C3 Crassulacean Acid Metabolism Research Articles

Mechanisms Underlying the C3–CAM Photosynthetic Shift in Facultative CAM Plants

Crassulacean acid metabolism (CAM), one of three kinds of photosynthesis, is a water-use efficient adaptation to an arid environment. CAM is characterized by CO2 uptake via open stomata during the nighttime and refixation CO2 via the Calvin cycle during the daytime. Facultative CAM plants can shift the photosynthesis from C3 to CAM and exhibit greater plasticity in CAM expression under different environments. Though leaf thickness is an important anatomical feature of CAM plants, there may be no anatomical feature changes during the C3–CAM transition for all facultative CAM plants. The shift from C3 photosynthesis to CAM in facultative CAM plants is accompanied by significant changes in physiology including stomata opening, CO2 gas exchange and organic acid fluxes; the activities of many decarboxylating enzymes increase during the shift from C3 to CAM; the molecular changes occur during the photosynthesis C3–CAM shift involved DNA hypermethylation, transcriptional regulation, post-transcriptional regulation and protein level regulation. Recently, omics approaches were used to discover more proceedings underling the C3–CAM transition. However, there are few reviews on the mechanisms involved in this photosynthetic shift in facultative CAM plants. In this paper, we summarize the progress in the comparative analysis of anatomical, physiological, metabolic and molecular properties of facultative CAM plants between C3 and CAM photosynthesis. Facultative CAM plants also show the potential for sustainable food crop and biomass production. We also discuss the implications of the photosynthesis transition from C3 to CAM on horticultural crops and address future directions for research.

Leaf vein density correlates with crassulacean acid metabolism, but not hydraulic capacitance, in the genus Clusia.

Many succulent species are characterized by the presence of Crassulacean acid metabolism (CAM) and/or elevated bulk hydraulic capacitance (CFT). Both CAM and elevated CFT substantially reduce the rate at which water moves through transpiring leaves. However, little is known about how these physiological adaptations are coordinated with leaf vascular architecture. The genus Clusia contains species spanning the entire C3-CAM continuum, and also is known to have >5-fold interspecific variation in CFT. We used this highly diverse genus to explore how interspecific variation in leaf vein density is coordinated with CAM and CFT. We found that constitutive CAM phenotypes were associated with lower vein length per leaf area (VLA) and vein termini density (VTD), compared to C3 or facultative CAM species. However, when vein densities were standardized by leaf thickness, this value was higher in CAM than C3 species, which is probably an adaptation to overcome apoplastic hydraulic resistance in deep chlorenchyma tissue. In contrast, CFT did not correlate with any xylem anatomical trait measured, suggesting CAM has a greater impact on leaf transpiration rates than CFT. Our findings strongly suggest that CAM photosynthesis is coordinated with leaf vein densities. The link between CAM and vascular anatomy will be important to consider when attempting to bioengineer CAM into C3 crops.

Evapotranspiration and Mineral Content of Sedum kamtschaticum Fischer Under Saline Irrigation

ABSTRACTAn alternative water resource such as graywater could be used for irrigation on green roofs during hot, dry summers, although it contains salt. In this study, the response to high-salt stress of a C3–CAM (Crassulacean acid metabolism) intermediate species, Sedum kamtschaticum Fischer, was evaluated over a 2-month experiment in terms of evapotranspiration (ET) and chemical compounds in plant tissue in triplicate for both experiments. High ET (10–15 mm day−1) was observed under non-stressed conditions. On the day following the first saline irrigation, the peak ET at noon decreased as much as one-third of the maximum. After 9 days, ET remained below 3 mm day−1, corresponding mostly to evaporation from the wet soil surface. The balance of chemical component contents in leaves changed depending on the electrical conductivity of irrigation water electrical conductivity (ECi). The potassium to sodium (K+/Na+) ratio, which indicates levels of sodium toxic for plant growth, decreased with higher ECi, while it excluded sodium from roots. However, based on enhanced water use efficiency under higher ECi regardless of reduced carbon dioxide (CO2) assimilation under salinity stress, the plant’s method of photosynthesis shifted from C3 to CAM metabolism. These findings show that S. kamtschaticum could survive for more than 2 months under low or moderate salinity of irrigation water in hot conditions.

