Editing Photorespiration

{{Short description|Process in plant metabolism}}
{{too detailed|lead|reason= It starts (and stays) too complex for the everyman encyclopedia user (without them ever giving them a chance to grasp the concept). Someone who understands the subject well needs to craft a plain English introductory sentence or two to explain the basic concepts in basic terms before it goes full scientific. After that, every man is on their own. But it needs to establish the topic in plain language before going on (as this is not an extremely rarefied or abstruse scientific matter for scientists only to approach and understand, though its chemistry is complex).|date=December 2023}}
[[File:Photorespiration allgemein.svg|class=skin-invert-image|thumb|upright=1.5| {{center|Simplified photorespiration cycle}}]]
[[File:Simplified photorespiration diagram.svg|class=skin-invert-image|thumb|upright=1.5|{{center|Simplified photorespiration and Calvin cycle}}]]

'''Photorespiration''' (also known as the '''oxidative photosynthetic carbon cycle''' or '''C<sub>2</sub> cycle''') refers to a process in [[plant physiology|plant metabolism]] where the [[enzyme]] [[RuBisCO]] oxygenates [[RuBP]], wasting some of the energy produced by photosynthesis. The desired reaction is the addition of [[carbon dioxide]] to RuBP ([[carboxylation]]), a key step in the [[Calvin–Benson cycle]], but approximately 25% of reactions by RuBisCO instead add [[oxygen]] to RuBP ([[Oxygenase|oxygenation]]), creating a product that cannot be used within the Calvin–Benson cycle. This process lowers the efficiency of photosynthesis, potentially lowering photosynthetic output by 25% in [[C3 carbon fixation|{{C3}} plants]].<ref name="Sharkey">{{cite journal | vauthors = Sharkey T |title=Estimating the rate of photorespiration in leaves |journal=Physiologia Plantarum |volume=73 |issue=1 |pages=147–152 |year=1988 |doi=10.1111/j.1399-3054.1988.tb09205.x|bibcode=1988PPlan..73..147S }}</ref> Photorespiration involves a complex network of enzyme reactions that exchange metabolites between [[chloroplast]]s, leaf [[peroxisome]]s and [[mitochondria]].

The oxygenation reaction of RuBisCO is a wasteful process because [[3-Phosphoglyceric acid|3-phosphoglycerate]]  is created at a lower rate and higher metabolic cost compared with [[Calvin cycle|RuBP carboxylase activity]]. While photorespiratory carbon cycling results in the formation of [[Glyceraldehyde 3-phosphate|G3P]] eventually, around 25% of carbon fixed by photorespiration is re-released as {{CO2}}<ref>{{cite journal | vauthors = Leegood RC | title = A welcome diversion from photorespiration | journal = Nature Biotechnology | volume = 25 | issue = 5 | pages = 539–40 | date = May 2007 | pmid = 17483837 | doi = 10.1038/nbt0507-539 | s2cid = 5015366 }}</ref> and nitrogen, as [[ammonia]]. [[Ammonia]] must then be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one [[Adenosine triphosphate|ATP]] and one [[NAD(P)H]].

While it is common to refer to the entire process as photorespiration, technically the term refers only to the metabolic network which acts to rescue the products of the oxygenation reaction (phosphoglycolate).

