Crassulacean acid metabolism


Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide, and allow it to diffuse into the mesophyll cells. The is stored as the four-carbon acid malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to, which is then used during photosynthesis. The pre-collected is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This mechanism of acid metabolism was first discovered in plants of the family Crassulaceae.

Historical background

Observations relating to CAM were first made by de Saussure in 1804 in his Recherches Chimiques sur la Végétation. Benjamin Heyne in 1812 noted that Bryophyllum leaves in India were acidic in the morning and tasteless by afternoon. These observations were studied further and refined by Aubert, E. in 1892 in his Recherches physiologiques sur les plantes grasses and expounded upon by Richards, H. M. 1915 in Acidity and Gas Interchange in Cacti, Carnegie Institution. The term CAM may have been coined by Ranson and Thomas in 1940, but they were not the first to discover this cycle. It was observed by the botanists Ranson and Thomas, in the succulent family Crassulaceae. Its name refers to acid metabolism in Crassulaceae, not the metabolism of "crassulacean acid", a nonexistent chemical entity.

Overview: a two-part cycle

CAM is an adaptation for increased efficiency in the use of water, and so is typically found in plants growing in arid conditions.

During the night

During the night, a plant employing CAM has its stomata open, allowing to enter and be fixed as organic acids by a PEP reaction similar to the pathway. The resulting organic acids are stored in vacuoles for later use, as the Calvin cycle cannot operate without ATP and NADPH, products of light-dependent reactions that do not take place at night.

During the day

During the day the stomata close to conserve water, and the -storing organic acids are released from the vacuoles of the mesophyll cells. An enzyme in the stroma of chloroplasts releases the, which enters into the Calvin cycle so that photosynthesis may take place.

Benefits

The most important benefit of CAM to the plant is the ability to leave most leaf stomata closed during the day. Plants employing CAM are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing such plants to grow in environments that would otherwise be far too dry. Plants using only carbon fixation, for example, lose 97% of the water they take up through the roots to transpiration - a high cost avoided by plants able to employ CAM.

Comparison with metabolism

The pathway bears resemblance to CAM; both act to concentrate around RuBisCO, thereby increasing its efficiency. CAM concentrates it temporally, providing during the day, and not at night, when respiration is the dominant reaction. plants, in contrast, concentrate spatially, with a RuBisCO reaction centre in a "bundle sheath cell" being inundated with. Due to the inactivity required by the CAM mechanism, carbon fixation has a greater efficiency in terms of PGA synthesis.

Biochemistry

Plants with CAM must control storage of and its reduction to branched carbohydrates in space and time.
At low temperatures, plants using CAM open their stomata, molecules diffuse into the spongy mesophyll's intracellular spaces and then into the cytoplasm. Here, they can meet phosphoenolpyruvate, which is a phosphorylated triose. During this time, the plants are synthesizing a protein called PEP carboxylase kinase, whose expression can be inhibited by high temperatures and the presence of malate. PEP-C kinase phosphorylates its target enzyme PEP carboxylase. Phosphorylation dramatically enhances the enzyme's capability to catalyze the formation of oxaloacetate, which can be subsequently transformed into malate by NAD+ malate dehydrogenase. Malate is then transported via malate shuttles into the vacuole, where it is converted into the storage form malic acid. In contrast to PEP-C kinase, PEP-C is synthesized all the time but almost inhibited at daylight either by dephosphorylation via PEP-C phosphatase or directly by binding malate. The latter is not possible at low temperatures, since malate is efficiently transported into the vacuole, whereas PEP-C kinase readily inverts dephosphorylation.
In daylight, plants using CAM close their guard cells and discharge malate that is subsequently transported into chloroplasts. There, depending on plant species, it is cleaved into pyruvate and either by malic enzyme or by PEP carboxykinase. is then introduced into the Calvin cycle, a coupled and self-recovering enzyme system, which is used to build branched carbohydrates. The by-product pyruvate can be further degraded in the mitochondrial citric acid cycle, thereby providing additional molecules for the Calvin Cycle. Pyruvate can also be used to recover PEP via pyruvate phosphate dikinase, a high-energy step, which requires ATP and an additional phosphate. During the following cool night, PEP is finally exported into the cytoplasm, where it is involved in fixing carbon dioxide via malate.