Spatial division of phosphoenolpyruvate carboxylase and nitrate reductase activity and its regulation by cytokinins in CAM-induced leaves of Guzmania monostachia (Bromeliaceae)

Crassulacean acid metabolism (CAM) is a physiological adaptation of plants that live in stress environment conditions. A good model of CAM modulation is the epiphytic bromeliad, Guzmania monostachia, which switches between two photosynthetic pathways (C3–CAM) in response to different environmental conditions, such as light stress and water availability. Along the leaf length a gradient of acidity can be observed when G. monostachia plants are kept under water deficiency. Previous studies showed that the apical portions of the leaves present higher expression of CAM, while the basal regions exhibit lower expression of this photosynthetic pathway. The present study has demonstrated that it is possible to induce the CAM pathway in detached leaves of G. monostachia kept under water deficit for 7 d. Also, it was evaluated whether CAM expression can be modulated in detached leaves of Guzmania and whether some spatial separation between NO3− reduction and CO2 fixation occurs in basal and apical portions of the leaf. In addition, we analyzed the involvement of endogenous cytokinins (free and ribosylated forms) as possible signal modulating both NO3− reduction and CO2 fixation along the leaf blade of this bromeliad. Besides demonstrating a clear spatial and functional separation of carbon and nitrogen metabolism along G. monostachia leaves, the results obtained also indicated a probable negative correlation between endogenous free cytokinins – zeatin (Z) and isopentenyladenine (iP) – concentration and PEPC activity in the apical portions of G. monostachia leaves kept under water deficit. On the other hand, a possible positive correlation between endogenous Z and iP levels and NR activity in basal portions of drought-exposed and control leaves was verified. Together with the observations presented above, results obtained with exogenous cytokinins treatments, strongly suggest that free cytokinins might act as a stimulatory signal involved in NR activity regulation and as a negative regulator of PEPC activity in CAM-induced leaves of G. monostachia during a diel cycle.

Limitation of C3–CAM shift in the common ice plant under high irradiance

In the halophytic plant Mesembryanthemum crystallinum salinity or drought can change the mode of photosynthesis from C3 to crassulacean acid metabolism (CAM). These two stress factors are linked to oxidative stress, however, the induction of CAM by oxidative stress per se is not straightforward. Treatment with high light (HL) did not lead to the induction of CAM, as documented by a low night/day difference in malate level and a low expression of the CAM-related form of phosphoenolcarboxylase (Ppc1), despite causing some oxidative damage (elevated MDA level, malondialdehyde). In contrast to the action of high salinity (0.4M NaCl), HL treatment did not activate neither the cytosolic NADP-malic enzyme nor the chloroplastic form of NADP-dependent malate dehydrogenase (NADP-MDH). In plastids of HL-treated plants a huge amount of starch was accumulated. This was associated with a weak stimulation of hydrolytic and phosphorolytic starch-degrading enzymes, in contrast to their strong up-regulation under high salinity. It is concluded that HL alone is not able to activate starch degradation necessary for CAM performance. Moreover, in the absence of salinity in C3M. crystallinum plants an age-dependent increase in energy dissipation from PSII was documented under high irradiance, as illustrated by non-photochemical quenching (NPQ). Obtained data suggest that in this halophytic species several photoprotective strategies are strictly salinity-dependent.

Effects of salinity, high irradiance, ozone, and ethylene on mode of photosynthesis, oxidative stress and oxidative damage in the C3/CAM intermediate plant Mesembryanthemum crystallinum L.