==Photorespiratory reactions==
[[File:Photorespiration.svg|class=skin-invert-image|thumb|350px|PhotorespirationFrom left to right: [[chloroplast]], [[peroxisome]], and [[mitochondrion]]]]
Addition of molecular oxygen to ribulose-1,5-bisphosphate produces [[3-phosphoglycerate]] (PGA) and [[2-phosphoglycolate]] (2PG, or PG). PGA is the normal product of carboxylation, and productively enters the [[Calvin cycle]]. Phosphoglycolate, however, inhibits certain enzymes involved in photosynthetic carbon fixation (hence is often said to be an 'inhibitor of photosynthesis').<ref>{{cite journal | vauthors = Peterhansel C, Krause K, Braun HP, Espie GS, Fernie AR, Hanson DT, Keech O, Maurino VG, Mielewczik M, Sage RF | title = Engineering photorespiration: current state and future possibilities | journal = Plant Biology | volume = 15 | issue = 4 | pages = 754–8 | date = July 2013 | pmid =  23121076| doi = 10.1111/j.1438-8677.2012.00681.x | bibcode = 2013PlBio..15..754P }}</ref> It is also relatively difficult to recycle: in higher plants it is salvaged by a series of reactions in the [[peroxisome]], [[mitochondria]], and again in the [[peroxisome]] where it is converted into [[glycerate]]. Glycerate reenters the [[chloroplast]] and by the same transporter that exports [[Glycolic acid|glycolate]]. A cost of 1 [[Adenosine triphosphate|ATP]] is associated with conversion to 3-phosphoglycerate (PGA) ([[Phosphorylation]]), within the [[chloroplast]], which is then free to re-enter the Calvin cycle.

Several costs are associated with this metabolic pathway; the production of [[hydrogen peroxide]] in the peroxisome (associated with the conversion of glycolate to glyoxylate). Hydrogen peroxide is a dangerously strong oxidant which must be immediately split into water and oxygen by the enzyme [[catalase]]. The conversion of 2× 2Carbon [[glycine]] to 1× {{C3}} [[serine]] in the mitochondria by the enzyme glycine-decarboxylase is a key step, which releases {{CO2}}, NH<sub>3</sub>, and reduces NAD to NADH. Thus, one {{chem|CO|2}} molecule is produced for every two molecules of {{chem|O|2}} (two deriving from RuBisCO and one from peroxisomal oxidations). The assimilation of NH<sub>3</sub> occurs via the [[Glutamine synthetase|GS]]-[[Glutamine oxoglutarate aminotransferase|GOGAT]] cycle, at a cost of one ATP and one NADPH.

[[Cyanobacteria]] have three possible pathways through which they can metabolise 2-phosphoglycolate. They are unable to grow if all three pathways are knocked out, despite having a carbon concentrating mechanism that should dramatically lower the rate of photorespiration [[#Biological adaptation to minimize photorespiration|(see below)]].<ref>{{cite journal | vauthors = Eisenhut M, Ruth W, Haimovich M, Bauwe H, Kaplan A, Hagemann M | title = The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plants | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 44 | pages = 17199–204 | date = November 2008 | pmid = 18957552 | pmc = 2579401 | doi = 10.1073/pnas.0807043105 | bibcode = 2008PNAS..10517199E | doi-access = free }}</ref>

==Substrate specificity of RuBisCO==
[[File:RuBisCO reaction O2.svg|class=skin-invert-image|thumb|350px|Oxygenase activity of RuBisCO]]

The oxidative photosynthetic carbon cycle reaction is [[catalyst|catalyzed]] by [[RuBisCO|RuBP oxygenase]] activity:

:[[Ribulose 1,5-bisphosphate|RuBP]] + {{chem|O|2}} → Phosphoglycolate + [[Glycerate 3-phosphate|3-phosphoglycerate]] + 2 {{chem|H|+}}

During the catalysis by RuBisCO, an 'activated' intermediate is formed (an enediol intermediate) in the RuBisCO active site. This intermediate is able to react with either {{chem|CO|2}} or {{chem|O|2}}. It has been demonstrated that the specific shape of the RuBisCO active site acts to encourage reactions with {{chem|CO|2}}. Although there is a significant "failure" rate (~25% of reactions are oxygenation rather than carboxylation), this represents significant favouring of {{chem|CO|2}}, when the relative abundance of the two gases is taken into account: in the current atmosphere, {{chem|O|2}} is approximately 500 times more abundant, and in solution {{chem|O|2}} is 25 times more abundant than {{chem|CO|2}}.<ref name="Griffiths_2006">{{cite journal | vauthors = Griffiths H | title = Plant biology: designs on Rubisco | journal = Nature | volume = 441 | issue = 7096 | pages = 940–1 | date = June 2006 | pmid = 16791182 | doi = 10.1038/441940a | bibcode = 2006Natur.441..940G | s2cid = 31190084 }}</ref>