Use by plants

Plants use CAM to different degrees. Some are "obligate CAM plants", i.e. they use only CAM in photosynthesis, although they vary in the amount of they are able to store as organic acids; they are sometimes divided into "strong CAM" and "weak CAM" plants on this basis. Other plants show "inducible CAM", in which they are able to switch between using either the or mechanism and CAM depending on environmental conditions. Another group of plants employ "CAM-cycling", in which their stomata do not open at night; the plants instead recycle produced by respiration as well as storing some during the day.
Plants showing inducible CAM and CAM-cycling are typically found in conditions where periods of water shortage alternate with periods when water is freely available. Periodic drought – a feature of semi-arid regions – is one cause of water shortage. Plants which grow on trees or rocks also experience variations in water availability. Salinity, high light levels and nutrient availability are other factors which have been shown to induce CAM.
Since CAM is an adaptation to arid conditions, plants using CAM often display other xerophytic characters, such as thick, reduced leaves with a low surface-area-to-volume ratio; thick cuticle; and stomata sunken into pits. Some shed their leaves during the dry season; others store water in vacuoles. CAM also causes taste differences: plants may have an increasingly sour taste during the night yet become sweeter-tasting during the day. This is due to malic acid being stored in the vacuoles of the plants' cells during the night and then being used up during the day.

Aquatic CAM

CAM photosynthesis is also found in aquatic species in at least 4 genera, including: Isoetes, Crassula, Littorella, Sagittaria, and possibly Vallisneria, being found in a variety of species e.g. Isoetes howellii, Crassula aquatica.
These plants follow the same nocturnal acid accumulation and daytime deacidification as terrestrial CAM species. However, the reason for CAM in aquatic plants is not due to a lack of available water, but a limited supply of. is limited due to slow diffusion in water, 10000x slower than in air. The problem is especially acute under acid pH, where the only inorganic carbon species present is, with no available bicarbonate or carbonate supply.
Aquatic CAM plants capture carbon at night when it is abundant due to a lack of competition from other photosynthetic organisms. This also results in lowered photorespiration due to less photosynthetically generated oxygen.
Aquatic CAM is most marked in the summer months when there is increased competition for, compared to the winter months. However, in the winter months CAM still has a significant role.

