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Crasulacean Acid Metabolism

M.Sc. Biotechnology

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Historical Background CAM was first discovered in the late 1940s. It

was observed by the botanists Ranson andThomas, in the Crassulaceae family ofsucculents.

Its name refers to acid metabolism inCrassulaceae, not the metabolism ofCrassulacean acid.

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Identifying 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 alow surface-area-to-volume ratio; thick cuticle;and stomata sunken into pits. Some shed theirleaves during the dry season; others (the

succulents) store water in vacuoles. CAM plants not only are good at retaining water

but also use nitrogen very efficiently.

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What is Crassulacean Acid Metabolism?

It is a carbon fixation pathway present insome plants.

Also known as CAM photosynthesis. These plants fix carbon dioxide (CO 2) during

the night, storing it as the four-carbonacid malate.

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A third mechanism for concentrating CO2 at the site of rubisco isfound in crassulacean acid metabolism (CAM). Despite its name,CAM is not restricted to the family Crassulaceae ( Crassula,Kalanchoe, Sedum); it is found in numerous angiosperm families.

Cacti and euphorbias are CAM plants, as well as pineapple,vanilla, and agave.

The CAM mechanism enables plants to improve water useefficiency. Typically, a CAM plant loses 50 to 100 g of water for

every gram of CO2 gained, compared with values of 250 to 300 gand 400 to 500 g for C4 and C3 plants, respectively. .Thus, CAMplants have a competitive advantage in dry environments.

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Historically, the discovery of CAM plants can beattributed to cyclical acidication / deacidication .

In a letter to the Linnean Society in 1813 BenjaminHeyne wrote the leaves of the plant Bryophyllumcalycinum , which on the whole have an herbaceoustaste, are in the morning as acid as sorrel, if not moreso.

As the day advances, they lose their acidity and aretasteless about noon

However, despite these early observations, it tookanother 150 years to unravel the complexities of theCAM pathway.

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Separate carbon fixation from Calvin cycle C4 plants

PHYSICALLY separate carbon fixation from Calvincycle

different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds

different enzyme to capture CO2 (fix carbon)

PEP carboxylase different leaf structure

CAM plants

separate carbon fixation from Calvin cycle by TIMEOF DAY fix carbon during night

store carbon in 4C compounds

perform Calvin cycle during day

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CAM Summarysolves CO 2 / O 2 gas exchange vs. H2O loss challenge

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CAM plants

succulents

cacti

pine apple

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The Stomata of CAM Plants Open at Nightand Close during the Day

CAM plants such as cacti achieve their highwater use efficiency by opening their stomataduring the cool, desert nights and closingthem during the hot, dry days.

Closing the stomata during the day minimizeswater loss, but because H 2O and CO 2 sharethe same diffusion pathway, CO 2 must thenbe taken up at night.

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The CAM mechanism is similar in many respectsto the C4 cycle. In C4 plants, formation of the C4acids in the mesophyll is spatially separatedfrom decarboxylation of the C4 acids and fromrefixation of the resulting CO2 by the Calvincycle in the bundle sheath.

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In CAM plants, formation of the C4 acids is bothtemporally and spatially separated. At night,CO2 is captured by PEP carboxylase in thecytosol, and the malate that forms from theoxaloacetate product is stored in the vacuole.

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During the day, the stored malate is transportedto the chloroplast and decarboxylated by NADP-malic enzyme, the released CO2 is fixed by theCalvin cycle, and the NADPH is used forconverting the decarboxylated triose phosphateproduct to starch.

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Nocturnal Acidification CO2 is incorporated via carboxylation of

phosphoenolpyruvate to oxaloacetate, which is thenreduced to malate.

The malate accumulates and is stored in the largevacuoles that are a typical, but not obligatory,anatomic feature of the leaf cells of CAM plants.

The accumulation of substantial amounts of malic

acid, equivalent to the amount of CO2 assimilated atnight, has long been recognized as a nocturnalacidification of the leaf (Bonner and Bonner 1948).

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With the onset of day, the stomata close,preventing loss of water and further uptake ofCO2.

The leaf cells deacidify as the reserves ofvacuolar malic acid are consumed.Decarboxylation is usually achieved by theaction of NADP-malic enzyme on malate.

Because the stomata are closed, the internallyreleased CO2 cannot escape from the leaf andinstead is fixed and converted to carbohydrateby the Calvin cycle.

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The elevated internal concentration of CO2effectively suppresses the photorespiratoryoxygenation of ribulose bisphosphate and

favors carboxylation. The C3 acid resulting from the

decarboxylation is thought to be convertedfirst to triose phosphate and then to starch orsucrose, thus regenerating the source of theoriginal carbon acceptor.

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The CAM mechanism enables plants toimprove water use efficiency. Typically, aCAM plant loses 50 to 100 g of water for

every gram of CO2 gained, compared withvalues of 250 to 300 g and 400 to 500 g for C4and C3 plants, respectively. .Thus, CAM plantshave a competitive advantage in dryenvironments.

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http://www.uic.edu/classes/bios/bios100/lectures/c4.htm

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CAM is an adaptation to drought!Epiphytes in Trinidad

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Taiz and Zeiger, Figure 9.24 CAM gas exchange