The ability of RuBisCO to specify between the two gases is known as its selectivity factor (or Srel), and it varies between species,<ref name="Griffiths_2006"/> with angiosperms more efficient than other plants, but with little variation among the [[vascular plant]]s.<ref name="Ehleringer 1991">{{cite journal | vauthors = Ehleringer JR, Sage RF, Flanagan LB, Pearcy RW | title = Climate change and the evolution of C(4) photosynthesis | journal = Trends in Ecology & Evolution | volume = 6 | issue = 3 | pages = 95–9 | date = March 1991 | pmid = 21232434 | doi = 10.1016/0169-5347(91)90183-x }}</ref>

A suggested explanation of RuBisCO's inability to discriminate completely between {{chem|CO|2}} and {{chem|O|2}} is that it is an evolutionary relic:{{citation needed|date=October 2014}} The early atmosphere in which primitive plants originated contained very little oxygen, the early evolution of [[RuBisCO]] was not influenced by its ability to discriminate between {{chem|O|2}} and {{chem|CO|2}}.<ref name="Ehleringer 1991"/>

==Conditions which affect photorespiration==
{{carbon cycle|respiration}}

Photorespiration rates are affected by:

===Altered substrate availability: lowered {{CO2}} or increased O<sub>2</sub>===
Factors which influence this include the atmospheric abundance of the two gases, the supply of the gases to the site of fixation (i.e. in land plants: whether the [[stomata]] are open or closed), the length of the liquid phase (how far these gases have to diffuse through water in order to reach the reaction site). For example, when the stomata are closed to prevent water loss during [[drought]]: this limits the {{CO2}} supply, while {{chem|O|2}} production within the leaf will continue. In algae (and plants which photosynthesise underwater); gases have to diffuse significant distances through water, which results in a decrease in the availability of {{CO2}} relative to {{chem|O|2}}. It has been predicted that the increase in [[wikt:ambient|ambient]] {{CO2}} concentrations predicted over the next 100 years may lower the rate of photorespiration in [[C3 carbon fixation|most plants]] by around 50%{{citation needed|date=March 2017}}. However, at temperatures higher than the photosynthetic thermal optimum, the increases in turnover rate are not translated into increased {{CO2}} assimilation because of the decreased affinity of Rubisco for {{CO2}}.<ref>{{cite journal |last1=Hermida-Carrera |first1=Carmen |last2=Kapralov |first2=Maxim V |last3=Galmés |first3=Jeroni |title=Rubisco catalytic properties and temperature response in crops |journal=Plant Physiology |volume=171 |issue=4 |date=21 June 2016 |pages=2549–61 |doi=10.1104/pp.16.01846 |pmid=27329223 |pmc=4972260 }}</ref>

===Increased temperature===
At higher temperatures RuBisCO is less able to discriminate between {{CO2}} and {{chem|O|2}}. This is because the enediol intermediate is less stable. Increasing temperatures also lower the solubility of {{CO2}}, thus lowering the concentration of {{CO2}} relative to {{chem|O|2}} in the [[chloroplast]].

==Biological adaptation to minimize photorespiration==
[[File:Field, corn, Liechtenstein, Mountains, Alps, Vaduz, sky, clouds, landscape.jpg|thumb|upright=1.2| {{center|Maize uses the C<sub>4</sub> pathway, minimizing photorespiration}}]]

The vast majority of plants are C3, meaning they photorespire when necessary. Certain species of plants or [[algae]] have mechanisms to lower the uptake of molecular oxygen by RuBisCO. These are commonly referred to as [[Photosynthesis#Carbon concentrating mechanisms|Carbon Concentrating Mechanisms]] (CCMs), as they increase the concentration of {{CO2|link=yes}} so that RuBisCO is less likely to produce glycolate through reaction with {{chem|O|2}}.

===Biochemical carbon concentrating mechanisms===
Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region, through [[metabolite]] exchange. C<sub>4</sub> and CAM photosynthesis both use the enzyme [[Phosphoenolpyruvate carboxylase]] (PEPC) to add {{chem|CO|2}} to a 4-carbon sugar. PEPC is faster than RuBisCO, and more selective for {{chem|CO|2}}.

