO prevails that it introduces to assimilation in the Calvin cycle.

In plants with the NADP-dependent malic enzyme, the resulting NADPH is used in the reductive part of the Calvin cycle. So, in the bundle sheath cells often relatively large chloroplasts without grana stack. Accordingly them photosystem II (PS II), which is normally localized in the grana membranes of the thylakoids is missing. This is missing the opportunity to linear electron transport of the light reaction of photosynthesis, which normally generates the NADPH required in the Calvin cycle.

After the release of CO2 are transported back to the mesophyll cell phosphoenolpyruvate ( PEP ) and pyruvate by plasmodesmata. In plants with the NAD - malic enzyme pyruvate type is previously converted to L-alanine by transamination and this is then transported. Pyruvate itself is implemented by a pyruvate phosphate dikinase in the chloroplasts of mesophyll cells with ATP to PEP and can thus condense with bicarbonate again.

C4 photosynthesis without Kranz anatomy

It terrestrial plants were discovered, namely operate a C4 - Phothosynthese but have no wreath anatomy like most C4 plants. Three species belonging to the goosefoot family, Bienertia sinuspersici, Bienertia cycloptera and Suaeda aralocaspica, perform this pathway within a cell.

These plants grow in desert areas, as sinuspersici in countries around the Persian Gulf, as cycloptera in the area of ​​Turkey, Afghanistan, Iran and S. aralocaspica, a salt plant, in Central Asia.

The C4 photosynthesis with Kranz anatomy achieved by a spatial allocation of CO2 Vorfinsierung and actual fixation in the Calvin cycle as in C4 plants, this being done here intracellularly. The plants mentioned above there is a C4 photosynthetic NAD -ME ​​type.

In S. aralocaspica boasts long palisade, which serves the outward-facing ( distal ) region for CO2 Vorfixierng and wherein the location inside ( proximal ) area for fixation in the Calvin cycle. The two Bienertia species show a different structure. There is a thin cytosolic compartment at the edge and an unusual central compartment with many chloroplasts in the middle. Here again there is the wreath Anatomy analog spatial division between pre-fixation by PEP and fixation by RuBisCO.

When occurring within a cell C4 photosynthesis, there are two different types of chloroplasts ( " dimorphogene chloroplasts "), which differ in their biochemistry, saving strength and in the ultrastructure. Thus, the grana are poorer performance in the outward facing chloroplasts, they hardly save strength and the enzyme PPDK is there for the construction of PEP active. The other, lying "inside " chloroplast RuBisCO have well-developed grana and save strength. These differences are analogous to those as for the individual chloroplasts in mesophyll and bundle sheath cells. The cytoskeleton ensuring the separation of these different functional chloroplasts within the cell.

With the "inner " chloroplasts mitochondria and peroxisomes are associated. This is placed in the decarboxylation of malate the released carbon dioxide in the immediate vicinity of RuBisCO. In addition, the likelihood that the same is reattached during the photorespiratory pathway liberated carbon dioxide.

Probably also lead the marine macroalgae Udotea flabellum and the unicellular diatom Thalassiosira weissflogii a C4 photosynthesis within the same cell.

Optional C4 photosynthesis

In the basic nettle, a freshwater plant, a C4 photosynthesis has been identified, although it has no rim anatomy. If the CO2 content of the water is high, there will be a C3 - photosynthesis. The basic Essel switches, declining CO2 concentrations to a C4 metabolism so that one speaks of a facultative C4 plant.

Although HCO3- in the aquatic environment in higher concentration than CO2, absorbs the carbon dioxide plant. This is because their adaxial walls have a low pH value, bringing the balance of HCO3- and CO2 in favor of CO2 is moved. CO2 is pre-fixed and placed in a C4 metabolism of the NADP -ME type in the chloroplasts and enriched there.

At high temperatures, the CO2 concentrating C4 metabolism of basic Essel advantages over other C3 freshwater plants. During the day, the O2 content of the water increases while the CO2 content decreases. In addition, CO2 can not diffuse rapidly in water and in the air, the losses increase by photorespiration. The basic nettle still able to operate an efficient photosynthesis.

