Crassulacean_acid_metabolism

The Pineapple is a CAM plant

Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some plants. These plants fix carbon dioxide (CO₂) during the night, storing it as the four carbon acid malate. The CO₂ is released during the day, where it is concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day; therefore it is especially common in plants adapted to arid conditions.

Historical background

CAM was first discovered in the late 1940s. It was observed by the botanists Ranson and Thomas, in the Crassulaceae family of succulents (which includes jade plants and sedums).^[1] Its name refers to acid metabolism in Crassulaceae, not the metabolism of Crassulacean acid.

Overview of CAM: a two-part cycle

CAM plants adapt to life in arid climates by conserving water.

During the night

During the night, the CAM plant's stomata are open, allowing CO₂ to enter and be fixated as organic acids that are stored in vacuoles. During the day the stomata are closed (thus preventing water loss) and the carbon is released to the Calvin Cycle so that photosynthesis may take place.

The carbon dioxide is fixed in the mesophyll cell's cytoplasm by a PEP reaction similar to that of C4 plants. But, unlike C4 plants, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin Cycle. Of course, the latter cannot operate during night because the light reactions which provide it with ATP and NADPH cannot take place without light.

During the day

The carbon in the organic acids is freed from the mesophyll cell's vacuoles and enters the chloroplast's stroma and thus into the Calvin Cycle.

The benefits of CAM

The most important benefit to the plant is the ability to leave most leaf stomata closed during the day^[2]. CAM plants 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 CAM plants to grow in environments that would otherwise be far too dry. C₃ plants, for example, lose 97% of the water they uptake through the roots to transpiration^[3] - a high cost avoided by CAM plants.

Comparison with C₄ metabolism

CAM is named after the family Crassulaceae, to which Jade plant belongs

The C₄ pathway bears resemblance to CAM; both act to concentrate CO₂ around RuBisCO, thereby increasing usefulness. CAM concentrates it in time, providing CO₂ during the day, and not at night, when respiration is the dominant reaction. C₄ plants, on the contrary, concentrate CO₂ spatially, with a RuBisCO reaction centre in a "bundle sheath cell" being inundated with CO₂.

How to spot a CAM plant

CAM can be considered an adaptation to arid conditions. CAM plants 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 (the succulents^{[verification needed]}) store water in vacuoles.

CAM plants are not only good at retaining water, but use nitrogen very efficiently.^{[citation needed]} However, due to their stomata being closed by day, they are less efficient at CO₂ absorption. This limits the amount of carbon they have available for growth.

CAM plants can also be recognised as plants which have sour tasting leaves increasing during nights but sweet tasting leaves increasing during days. This is due to the malic acid being stored in the vacuoles of the plant cells during the night, and its being used up during the day^[4].

Biochemistry of Crassulacean Acid Metabolism

Biochemistry of CAM

Plants with Crassulacean Acid Metabolism (CAM plants) must control storage of carbon dioxide and its reduction to branched carbohydrates in space and time.

At low temperatures (frequently at night), when CAM plants open their guard cells, carbon dioxide molecules diffuse into the spongy mesophyll's intracellular spaces and finally get into the cytoplasm. Here, they can meet phosphoenolpyruvate (PEP), which is a phosphorylated triose. During this time, CAM plants are synthesizing a protein called PEP carboxylase kinase (PEP-C kinase), which expression can be inhibited by high temperatures (frequently at daylight) and the presence of malate. PEP-C kinase phosphorylates its target enzyme PEP carboxylase (PEP-C). Phosphorylation dramatically enhanced the enzyme‘s capability to catalyze the formation of oxalacetate that 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.

At daylight, CAM plants close their guard cells and discharged malate that is subsequently transported into chloroplasts. There, depending on plant species, it is cleaved into pyruvate and carbon dioxide either by malic enzyme or PEP carboxykinase. Carbon dioxide 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 and therefore, provides additional carbon dioxide molecules for the calvin cycle. Alternatively, pyruvate can be also used to recover PEP via pyruvate phosphate dikinase, a high energy step, which requires ATP and an additional phosphate. In the following cold night, PEP is finally exported into the cytoplasm, where it is involved in fixing carbon dioxide via malate.

Ecological and Taxonomic Distribution of CAM Plants

The majority of plants possessing Crassulacean Acid Metabolism are either epiphytes (e.g. orchids, bromeliads) or succulent xerophytes (e.g. cacti, cactoid Euphorbias), but it is also found in hemiepiphytes (e.g. Clusia), lithophytes (e.g. Sedum, Sempervivum), terrestrial bromeliads, hydrophytes (e.g. Isoetes, Crassula (Tillaea), and from a halophyte (Mesembryanthemum crystallinum), a non-succulent terrestrial plant (Dodonaea viscosa) and a mangrove associate (Sesuvium portulacastrum). Portulacaria afra is the only plant known to display both CAM and C4 pathways.

Crassulacean Acid Metabolism has evolved convergently many times^[5]. It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families. It is found in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of CAM plants are angiosperms (flowering plants).

The following list summarises the taxonomic distribution of CAM plants.

