Crassulacean Acid Metabolism Photosynthetic Pathway Research Articles

Exploring molecular evolution of Rubisco in C3 and CAM Orchidaceae and Bromeliaceae

BackgroundThe CO2-concentrating mechanism associated to Crassulacean acid metabolism (CAM) alters the catalytic context for Rubisco by increasing CO2 availability and provides an advantage in particular ecological conditions. We hypothesized about the existence of molecular changes linked to these particular adaptations in CAM Rubisco. We investigated molecular evolution of the Rubisco large (L-) subunit in 78 orchids and 144 bromeliads with C3 and CAM photosynthetic pathways. The sequence analyses were complemented with measurements of Rubisco kinetics in some species with contrasting photosynthetic mechanism and differing in the L-subunit sequence.ResultsWe identified potential positively selected sites and residues with signatures of co-adaptation. The implementation of a decision tree model related Rubisco specific variable sites to the leaf carbon isotopic composition of the species. Differences in the Rubisco catalytic traits found among C3 orchids and between strong CAM and C3 bromeliads suggested Rubisco had evolved in response to differing CO2 concentration.ConclusionsThe results revealed that the variability in the Rubisco L-subunit sequence in orchids and bromeliads is composed of coevolving sites under potential positive adaptive signal. The sequence variability was related to δ13C in orchids and bromeliads, however it could not be linked to the variability found in the kinetic properties of the studied species.

Expression Profiles of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxylase Kinase Genes in Phalaenopsis, Implications for Regulating the Performance of Crassulacean Acid Metabolism.

Phalaenopsis is one of the most important potted plants in the ornamental market of the world. Previous reports implied that crassulacean acid metabolism (CAM) orchids at their young seedling stages might perform C3 or weak CAM photosynthetic pathways, but the detailed molecular evidence is still lacking. In this study, we used a key species in white Phalaenopsis breeding line, Phalaenopsis aphrodite subsp. formosana, to study the ontogenetical changes of CAM performance in Phalaenopsis. Based on the investigations of rhythms of day/night CO2 exchange, malate contents and phosphoenolpyruvate carboxylase (PEPC) activities, it is suggested that a progressive shift from C3 to CAM occurred as the protocorms differentiated the first leaf. To understand the role of phosphoenolpyruvate carboxylase kinase (PEPC kinase) in relation to its target PEPC in CAM performance in Phalaenopsis, the expression profiles of the genes encoding PEPC (PPC) and PEPC kinase (PPCK) were measured in different developmental stages. In Phalaenopsis, two PPC isogenes were constitutively expressed over a 24-h cycle similar to the housekeeping genes in all stages, whereas the significant day/night difference in PaPPCK expression corresponds to the day/night fluctuations in PEPC activity and malate level. These results suggest that the PaPPCK gene product is most likely involved in regulation of CAM performance in different developmental stages of Phalaenopsis seedlings.

Unified representation of the C3, C4, and CAM photosynthetic pathways with the Photo3 model

Recent interest in crassulacean acid metabolism (CAM) photosynthesis has resulted in new, physiologically based CAM models. These models show promise, yet typically are not developed with a basis that is compatible with widely used models of C3 and C4 photosynthesis. Indeed, most efforts to assess the potential of CAM still rely on empirically based environmental productivity indices, which makes uniform comparisons between CAM and non-CAM species difficult. In order to represent C3, C4, and CAM photosynthesis in a consistent, physiologically based manner, we introduce the Photo3 model. Photo3 unites a common photosynthetic and hydraulic core with components depicting the circadian rhythm of CAM photosynthesis and the carbon-concentrating mechanism of C4 photosynthesis. This work allows consistent comparisons of the three photosynthetic types for the first time. It also allows the representation of intermediate C3-CAM behavior through the adjustment of a single model parameter. Model simulations of Opuntia ficus-indica (CAM), Sorghum bicolor (C4), and Triticum aestivum (C3) capture the diurnal behavior of each species as well as the cumulative effects of long-term water limitation. These results show the model's potential for evaluating the tradeoffs between C3, C4, and CAM photosynthesis, and for better understanding CAM productivity, ecology, and climate feedbacks.