====C<sub>4</sub>====
[[C4 carbon fixation|C<sub>4</sub>]] plants capture carbon dioxide in their mesophyll cells (using an enzyme called [[phosphoenolpyruvate carboxylase]] which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where [[oxygen]] concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the [[Calvin cycle]] proceeds as normal. The {{chem|CO|2}} concentrations in the Bundle Sheath are approximately 10–20 fold higher than the concentration in the mesophyll cells.<ref name="Ehleringer 1991" />

This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments, wherein stomata are closed and internal carbon dioxide levels are low. Under these conditions, photorespiration does occur in C<sub>4</sub> plants, but at a much lower level compared with C<sub>3</sub> plants in the same conditions. C<sub>4</sub> plants include [[sugar cane]], [[maize|corn (maize)]], and [[sorghum]].

====CAM (Crassulacean acid metabolism)====
[[File:CAMplantgraph.jpg|class=skin-invert-image|thumb|300px|left|Overnight graph of {{CO2}} absorbed by a CAM plant]]
CAM plants, such as [[cacti]] and [[succulent plant]]s, also use the enzyme PEP carboxylase to capture carbon dioxide, but only at night. [[Crassulacean acid metabolism]] allows plants to conduct most of their gas exchange in the cooler night-time air, [[Carbon sequestration|sequestering carbon]] in 4-carbon sugars which can be released to the photosynthesizing cells during the day. This allows CAM plants to minimize water loss ([[transpiration]]) by maintaining closed stomata during the day. CAM plants usually display other water-saving characteristics, such as thick cuticles, stomata with small apertures, and typically lose around 1/3 of the amount of water per {{chem|CO|2}} fixed.<ref>{{cite book | title = Plant Physiology | edition = Fifth | publisher = Sinauer Associates, Inc. | vauthors = Taiz L, Zeiger E | date = 2010 | chapter = Chapter 8: Photosynthesis: The Carbon Reactions: Inorganic Carbon–Concentrating Mechanisms: Crassulacean Acid Metabolism (CAM) | page = 222 }}</ref>


====C<sub>2</sub>====
[[File:C2 Photosynthesis.svg|thumb|right|In C<sub>2</sub> plants, the mitochondria of mesophyll cells have no glycine decarboxylase (GDC).]]
'''C<sub>2</sub> photosynthesis''' (also called '''glycine shuttle''' and '''photorespiratory CO<sub>2</sub> pump''') is a CCM that works by making use of &ndash; as opposed to avoiding &ndash; photorespiration. It performs ''carbon refixation'' by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the [[mesophyll]] into the [[bundle sheath]]. Once there, the glycine is decarboxylated in [[mitochondria]] as usual, releasing CO<sub>2</sub> and concentrating it to triple the usual concentration.<ref name=Cee2>{{cite journal |last1=Lundgren |first1=Marjorie R. |title=C 2 photosynthesis: a promising route towards crop improvement? |journal=New Phytologist |date=December 2020 |volume=228 |issue=6 |pages=1734–1740 |doi=10.1111/nph.16494 |pmid=32080851 |doi-access=free}}</ref>

Although C<sub>2</sub> photosynthesis is traditionally understood as an intermediate step between C<sub>3</sub> and C<sub>4</sub>, a wide variety of plant lineages do end up in the C<sub>2</sub> stage without further evolving, showing that it is an evolutionary steady state of its own. C<sub>2</sub> may be easier to engineer into crops, as the phenotype requires fewer anatomical changes to produce.<ref name=Cee2/>

====Algae====
There have been some reports of algae operating a biochemical CCM: shuttling metabolites within single cells to concentrate {{CO2}} in one area. This process is not fully understood.<ref>{{cite journal | vauthors = Giordano M, Beardall J, Raven JA | title = {{CO2}} concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution | journal = Annual Review of Plant Biology | volume = 56 | issue = 1 | pages = 99–131 | date = June 2005 | pmid = 15862091 | doi = 10.1146/annurev.arplant.56.032604.144052 | bibcode = 2005AnRPB..56...99G }}</ref>

