Mehler reaction

The Mehler reaction is named after Alan H. Mehler, who, in 1951, presented data to the effect that isolated chloroplasts reduce oxygen to form hydrogen peroxide (H
2O
2).[1] Mehler observed that the H
2O
2 formed in this way does not present an active intermediate in photosynthesis; rather, as a reactive oxygen species, it can be toxic to surrounding biological processes as an oxidizing agent. In scientific literature, the Mehler reaction often is used interchangeably with the Water-Water Cycle[2] to refer to the formation of H
2O
2 by photosynthesis. Sensu stricto, the Water Water Cycle encompasses the Hill reaction, in which water is split to form oxygen, as well as the Mehler Reaction, in which oxygen is reduced to form H
2O
2 and, finally, the scavenging of this H
2O
2 by antioxidants to form water.

Beginning in the 1970s, Professor Kozi Asada elucidated that oxygen can be reduced by electrons emerging from ferredoxin of photosystem I, to form superoxide, which is then reduced by superoxide dismutase to form H
2O
2. This photochemical H
2O
2 is then reduced by the action of ascorbate peroxidase to form water and oxidized ascorbate. Asada argued that oxygen presents an important sink for excess excitation energy acquired during plant exposure to bright light. He would often begin seminars by asking: 'Why aren't plants sunburnt despite being exposed to light?'.[3]

How much of a photoprotective role the Water Water Cycle plays has been occasion for some debate. In terrestrial plants, transfer of electrons to oxygen from ferredoxin at PSI accounts for easily less than 10% of total photosynthetic electron transport.[4][5][6] In algae and other uni-cellular photosynthetic organisms, however, this amount can account for 20 to 30% of total electron transport. It is possible that the reduction of oxygen by free electrons emerging from PSI prevents components of the electron transport chain from becoming over-reduced.[7]

The Water Water Cycle is not related to photorespiration, as it comprises different reactions and results in no net oxygen consumption.

References
  1. Mehler, Alan (1951). "Studies on reactions of illuminated chloroplasts: I. Mechanism of the reduction of oxygen and other hill reagents". Archives of Biochemistry and Biophysics. 33 (1): 65–77. doi:10.1016/0003-9861(51)90082-3.
  2. Asada, Kozi (June 1999). "The Water-Cycle in Chloroplasts: Scavenging of Active Oxygens and Dissipation of Excess Photons". Annual Review of Plant Physiology and Plant Molecular Biology. 50: 601-639. doi:10.1146/annurev.arplant.50.1.601.
  3. Mano, Endo and Miyake (2016). "How do photosynthetic organisms manage light stress? A tribute to the late Professor Kozi Asada". Plant and Cell Physiology. 57 (7): 1351-1353. doi:10.1093/pcp/pcw116.
  4. Badger, M.; von Caemmerer, S.; Ruuska, S.; Nakano, H. (2000). "Electron flow to oxygen in higher plants and algae: rates and control of direct photoreduction (Mehler reaction) and rubisco oxygenase". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 355 (1402): 1433–1446. doi:10.1098/rstb.2000.0704. PMC 1692866. PMID 11127997.
  5. Ruuska, S.A.; Badger, M.R.; von Caemmerer, S. (2000). "Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction". Journal of Experimental Botany. 51: 357–368. doi:10.1093/jexbot/51.suppl_1.357.
  6. Driever, S.M.; Baker, N. (2011). "The water–water cycle in leaves is not a major alternative electron sink for dissipation of excess excitation energy when CO
    2     assimilation is restricted"    . Plant, Cell & Environment. 34 (5): 837–846. doi:10.1111/j.1365-3040.2011.02288.x.
  7. Heber, Ulrich (2002-01-01). "Irrungen, Wirrungen? The Mehler reaction in relation to cyclic electron transport in C3 plants". Photosynthesis Research. 73 (1–3): 223–231. doi:10.1023/A:1020459416987. ISSN 1573-5079. PMID 16245125.
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