Economic and ecological aspects

• C4 plants can be used for the production of biomass for energy. Miscanthus yields reached 15 to 25 tonnes of dry matter per hectare per year.
• Although belonging to the grasses, rice belongs to the C3 plants. Attempts by the introduction of various genes from C4 plants to increase the rate of photosynthesis were until now not very promising.

A problem of the growing world population ( overpopulation ) is the shortage of food supplies, especially since less land will be available for agricultural use. One way to increase yields would be a C4 photosynthesis in C3 plants. Especially in warmer regions of the world, this is an advantage because there are C3 plants to C4 plants inferior.

In principle, two ways are possible to transform a C3 plant into a C4 plant:

• Either going from a Einzellenmodell from, wherein the C4 photosynthetic not as usual takes place in two different cells, but within the same cell. Even in nature, there are plants that will bring about the spatial separation between CO2 and pre-fixing Calvin cycle in the same cell ( see section below). For the conversion you would have to perform several steps to bring C4 - specific characteristics in the cell. So shall inter alia the Carboxylierungsenzym PEPC are present in the cytosol, the CA activity of chloroplasts moved to the cytosol, introduced efficient transporter in the chloroplast and will achieve a spatial separation of PEPC and RubisCO.

• The other model is a two cell model, in which an attempt is made to achieve anatomical specialization as in most plants with C4 - ring anatomy. For this purpose, but much more genetic interventions would be required than for Einzellenmodell, especially as a new, more specialized cell type would have to be formed.

Water demand

C4 plants are superior to most C3 plants in that they can be used more economically their Kohlenstoffdioxidanreichung water ( WUE, water use efficiency, dt: water use efficiency ): The optimum growth temperature is between 30 and 40 ° C, for C3 plants in contrast 20-30 ° C.

With increasing temperature, oxygen dissolves better in comparison to CO2, so it comes in C3 plants to greater losses by photorespiration due to the oxygenase activity of RubisCO, which reduces in C4 plants until it can be completely suppressed.

While C4 plants need to form 1 g dry mass 230-250 ml of water, the need for C3 plants is two to three times as high.

Nitrogen requirements

The nitrogen requirements for C4 plants is lower because they require less RubisCO. This can work more efficiently namely due to the higher CO2 saturation, loss by photorespiration is minimal. It has been calculated that at 30 ° C, a C4 - sheet requires approximately 13-20 % of the amount of RubisCO a C3 - sheet to achieve the same rate of photosynthesis ( in saturated intensity ). It has to be noted, however, that typical C4 enzymes - such as PPDK and PEPC - attract a higher nitrogen requirement by itself.

Overall, it is estimated that the so-called nitrogen use efficiency ( NUE nitrogen, use efficiency ) in C4 plants is at least twice as high as in C3 plants.

Increasing attention also gain tropical C4 forage grasses that are associated with nitrogen-fixing bacteria and thus hardly require additional fertilization.

Energy demand

The energy requirements of a C4 plant ( NADP -ME and NAD -ME ​​- type type) is 5 ATP and 2 NADPH per CO2 molecule fixed and is thus higher than that of a C3 plant. C3 plants need 3 ATP and 2 NADPH per CO2 molecule fixed, these values ​​allow the photorespiratory losses into account.

C4 plants of the PEPCK - type need 3.6 ATP and 2.3 NADPH per CO2 molecule fixed.

Isotope discrimination

C4 plants can be identified by the ratio of two carbon isotopes 12C and 13C. The two isotopes are used in the atmosphere with 98.89 % and 1.11 % before ( the radioactive isotope 14C is irrelevant in this context). The enzyme RubisCO reacts with 12CO2 and 13CO2 discriminated against, so the 13C/12C ratio is higher than in C3 plants with C4 plants. It is expressed as δ 13C value:

As standard a particular limestone is defined ( Pee Dee Belemnite ). Products of C3 photosynthesis have δ 13C values ​​of around -28 ‰.

The PEP carboxylase prefers 12CO2 less than RubisCO, but almost all the CO2 is pre-fixed by PEP carboxylase in C4 plants. Due to the high internal CO2 concentration in the bundle sheath cells and the discrimination of RubisCO does not come to fruition. The result for C4 plants a δ 13C value of -14 ‰ on average. By determining the δ 13C value by mass spectrometry can therefore distinguish whether sugar from sugar beet (C3 ) or sugar cane (C4 ) originates.