===Biophysical carbon-concentrating mechanisms===
This type of carbon-concentrating mechanism (CCM) relies on a contained compartment within the cell into which {{CO2}} is shuttled, and where RuBisCO is highly expressed. In many species, biophysical CCMs are only induced under low carbon dioxide concentrations. Biophysical CCMs are more evolutionary ancient than biochemical CCMs. There is some debate as to when biophysical CCMs first evolved, but it is likely to have been during a period of low carbon dioxide, after the [[Great Oxygenation Event]] (2.4 billion years ago). Low {{chem|CO|2}} periods occurred around 750, 650, and 320–270 million years ago.<ref>{{cite journal | vauthors = Raven JA, Giordano M, Beardall J, Maberly SC | title = Algal evolution in relation to atmospheric {{CO2}}: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 367 | issue = 1588 | pages = 493–507 | date = February 2012 | pmid = 22232762 | pmc = 3248706 | doi = 10.1098/rstb.2011.0212 }}</ref>

====Eukaryotic algae====
In nearly all species of [[Algae|eukaryotic algae]] (''[[Chloromonas]]'' being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the [[pyrenoid]]. Carbon dioxide is concentrated in this compartment using a combination of CO<sub>2</sub> pumps, [[Carbonic acid|bicarbonate]] pumps, and [[carbonic anhydrase]]s. The pyrenoid is not a membrane-bound compartment but is found within the chloroplast, often surrounded by a starch sheath (which is not thought to serve a function in the CCM).<ref>{{Cite journal | vauthors = Villarejo A, Martinez F, Pino Plumed M, Ramazanov Z | title = The induction of the CO<sub>2</sub> concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii | doi = 10.1111/j.1399-3054.1996.tb06687.x | journal = Physiologia Plantarum | volume = 98 | issue = 4 | pages = 798–802 | year = 1996 | bibcode = 1996PPlan..98..798V }}</ref>

====Hornworts====
Certain species of [[hornwort]] are the only land plants that are known to have a biophysical CCM involving concentration of carbon dioxide within [[pyrenoid]]s in their chloroplasts.<ref>{{cite journal | vauthors=((Robison, T. A.)), ((Oh, Z. G.)), ((Lafferty, D.)), ((Xu, X.)), ((Villarreal, J. C. A.)), ((Gunn, L. H.)), ((Li, F.-W.)) | journal=Nature Plants | title=Hornworts reveal a spatial model for pyrenoid-based CO2-concentrating mechanisms in land plants | pages=63–73 | publisher=Nature Publishing Group | date=3 January 2025 | volume=11 | issue=1 | issn=2055-0278 | doi=10.1038/s41477-024-01871-0| pmid=39753956 }}
</ref>

====Cyanobacteria====
[[Cyanobacteria]]l CCMs are similar in principle to those found in eukaryotic algae and hornworts, but the compartment into which carbon dioxide is concentrated has several structural differences. Instead of the pyrenoid, cyanobacteria contain [[carboxysome]]s, which have a protein shell, and linker proteins packing RuBisCO inside with a very regular structure.
Cyanobacterial CCMs are much better understood than those found in [[eukaryote]]s, partly due to the ease of genetic manipulation of [[prokaryote]]s.

