Crassulacean Acid Metabolism. A Plastic Photosynthetic Adaptation to Arid Environments
Year: 2001
Type: article
Abstract: Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO2initially at night using phosphoenolpyruvate carboxylase (PEPC) in the cytosol. This leads to the formation of C4 organic acids (usually malate), which are stored in the vacuole. Subsequent daytime decarboxylation of these organic acids behind closed stomata creates an internal CO2 source that is reassimilated by Rubisco in the chloroplast. The refixation of this internal CO2 generates carbohydrates via the conventional photosynthetic carbon reduction cycle. Thus, CAM involves a temporal separation of carbon fixation modes in contrast to the spatial separation found in C4 plants. The first recognition of the nocturnal acidification process can be traced to the Romans, who noted that certain succulent plants taste more bitter in the morning than in the evening (Rowley, 1978). However, formal descriptions of the ability of succulent plants to conduct nocturnal CO2 fixation or to acidify photosynthetic tissues at night and deacidify them during the day did not appear until the early 19th century (de Saussure, 1804; Heyne, 1815). The term CAM was coined to give credit to Heyne's observations that were made usingBryophyllum calycinum, a succulent member of the Crassulaceae. Since these early descriptions, a detailed account of the sequence of biochemical reactions of the CAM cycle (Ranson and Thomas, 1960), the complexity of the biochemical variations in the pathway among different CAM species, and its regulation by the environment have been achieved (Osmond, 1978; Ting, 1985). Initial nocturnal CO2fixation by PEPC occurs when stomata are open and transpirational water losses are low. CO2 release during the day promotes stomatal closure and concentrates CO2around Rubisco, suppressing its oxygenase activity, thereby minimizing photorespiration. The net effect of this CO2-concentrating strategy is that CAM plants exhibit water use efficiency (WUE) rates severalfold higher than C3 and C4 plants under comparable conditions (Drennan and Nobel, 2000). Thus, CAM is typically, although not exclusively, associated with plants that inhabit extremely arid environments (e.g. deserts), semi-arid regions with seasonal water availability (e.g. Mediterranean climates), or habitats with intermittent water supply (e.g. tropical epiphytic habitats). Most notable among these are commercially or horticulturally important plants such as pineapple (Ananas comosus), agave (Agavesubsp.), cacti (Cactaceae), and orchids (Orchidaceae). CAM is also correlated with various anatomical or morphological features that minimize water loss, including thick cuticles, low surface-to-volume ratios, large cells and vacuoles with enhanced water storage capacity (i.e. succulence), and reduced stomatal size and/or frequency. The selective advantage of high WUE likely accounts for the extensive diversification and speciation among CAM plants principally in water-limited environments. Intensive ecophysiological studies over the last 20 years have documented that CAM is present in approximately 7% of vascular plant species, a much larger percentage than the percentage of C4 species (Winter and Smith, 1996a). The widespread distribution of CAM among 33 taxonomically diverse families (Smith and Winter, 1996) suggests that CAM most likely evolved independently on numerous occasions in different families and even within individual families (Griffiths, 1989; Ehleringer and Monson, 1993; Pilon-Smits et al., 1996). More recent phylogenetic reconstructions using PEPC sequence information have provided more convincing support for the polyphyletic origins of CAM (Gehrig et al., 1998b, 2001). It is curious that CAM is also found in aquatic vascular plants where it presumably enhances inorganic carbon acquisition in certain aquatic environments where CO2availability can become rate limiting for photosynthesis (Keeley, 1996,1998). Thus, the daytime limitation of CO2availability, brought about by water-conserving stomatal closure in arid terrestrial habitats or by competition from other species and the high diffusional resistances limiting access to CO2 in aquatic habitats, appears to be the common factor responsible for the evolution of CAM. One of most striking themes to emerge in recent years is the extent to which the phylogenetic and ecological diversity of CAM plants is also reflected in a remarkable plasticity of the basic metabolic scheme described above. Genotypic, ontogenetic, and environmental factors such as light intensity, relative humidity, and water availability combine to govern the extent to which the biochemical and physiological attributes of CAM are expressed (Cushman and Borland, 2001). The photosynthetic plasticity of CAM occurs within a continuum of diel gas exchange patterns that fall into four phases as defined byOsmond (1978). The nocturnal uptake of atmospheric and respiratory CO2 via PEPC to form C4acids (phase I) and daytime organic acid decarboxylation to generate elevated Ci and stomatal closure (phase III) are interspersed with transitional periods of net CO2uptake at the start (phase II) and end of the day (phase IV) when both PEPC- and Rubisco-mediated carboxylation can contribute to CO2 assimilation. The proportion of CO2 taken up via PEPC at night or directly during the day by Rubisco (net CO2 assimilation) is dictated by the integration of stomatal behavior, fluctuations in organic acid and storage carbohydrate accumulation, and the abundance and activity of primary (PEPC) and secondary (Rubisco) carboxylating and decarboxylating enzymes (e.g. malic enzyme or PEP carboxykinase), as well as gluconeogenic/glycolytic enzymes responsible for the synthesis and breakdown of C3 carbon skeletons. Depending on developmental and/or environmental influences, a variety of CO2 assimilation, acid flux, and stomatal behavior characteristics may be observed outside the conventional pattern of the four phases (Table I). “Nearly-C3 ” or “CAM cycling” species display daytime net CO2 uptake with refixation of respiratory CO2 at night accompanied by only small diel C4 acid fluctuations. In plants growing in thin soils or rock outcrops, this nocturnal recapture of respiratory CO2 is thought to help maintain a positive carbon balance during frequent episodes of drought (Martin, 1996). However, the potential conservation of water resulting from the induction of CAM cycling varies widely (5%–70%) in various species (Borland, 1996; Martin, 1996). In C3-CAM intermediate species, such improvements in WUE are not always associated with CAM induction (Eller and Ferrari, 1997; Cushman and Borland, 2001). In “obligate” or “constitutive” CAM species, net CO2 uptake occurs almost exclusively at night (phase I), with some net CO2 assimilation occurring during phases II and IV, even under well-watered conditions, accompanied by large diel C4 acid fluctuations. Under severe drought conditions, many CAM species will undergo “CAM-idling” wherein stomata remain closed day and night, preventing net CO2 uptake, yet the plants will continue to conduct diel fluctuations in organic acids. Other modes of CAM such as latent CAM, indicated by organic acid concentrations elevated above those normally present in C3plants but without diel fluctuation, may represent a nascent C3-to-CAM progression in some species (Schuber and Kluge, 1981). A hypothetical variation of CAM called “rapid-cycling CAM” has also been proposed in which the CO2 acquisition and reduction phases of CAM may occur over time periods shorter than the normal diel cycle (Cockburn, 1998). Plasticity of CAM modes in relationship to environmental and developmental influences Dashes indicate no substantial occurrence or effect. Question marks indicate that no information is available. Plasticity of CAM modes in relationship to environmental and developmental influences Dashes indicate no substantial occurrence or effect. Question marks indicate that no information is available. The best examples of CAM plasticity are the C3-CAM intermediate species found predominantly among the Aizoaceae, Crassulaceae, Portulaceae, and Vitaceae (Smith and Winter, 1996). These facultative or inducible CAM species use the C3 pathway to maximize growth when water is abundant, but then they undergo a gradual C3-to-CAM transition often coincident with seasonal moisture availability (Winter et al., 1978). The C3-to-CAM transition reduces water loss and maintains photosynthetic integrity under water-limited conditions that ultimately translates into reproductive success (Winter and Ziegler, 1992). Among facultative CAM species, the common ice plant,Mesembryanthemum crystallinum, has been most intensively studied (Adams et al., 1998; Bohnert and Cushman, 2001). This model species undergoes a gradual, largely irreversible, and partially developmentally regulated transition into CAM following water stress (Cushman et al., 1990; Herppich et al., 1992). In contrast, other inducible CAM species (e.g. Clusiaceae and Bromeliaceae) display more rapid and reversible shifts between C3photosynthesis and CAM in response to changes in water deficit, regardless of leaf or plant ontogeny (Schmitt et al., 1988; Zotz and Winter, 1993; Lüttge, 1996; Borland et al., 1998). The magnitude of CAM induction in facultative CAM plants tends not only to be influenced by water deficit, but also by associated environmental conditions such as temperature, light intensity, and humidity (Lüttge, 2000). For example, it is well established that high light intensity or light quality can enhance CAM induction in the ice plant in the presence or absence of salinity stress (McElwain et al., 1992; Cockburn et al., 1996; Miszalski et al., 2001). Since the first molecular characterization of the common ice plantPpc1 gene encoding a CAM-specific isoform of PEPC more than a decade ago (Cushman et al., 1989), a large number of enzymes, transporters, and regulatory proteins required for CAM have been identified and characterized (for review, see Cushman and Bohnert, 1999, 2001; Cushman and Borland, 2001). Most studies have been restricted to inducible C3-CAM models (e.g. common ice plant and Kalanchoë sp.) because the differential expression of genes induced in response to water deficit serves as a convenient and reliable indicator of their potential functional role(s) in CAM. Greater investments have been made in establishing molecular genetic resources for common ice plant than other CAM models because this species is also a halophyte and has been extensively investigated to understand salinity stress tolerance mechanisms (Bohnert and Cushman, 2001; Bohnert et al., 2001). CAM induction in response to salinity, water deficit, osmotic stress, or abscisic acid treatment is controlled primarily by transcriptional activation (Cushman et al., 1989, 2000b) initiated through a signaling cascade with apparent requirements for calcium and calcium-dependent protein kinase activities (Taybi and Cushman, 1999;Golldack and Dietz, 2001). In general, transcript and protein accumulation patterns are well correlated; however, discrepancies between transcript and protein abundance