See also

• C4 plants
• C3 carbon fixation
• RuBisCO

References

1. ^ Ranson S. L.; Thomas M (1960). "Crassulacean acid metabolism". Annual Rev Plant Physiol 11: 81–110. doi:10.1146/annurev.pp.11.060160.000501. 
2. ^ Ting I. P. (1985) Crassulacean Acid Metabolism. Annual Review of Plant Physiology 36: 595-622.
3. ^ "Roots: evolutionary origins and biogeochemical significance". Journal of Experimental Botany 52 (90001): 381–401. 2001. doi:10.1093/jexbot/52.suppl_1.381 (inactive 2008-06-20). 
4. ^ Raven, P & Evert, R & Eichhorn, S, 2005, "Biology of Plants" (seventh edition), pp. 135 (Figure 7-26), W.H. Freeman and Company Publishers
5. ^ http://www.werc.usgs.gov/seki/pdfs/IJPS_Keeley_Rundel.pdf
6. ^ ^{a b} Boston & Adams, Evidence of crassulacean acid metabolism in two North American isoetids, Aquatic Botany 15(4): 381-386 (1983)
7. ^ Holtum & Winter, Degrees of crassulacean acid metabolism in tropical epiphytic and lithophytic ferns, Australian Journal of Plant Physiology 26(8): 749-757 (1999)
8. ^ Wong & Hew, Diffusive Resistance, Titratable Acidity, and CO₂ Fixation in Two Tropical Epiphytic Ferns, American Fern Journal 66(4): 121-124 (1976)
9. ^ ^{a b} Crassulacean Acid Metabolism
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11. ^ abstract to Martin et al., The Occurrence of Crassulacean Acid Metabolism in Epiphytic Ferns, with an Emphasis on the Vittariaceae, International Journal of Plant Sciences 166(4): 623-630 (2005)
12. ^ Vovides et al., CAM-cycling in the cycad Dioon edule Lindl. in its natural tropical deciduous forest habitat in central Veracruz, Mexico, Botanical Journal of the Linnean Society 138(2): 155–162 (2002)
13. ^ Schultze, Ziegler & Stichler, Environmental control of crassulacean acid metabolism in Welwitschia mirabilis Hook. Fil. in its range of natural distribution in the Namib desert, Oecologia 24(4): 323-334 (1976)
14. ^ Sipes & Ting, Crassulacean Acid Metabolism and Crassulacean Acid Metabolism Modifications in Peperomia camptotricha, Plant Physiol. 77(1): 59-63 (1985)
15. ^ Chu, Dai, Ku & Edwards, Induction of Crassulacean Acid Metabolism in the Facultative Halophyte Mesembryanthemum crystallinum by Abscisic Acid, Plant Physiol. 93(3): 1253–1260 (1990)
16. ^ Guralnick & Jackson, The Occurrence and Phylogenetics of Crassulacean Acid Metabolism in the Portulacaceae, Int. J Plant Sci. 162(2): 257–262 (2001)
17. ^ ^{a b c d e f g} Cockburn, Variation in Photosynthetic Acid Metabolism in Vascular Plants: CAM and Related Phenomena, New Phytologist 101(1): 3-24 (1985)
18. ^ ^{a b c d e f g h i j} Nelson, Sage & Sage, Functional Leaf Anatomy of plants with Crassulacean Acid Metabolism, Functional Plant Biology 32: 409-419 (2005)
19. ^ ^{a b} Lüttge, Ecophysiology of Crassulacean Acid Metabolism (CAM), Annals of Botany 93: 629-652 (2004)
20. ^ ^{a b c} Bender et al., 13C/12C Ratio Changes in Crassulacean Acid Mechanism Plants, Plant Physiology 52: 427-430 (1973)
21. ^ Jones, Cardon & Czaja, A phylogenetic view of low-level CAM in Pelargonium (Geraniaceae), American Journal of Botany 90: 135-142 (2003)
22. ^ Bastide, Sipes, Hann & Ting, Plant Physiol. 103(4): 1089–1096 (1993)
23. ^ abstract to Lange & Zuber, Frerea indica, a stem succulent CAM plant with deciduous C₃ leaves, Oecologia 31(1): 67-72 (1977)
24. ^ [abstract to http://links.jstor.org/sici?sici=0002-9122(198603)73:3%3C336:CAMITG%3E2.0.CO;2-H Guralnick et al., Crassulacean Acid Metabolism in the Gesneriaceae, American Journal of Botany 73(3): 336-345 (1986)]
25. ^ Fioretti & Alfani, Anatomy of Succulence and CAM in 15 Species of Senecio, Botanical Gazette 149(2): 142-152 (1988)
26. ^ Holtum, Winter, Weeks and Sexton, Crassulacean acid metabolism in the ZZ plant, Zamioculcas zamiifolia (Araceae), American Journal of Botany 94: 1670-1676 (2007)
27. ^ Winter & Smith, Multiple origins of crassulacean acid metabolism and the epiphytic habit in the Neotropical family Bromeliaceae, PNAS 101(10): 3703-3708 (2004)