==Possible purpose of photorespiration==
Lowering photorespiration may not result in increased growth rates for plants. Photorespiration may be necessary for the assimilation of nitrate from soil. Thus, a lowering in photorespiration by genetic engineering or because of increasing atmospheric carbon dioxide may not benefit plants as has been proposed.<ref>{{cite journal | vauthors = Rachmilevitch S, Cousins AB, Bloom AJ | title = Nitrate assimilation in plant shoots depends on photorespiration | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 31 | pages = 11506–10 | date = August 2004 | pmid = 15272076 | pmc = 509230 | doi = 10.1073/pnas.0404388101 | bibcode = 2004PNAS..10111506R | doi-access = free }}</ref> Several physiological processes may be responsible for linking photorespiration and nitrogen assimilation. Photorespiration increases availability of NADH, which is required for the conversion of [[nitrate]] to [[nitrite]]. Certain nitrite transporters also transport [[bicarbonate]], and elevated {{CO2}} has been shown to suppress nitrite transport into chloroplasts.<ref>{{cite journal | vauthors = Bloom AJ, Burger M, Rubio Asensio JS, Cousins AB | title = Carbon dioxide enrichment inhibits nitrate assimilation in wheat and Arabidopsis | journal = Science | volume = 328 | issue = 5980 | pages = 899–903 | date = May 2010 | pmid = 20466933 | doi = 10.1126/science.1186440 | bibcode = 2010Sci...328..899B | s2cid = 206525174 }}</ref> However, in an agricultural setting, replacing the native photorespiration pathway with [[C3 photosynthesis#Synthetic glycolate pathway|an engineered synthetic pathway]] to metabolize glycolate in the chloroplast resulted in a 40 percent increase in crop growth.<ref>{{cite journal | vauthors = South PF, Cavanagh AP, Liu HW, Ort DR | title = Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field | journal = Science | volume = 363 | issue = 6422 | pages = eaat9077 | date = January 2019 | pmid = 30606819 | doi = 10.1126/science.aat9077 | pmc = 7745124 | doi-access = free }}</ref><ref name="John_Timmer_2017">{{cite web|url=https://arstechnica.com/science/2017/12/key-plant-proteins-that-grab-co%e2%82%82-finally-made-in-bacteria/|last=Timmer|first= John | name-list-style = vanc |title=We may now be able to engineer the most important lousy enzyme on the planet|date=7 December 2017|work=Ars Technica|access-date=5 January 2019}}</ref><ref name="John_Timmer_2019">{{cite web|url=https://arstechnica.com/science/2019/01/re-engineering-photosynthesis-gives-plants-a-40-growth-boost/|last=Timmer|first=John | name-list-style = vanc |title=Fixing photosynthesis by engineering it to recycle a toxic mistake|date=3 January 2019|work=Ars Technica|access-date=5 January 2019}}</ref>

Although photorespiration is much lower in C<sub>4</sub> species, it is still an essential pathway{{snd}}mutants without functioning 2-phosphoglycolate metabolism cannot grow in normal conditions. One mutant was shown to rapidly accumulate glycolate.<ref>{{cite journal | vauthors = Zabaleta E, Martin MV, Braun HP | title = A basal carbon concentrating mechanism in plants? | journal = Plant Science | volume = 187 | pages = 97–104 | date = May 2012 | pmid = 22404837 | doi = 10.1016/j.plantsci.2012.02.001 | bibcode = 2012PlnSc.187...97Z | url = https://www.repo.uni-hannover.de/handle/123456789/11750 | hdl = 11336/13266 | s2cid = 25371100 | hdl-access = free }}</ref>

Although the functions of photorespiration remain controversial,<ref name="Foyer">{{cite journal | vauthors = Foyer CH, Bloom AJ, Queval G, Noctor G | title = Photorespiratory metabolism: genes, mutants, energetics, and redox signaling | journal = Annual Review of Plant Biology | volume = 60 | issue = 1 | pages = 455–84 | year = 2009 | pmid = 19575589 | doi = 10.1146/annurev.arplant.043008.091948 | bibcode = 2009AnRPB..60..455F }}</ref> it is widely accepted that this pathway influences a wide range of processes from bioenergetics, photosystem II function, and carbon metabolism to nitrogen assimilation and respiration. The oxygenase reaction of RuBisCO may prevent {{CO2}} depletion near its active sites<ref>{{cite journal | vauthors = Igamberdiev AU | title = Control of Rubisco function via homeostatic equilibration of {{CO2}} supply | journal = Frontiers in Plant Science | volume = 6 | pages = 106 | date = 2015 | pmid = 25767475 | pmc = 4341507 | doi = 10.3389/fpls.2015.00106 | doi-access = free }}</ref> and contributes to the regulation of CO<sub>2.</sub> concentration in the atmosphere<ref>{{cite journal | vauthors = Igamberdiev AU, Lea PJ | title = Land plants equilibrate O<sub>2</sub> and CO<sub>2</sub> concentrations in the atmosphere | journal = Photosynthesis Research | volume = 87 | issue = 2 | pages = 177–94 | date = February 2006 | pmid = 16432665 | doi = 10.1007/s11120-005-8388-2 | bibcode = 2006PhoRe..87..177I | s2cid = 10709679 }}</ref> The photorespiratory pathway is a major source of [[hydrogen peroxide]] ({{chem|H|2|O|2}}) in photosynthetic cells. Through {{chem|H|2|O|2}} production and pyrimidine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signalling pathways, in particular, those that govern plant hormonal responses controlling growth, environmental and defense responses, and programmed cell death.<ref name="Foyer"/>