have suggested that changes in mRNA stability and utilization or translational efficiency are also likely to govern gene expression changes during the C3-to-CAM transition (Cushman et al., 1990;DeRocher and Bohnert, 1993). Detailed analysis of the PEPC gene families from facultative and obligate CAM species including pineapple (Ananas comosus),K. blossfeldiana, K. daigremontiana, common ice plant, and Vanilla planifolia has indicated that a single member of a four- to six-member PEPC gene family is typically recruited to fulfill the primary carboxylation and carbon flux requirements of CAM, as demonstrated by its enhanced expression in CAM-performing leaves (Cushman et al., 1989; Gehrig et al., 1995, 1998a). Remaining isoforms, which presumably fulfill anapleurotic “housekeeping” or tissue-specific functional roles, generally show lower transcript or protein abundance and remain unaffected in their expression following CAM induction. This “gene recruitment” paradigm likely pertains to other gene families as well. Enhanced expression of enzymes for C4 acid metabolism is accompanied by corresponding increases in carbohydrate-forming and -degrading enzymes and transcripts (Holtum and Winter, 1982; Paul et al., 1993;Häusler et al., 2000). Elevated organellar PEP (Kore-eda et al., 1996) and triose and hexose phosphate transport activities (Neuhaus and Schulte, 1996; Kore-eda and Kanai, 1997) associated with CAM induction in common ice plant are matched by light-enhanced increases in transcript abundance and diurnal gene expression patterns of a PEP phosphate translocator and a Glc-6-P phosphate translocator (Häusler et al., 2000). However, the expression of a chloroplast Glc transporter and a triose phosphate transporter remain largely unchanged (Häusler et al., 2000; S. Kore-eda and J.C. Cushman, unpublished data). Tonoplast H+-translocating ATPase transport activity and expression of corresponding tonoplast H+-translocating ATPase subunit genes for energizing vacuolar malate storage is enhanced during the C3-CAM transition in common ice plant (Rockel et al., 1998a, 1998b; Barkla et al., 1999; Golldack and Dietz, 2001). Molecular characterization of the vacuolar malate transporters, carriers, and channels for malate influx and efflux has remained a challenge (Lüttge et al., 2000). Recent measurements of vacuolar malate transport activities demonstrate an approximate 3-fold increase following CAM induction in common ice plant (Lüttge et al., 2000). A strategy to analyze differences in polypeptide expression patterns in C3- versus CAM-performing leaves of common ice plant is being used to identify candidate vacuolar malate transporters. Antisera raised against affinity chromatography-purified tonoplast vesicle fractions fromK. daigremontiana enriched for malate transport activity has been used to identify 32- and 33-kD common ice plant polypeptides that are induced or enhanced in the CAM state (Steiger et al., et al., 2000). These low abundance polypeptides be for the vacuolar malate acid sequence information from these polypeptides may the of the corresponding The of CO2 primarily studied in K. is of the and best documented examples of in higher 1992). in the activity of controlled in by changes in its a in carbon flux through the CAM pathway by the to such as malate et al., 1998). PEPC state is controlled largely by changes in the activity of PEPC kinase et al., In common ice plant, activity is induced with a CAM-specific isoform of PEPC and Recent of the gene for first in K. et al., and then in common ice plant (Taybi et al., demonstrated directly that this kinase is regulated at the of transcript abundance by a A protein of activity has also been described fromK. that may to kinase activity during the light and early of the when carbon flux through PEPC is not et al., In contrast to C4 in appear to have et al., or no and on activity in common ice plant or K. However, of transcript abundance may be a secondary response to other factors such as the malate which has been to the transcript abundance and activity of et al., 1999; 2000). malate concentrations are likely to be controlled by transport of malate the a that is well by on tonoplast and studies et al., 2000). Thus, response to environmental factors that organic acid or malate between the and may be to of activity, a for the rapid in PEPC activity observed in some CAM species et al., 2000). In of K. and common ice plant leaves with various the of a tonoplast calcium a protein and and protein synthesis as in the signaling cascade that activity on a et al., 1999; et al., 2001; et al., However, these studies to the of such on the of the and observed changes in PEPC activity may not in the PEPC cascade One challenge to regulation of CAM will be to the mechanisms responsible for the in malate uptake and release the tonoplast In it will be important to understand tonoplast malate transport is controlled by an molecular of malate transport in the tonoplast and from CAM species will be for this ecophysiological have a variety of CAM species to which CAM. studies have on analysis of of CAM such as the of CAM induction by water limitation (Cushman and Borland, of carboxylation and decarboxylation et al., or the patterns of carbohydrate within a family and 1998). However, C3 and C4 which have the genetic models and has until no in the of a genetic model for CAM. This has of many of the molecular mechanisms that CAM. In the CAM models were for their physiological For example, certain obligate CAM species such as K. daigremontiana are often for gas exchange and biochemical studies to their