Ecological and taxonomic distribution of CAM-using plants

The majority of plants possessing CAM are either epiphytes or succulent xerophytes, but CAM is also found in hemiepiphytes ; lithophytes ; terrestrial bromeliads; wetland plants, Lobelia; and in one halophyte, Mesembryanthemum crystallinum; one non-succulent terrestrial plant, and one mangrove associate.
Plants which are able to switch between different methods of carbon fixation include Portulacaria afra, better known as Dwarf Jade Plant, which normally uses C3 fixation but can use CAM if it is drought-stressed, and Portulaca oleracea, better known as Purslane, which normally uses fixation but is also able to switch to CAM when drought-stressed.
CAM has evolved convergently many times. It occurs in 16,000 species, belonging to over 300 genera and around 40 families, but this is thought to be a considerable underestimate. It is found in quillworts, in ferns, and in Gnetopsida, but the great majority of plants using CAM are angiosperms.
The following list summarizes the taxonomic distribution of plants with CAM:
DivisionClass/Angiosperm groupOrderFamilyPlant TypeClade involved
LycopodiophytaIsoetopsidaIsoetalesIsoetaceaehydrophyteIsoetes - I. howellii, I. macrospora, I. bolanderi, I. engelmanni, I. lacustris, I. sinensis, I. storkii, I. kirkii
PteridophytaPolypodiopsidaPolypodialesPolypodiaceaeepiphyte, lithophyteCAM is recorded from Microsorum, Platycerium and Polypodium, Pyrrosia and Drymoglossum and Microgramma
PteridopsidaPolypodialesPteridaceaeepiphyteVittaria
Anetium citrifolium
CycadophytaCycadopsidaCycadalesZamiaceaeDioon edule
PinophytaGnetopsidaWelwitschialesWelwitschiaceaexerophyteWelwitschia mirabilis
MagnoliophytamagnoliidsMagnolialesPiperaceaeepiphytePeperomia camptotricha
eudicotsCaryophyllalesAizoaceaexerophytewidespread in the family; Mesembryanthemum crystallinum is a rare instance of an halophyte that displays CAM
CactaceaexerophyteAlmost all cacti have obligate Crassulacean Acid Metabolism in their stems; the few cacti with leaves may have C3 Metabolism in those leaves; seedlings have C3 Metabolism.
Portulacaceaexerophyterecorded in approximately half of the genera
Didiereaceaexerophyte
SaxifragalesCrassulaceaehydrophyte, xerophyte, lithophyteCrassulacean acid metabolism is widespread among the Crassulaceae.
eudicots VitalesVitaceaeCissus, Cyphostemma
MalpighialesClusiaceaehemiepiphyteClusia
EuphorbiaceaeCAM is found is some species of Euphorbia including some formerly placed in the sunk genera Monadenium, Pedilanthus and Synadenium. C4 photosynthesis is also found in Euphorbia.
PassifloraceaexerophyteAdenia
GeranialesGeraniaceaeCAM is found in some succulent species of Pelargonium, and is also reported from Geranium pratense
CucurbitalesCucurbitaceaeXerosicyos danguyi, Dendrosicyos socotrana, Momordica
CelastralesCelastraceae
OxalidalesOxalidaceaeOxalis carnosa var. hirta
BrassicalesMoringaceaeMoringa
SalvadoraceaeCAM is found in Salvadora persica. Salvadoraceae were previously placed in order Celastrales, but are now placed in Brassicales.
SapindalesSapindaceaeDodonaea viscosa
FabalesFabaceaeCAM is found in Prosopis juliflora but is currently in the family Fabaceae.
ZygophyllaceaeZygophyllum
eudicots EricalesEbenaceae
SolanalesConvolvulaceaeIpomoea
GentianalesRubiaceaeepiphyteHydnophytum and Myrmecodia
ApocynaceaeCAM is found in subfamily Asclepidioideae , and also in Carissa and Acokanthera
LamialesGesneriaceaeepiphyteCAM was found Codonanthe crassifolia, but not in 3 other genera
LamiaceaePlectranthus marrubioides, Coleus
PlantaginaceaehydrophyteLittorella uniflora
ApialesApiaceaehydrophyteLilaeopsis lacustris
AsteralesAsteraceaesome species of Senecio
MagnoliophytamonocotsAlismatalesHydrocharitaceaehydrophyteHydrilla, Vallisneria
AlismataceaehydrophyteSagittaria
AraceaeZamioculcas zamiifolia is the only CAM plant in Araceae, and the only non-aquatic CAM plant in Alismatales
PoalesBromeliaceaeepiphyteBromelioideae, Puya, Dyckia and related genera, Hechtia, Tillandsia
CyperaceaehydrophyteScirpus, Eleocharis
AsparagalesOrchidaceaeepiphyteOrchidaceae has more CAM species than any other family
AgavaceaexerophyteAgave, Hesperaloe, Yucca and Polianthes
AsphodelaceaexerophyteAloe, Gasteria, and Haworthia
RuscaceaeSansevieria, Dracaena
CommelinalesCommelinaceaeCallisia, Tradescantia, Tripogandra''