ABSTRACTThe occurrence of oxidative stress and oxidative damage in the halophyte Mesembryanthemum crystallinum during C3/crassulacean acid metabolism (CAM) transition evoked by salinity and high irradiance was investigated. The hypothesis tested was that CAM induction will effectively protect M. crystallinum against oxidative damage even under additionally applied oxidative stress via ozone (150 p.p.b.) exposure. Salinity (0.4 m NaCl), high irradiance and ozone induced the cytosolic CuZn‐superoxide dismutase (SOD; EC 1.15.1.1) transcript abundance and activity indicating oxidative stress. However, ozone did not induce CAM. Contrary to the hypothesis, that ethylene may be involved in CAM induction, gassing with ethylene (200 p.p.b.) did not lead to CAM, but revealed that in M. crystallinum ethylene biosynthesis is induced by autostimulation. A putative fragment of the hitherto unknown nu‐cleotide sequence of 1‐aminocyclopropane‐1‐carboxylate oxidase (ACC; EC 4.4.1.14) was isolated and used as a specific probe against the ACC oxidase transcript to monitor oxidative damage. Ozone exposure increased the ACC oxidase expression only in plants with C3‐photosynthesis which also showed severe necrosis and reduced photosynthetic activity, whereas plants in CAM mode (salt induced) remained unaffected. Natural stressors such as salinity and high irradiance induce CAM and up‐regulate the ARS component CuZn‐SOD very effectively to protect M. crystallinum from oxidative damage even when additional oxidative stress such as ozone is present.

Resistance is useful: diurnal patterns of photosynthesis in C3 and crassulacean acid metabolism epiphytic bromeliads.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001 Diurnal patterns of photosynthesis in response to environmental variables were investigated in an obligate C3 and a facultative C3-crassulacean acid metabolism (CAM) bromeliad species. A midday depression of photosynthesis occurred in both C3 groups, mediated as a decrease in stomatal conductance in response to increased vapour pressure difference. The response was associated with a reduction in Rubisco activation state during the period of maximum photon flux density. In contrast, the switch to CAM resulted in a strong shift in the pattern of Rubisco carbamylation, with full enzyme activation delayed until the midday period. For the first time it is demonstrated that the pattern of Rubisco activation differs between C3 and CAM plants of the same species under identical conditions. Despite large differences in Rubisco content between C3 and CAM plants, neither the amount of Rubisco or enzyme activity is thought to be limiting for photosynthesis, and it is suggested that Rubisco may function as a nitrogen store. Extreme CO2 diffusion limitation resulted in low rates of atmospheric CO2 assimilation that were associated with high rates of photosynthetic electron transport, and it is likely that photorespiration constitutes a significant electron sink over the entire diurnal course. Leaf morphological and physiological adaptations to drought stress are necessary for the epiphytic lifestyle but limit CO2 assimilation and confound the likelihood of high productivity.

Carbon-Isotope Composition of Biochemical Fractions and the Regulation of Carbon Balance in Leaves of the C3-Crassulacean Acid Metabolism Intermediate Clusia minor L. Growing in Trinidad.

Carbon-isotope ratios ([delta]13Cs) were measured for various bio-chemical fractions quantitatively extracted from naturally exposed and shaded leaves of the C3-Crassulacean acid metabolism (CAM) intermediate Clusia minor, sampled at dawn and dusk on days during the wet and dry seasons in Trinidad. As the activity of CAM increased in response to decreased availability of water and higher photon flux density, organic acids and soluble sugars were enriched in 13C by approximately 3.5 to 4%[per mille (thousand) sign] compared to plants sampled during the wet season. The induction of CAM was accompanied by a doubling in size of the reserve carbohydrate pools. Moreover, stoichiometric measurements indicated that degradation of both chloroplastic reserves and soluble sugars were necessary to supply phosphoenolpyruvate for the synthesis of organic acids at night. Results also suggest that two pools of soluble sugars exist in leaves of C. minor that perform CAM, one a vacuolar pool enriched in 13C and the second a transport pool depleted in 13C. Estimates of carbon-isotope discrimination expressed during CAM, derived from the trafficking among inorganic carbon, organic acids, and carbohydrate pools overnight, ranged from 0.9 to 3.1%[per mille (thousand) sign]. The [delta]13C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C3 carboxylation.