Other CAM models such as common ice plant can show stress to changes in growth conditions, which can be a for physiological species, however, potential for as a genetic as well as molecular genetic A of the attributes of or commercially important CAM models from diverse families that common ice plant has many features that it an genetic model (Table This large of small under or growth conditions in The plant is yet is In contrast, the or species more and are the common ice plant more than models such as with other CAM the common ice plant cycle is it is to the normal cycle of common ice plant from to under conditions (Winter et al., to approximately under growth conditions under light or and and of the cycle is accompanied by a of the This is an important when genetic because growth or is often a limiting genetic be in a identified that CAM have been established in common ice plant from or (Cushman et al., 2000b) or (Adams et al., 1998). of a for a genetic in the common ice plant, to resources in and et al., 2001). have been for the of (Cushman et al., of is on a that a in nocturnal C4 acid are not in other CAM A of such is the of with morphological (e.g. and absence of or physiological (e.g. J.C. Cushman, unpublished data). of attributes of CAM models plants under conditions in Question marks indicate that no information is available. of attributes of CAM models plants under conditions in Question marks indicate that no information is available. of the CAM model is the availability of an that a such as or in et al., and 1998). A with efficiency functional to be genetic for gene or and ultimately gene of regulatory or genes of with in CAM. the candidate model CAM species, are to genetic using an et al., However, the ice to in et al., and the availability of a high efficiency (Cushman et al., an for analysis in common ice plant appears The other limitation for CAM has been the a genetic model with a of molecular genetic such as the sequence of the or at the of The common ice plant is approximately as by et al., in et al., or approximately larger than the and than the The common ice plant is also than other CAM models for which such are available. For example, the pineapple is with a approximately the size of the and and K. and K. are and four the of the common ice plant et al., Thus, the small size of the common ice plant it a most for In of sequence the availability of sequence from for to in a CAM are for K. et al., 1996) and K. et al., the most of for CAM plant is for the common ice More than from tissues that the from to and as well as different tissues such as and and different stress (Bohnert and Cushman, 2001). more than are Bohnert and Cushman, 2001). In a gene has been that allows access to the sequence information in the form of genes and from However, investments in other intensively studied models such as K. daigremontiana and in which are also under and Borland, will be for of the functional of genes encoding signaling and regulatory enzymes, and and to physiological or biochemical molecular genetic resources will to from gene expression to gas exchange integration is required to identify and the functional and regulation of gene among regulated and associated the of sequence information on which to as well as with which to or to for gene or expression can often important about the of in CAM or in CAM for of the common ice plant from plants the presence of large of up to that are not in other plant (Bohnert and Cushman, 2001). or in the common ice plant may in from the between common ice plant and the other plant also that gene family has in the common ice plant, a of the to the requirements of CAM for and reproductive success in arid environments. for this can be for example, the PEPC gene In this gene family is of four In the common ice plant, however, at up this gene with only recruited to in CAM (Cushman and Borland, 2001). Recent improvements in and access to large capacity it to for the of the of a CAM plant in the The common ice plant is a for such an because it has the among CAM models and the for gene (Table This will also important information for studies of a species within the Most the species in the and In contrast, which such plant families as the Aizoaceae, and are for because most are or species of many species in the have evolved to environments characterized by water deficit, salinity, or these species can be to be of genes in biochemical for plant secondary or stress For example, many species of the of and other Thus, access to sequence information for the common ice plant of genes with CAM-specific or regulation (e.g. regulated as well as of gene for stress and and et al., The C3 and have been extensively investigated at the molecular genetic of this has been by the availability of and genetic models and an abundance of and sequence In contrast, of the regulation of the CAM photosynthetic pathway has behind these other However, recent the of or more genetic models for CAM, with availability of gene sequence and expression a and for CAM and of and metabolic in CAM models such as the common ice plant is to of CAM induction by environmental and developmental influences and the that the diel patterns of CO2 fixation of CAM plants. Thus, the challenge CAM in the will be to of using and physiological in CAM This will an of the regulatory that such to the environment that has become of the of CAM plants. to Cushman and for their of the
Source: PLANT PHYSIOLOGY
DOI: 10.1104/pp.010818
Author: John C. Cushman
Institution: University of Nevada, Reno
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2084179913