It has been postulated that photorespiration may function as a "safety valve",<ref>{{cite journal | vauthors = Stuhlfauth T, Scheuermann R, Fock HP | title = Light Energy Dissipation under Water Stress Conditions: Contribution of Reassimilation and Evidence for Additional Processes | journal = Plant Physiology | volume = 92 | issue = 4 | pages = 1053–61 | date = April 1990 | pmid = 16667370 | pmc = 1062415 | doi = 10.1104/pp.92.4.1053 }}</ref> preventing the excess of reductive potential coming from an overreduced [[NADPH]]-pool from reacting with oxygen and producing [[free radicals]] (oxidants), as these can damage the metabolic functions of the cell by subsequent oxidation of membrane lipids, proteins or nucleotides. The mutants deficient in photorespiratory enzymes are characterized by a high redox level in the cell,<ref>{{cite journal | vauthors = Igamberdiev AU, Bykova NV, Lea PJ, Gardeström P | title = The role of photorespiration in redox and energy balance of photosynthetic plant cells: A study with a barley mutant deficient in glycine decarboxylase | journal = Physiologia Plantarum | volume = 111 | issue = 4 | pages = 427–438 | date = April 2001 | pmid = 11299007 | doi = 10.1034/j.1399-3054.2001.1110402.x | bibcode = 2001PPlan.111..427I }}</ref> impaired stomatal regulation,<ref>{{cite journal| vauthors = Igamberdiev AU, Mikkelsen TN, Ambus P, Bauwe H, Lea PJ, Gardeström P |date=2004|title=Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation: A Study with Barley, Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase |journal=Photosynthesis Research |volume=81|issue=2|pages=139–152|doi=10.1023/B:PRES.0000035026.05237.ec |bibcode=2004PhoRe..81..139I |s2cid=9485316}}</ref> and accumulation of [[formate]].<ref>{{Cite journal| vauthors = Wingler A, Lea PJ, Leegood RC |date=1999|title=Photorespiratory metabolism of glyoxylate and formate in glycine-accumulating mutants of barley and Amaranthus edulis 2|journal=Planta|volume=207|issue=4|pages=518–526|doi=10.1007/s004250050512 |bibcode=1999Plant.207..518W |s2cid=34817815}}</ref>

== See also ==
* [[C3 photosynthesis|{{C3}} photosynthesis]]
* [[C4 photosynthesis|{{C4}} photosynthesis]]
* [[Crassulacean acid metabolism|CAM photosynthesis]]

{{Clear}}

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{cite book | last = Stern | first = Kingsley | name-list-style = vanc | title = Introductory Plant Biology | publisher = McGraw-Hill | location = New York | year = 2003 | isbn = 978-0-07-290941-8 }}
* {{cite book | last1 = Siedow | first1 = James N. | last2 = Day | first2 = David | name-list-style = vanc |chapter= Chapter 14: Respiration and Photorespiration |title=Biochemistry and Molecular Biology of Plants |publisher=American Society of Plant Physiologists |year=2000 }}
{{refend}}

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[[Category:Plant physiology]]
[[Category:Photosynthesis]]
[[Category:Metabolism]]