Alarm photosynthesis is a variation of

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

where

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...

crystals A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

function as dynamic carbon pools, supplying carbon dioxide ( CO₂) to photosynthetic cells when

stomata In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...

are partially or totally closed. This biochemical appendance of the photosynthetic machinery is a means to alleviate the perpetual plant dilemma of using atmospheric CO₂ for photosynthesis and losing water vapor, or saving water and reducing photosynthesis. The function of alarm photosynthesis seems to be rather auxiliary to the overall photosynthetic performance. It supports a low photosynthetic rate, aiming at the maintenance and

photoprotection Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stre ...

of the photosynthetic apparatus rather than a substantial carbon gain. Scheme_of_the_biochemical_reactions_involved_in_alarm_photosynthesis

History

The alarm photosynthesis process was first evidenced in pigweed ''

Amaranthus hybridus ''Amaranthus hybridus'', commonly called green amaranth, slim amaranth, smooth amaranth, smooth pigweed, or red amaranth, is a species of annual flowering plant. It is a weedy species found now over much of North America and introduced into Europ ...

'' plant in 2016 when ''A. hybridus'' leaves were exposed to

drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...

conditions or exogenous application of

abscisic acid Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental s ...

. The same study showed similar results in '' Dianthus chinensis'', ''

Pelargonium peltatum ''Pelargonium peltatum'' is a scrambling perennial plant with five shallow or deeply lobed, circular- to heart-shaped, somewhat fleshy leaves, sometimes with a differently coloured semicircular band, that has been assigned to the Geraniaceae, cra ...

'', and ''

Portulacaria afra ''Portulacaria afra'' (known as elephant bush, porkbush, purslane tree and ' in Afrikaans) is a small-leaved succulent plant found in South Africa. These succulents commonly have a reddish stem and leaves that are green, but also a variegated cu ...

'' plants under drought stress. In 2018, the alarm photosynthesis process was exhibited in ''A. hybridus'' plants under controlled CO₂ starvation conditions. In 2020, evidence of this process was shown in the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...

plant ''

Colobanthus quitensis ''Colobanthus quitensis'', the Antarctic pearlwort, is one of two native flowering plants found in the Antarctic region. It has yellow flowers and grows about 5 cm (two inches) tall, with a cushion-like growth habit that gives it a moss-like appe ...

'' under CO₂ limiting conditions.

Mechanism

Under stress conditions (e.g. water deficit) oxalate released from calcium oxalate crystals is converted to CO₂ by an

oxalate oxidase In enzymology, an oxalate oxidase () is an oxalate degrading enzyme that catalyzes the chemical reaction: :oxalate + O2 + 2 H+ \rightleftharpoons 2 CO2 + H2O2 The 3 substrates of this enzyme are oxalate, O2, and H+, whereas its two products a ...

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

and the produced CO₂ can support the

Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...

reactions. Reactive hydrogen peroxide ( H₂O₂), the byproduct of oxalate oxidase reaction, can be neutralized by

catalase Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting t ...

Differences against CAM and C4

Alarm photosynthesis represents an unknown photosynthetic variation to be added to the already known C4 and

CAM Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bind ...

pathways. However, alarm photosynthesis, in contrast to these pathways, operates as a biochemical pump that collects carbon from the organ interior (or from the soil) and not from the atmosphere.

References

Photosynthesis {{plant-stub

History

Mechanism

Differences against CAM and C4

See also

References