Potential of Plants from the Genus Agave as Bioenergy Crops

Agave is a succulent genus within the monocot family Agavaceae. The plants have a large rosette of thick fleshy leaves, each ending generally in a sharp point, and are native to arid and semi-arid regions from the southern USA to northern South America. The most important commercial species is Agave tequilana grown for production of tequila. Several cultivated species of Agave such as Agave sislana and Agave salmiana can perform well in areas where rainfall is insufficient for the cultivation of many C3 and C4 crops. The key feature of the crassulacean acid metabolism photosynthetic pathway used by agaves is the stomata opening and CO2 uptake during the night, thus allowing less water to be lost by transpiration. Alcoholic beverages, sweeteners, fibers, and some specialty chemicals are currently the main products coming from agave plants. The recovered information related to productivity, biofuel processability, by-products, etc. suggests that some Agave species have a real potential to compete economically with other bioenergy crops. But more than compete, it could complement the list of bioenergy crops due to its capacity to grow with very little rainfall and/or inputs and still reach good amount of biomass, so unused semi-arid land could be productive. Although Agave has great potential to be developed as a bioenergy crop, more laboratory and field research are needed.

Soil salinity determines the relative abundance of C3/C4 species in Argentinean grasslands

ABSTRACTAim The C4 and crassulacean acid metabolism (CAM) pathways are adaptations to compensate for high rates of photorespiration and water and carbon deficiency. This is the first attempt to compare the relative abundance of C3 vs. C4 + CAM species in temperate and subtropical grasslands across a latitudinal gradient in central Argentina. We predict that under the same rainfall regime, C4 + CAM plants will have larger soil coverage in highly saline soils than in neighbouring non‐saline ones.Location Data were taken from three phytogeographical provinces in the Santa Fe province of Argentina: Chaquenian, Pampean and Espinal.Methods The salinity of the soil was estimated through aqueous solution conductivity. The proportions of species belonging to C3/C4 + CAM photosynthetic pathways were compared among halophyte and non‐halophyte communities with a χ2 homogeneity test. The sum of cover percentages corresponding to the C3 and C4 + CAM photosynthetic pathways were calculated and compared using analysis of variance (ANOVA).Results The soil conductivity values were higher in the halophyte than in the non‐halophyte communities for the same phytogeographical area. The C4 + CAM plants had much higher soil coverage values in halophyte than in non‐halophyte communities in the Pampean and Espinal phytogeographical provinces. The differences were not statistically significant in the Chaquenian province.Main conclusions Soil drought provoked by soil salinity results in a much higher soil cover by C4 + CAM plants in regions with positive to neutral water balance (i.e. Pampean and Espinal). This differential abundance pattern in C4 + CAM functional group is not observed in areas where a pronounced water deficit exists per se (Chaquenian region), and therefore C4 + CAM plants predominate in all environments regardless of soil salinity. Our results suggest that one of the main environmental forces driving the upsurge of C4 species in Argentinean grasslands might have been the strong local soil salinity gradient.

The Occurrence and Phylogenetics of Crassulacean Acid Metabolism in the Portulacaceae

The plant family Portulacaceae is a relatively small family (∼19 genera and ∼425 species) with a widespread distribution. Species of the Portulacaceae range in size from small herbaceous plants to shrubs that measure up to 4 m in height. Most members of the family have leaves that range from fleshy to fully succulent and live in very diverse habitats ranging from shady understory plants to plants exposed to full sunlight. The Portulacaceae have members that utilize different photosynthetic pathways ranging from C3 to C4 to Crassulacean acid metabolism (CAM) to CAM‐cycling plants. We are interested in the prevalence of the CAM photosynthetic pathway in this family. We studied the nocturnal CO2 uptake, diurnal acid fluctuation, and phosphoenolpyruvate (PEP) carboxylase activity of different members of the Portulacaceae. Our results indicate the highest levels of CAM activity in the genera of Anacampseros, Portulacaria, Ceraria, and Talinopsis. Intermediate in their CAM activity were the genera Talinum, Port...

Assessing the carbon dioxide content of ancient atmospheres using palaeocalcretes: theoretical and empirical contraints

Recent suggestions that the δ 13 C composition of calcretes can be used to determine the P (CO 2 ) of ancient atmospheres is critically evaluated. The problem of differentiating pedogenic calcretes from groundwater ones (which could not be used for P (CO 2 ) determinations) is considered a critical problem. A basic tenet of the use of calcretes for assessing P(CO 2 ) is that plants using C 4 or cras-sulacean acid metabolism photosynthetic pathways were unimportant until the Miocene. This view is questioned, and evidence is presented from Carboniferous calcified root mats that non-C 3 plants were probably present at this time, confirming speculations by palaeobotanists that crassulacean acid metabolism or C 4 photosynthesis may have arisen numerous times during the evolution of higher plants.

Eco-physiological Differences between High and Low Elevation CAM Species in Big Bend National Park, Texas

There are distinct high and low elevation assemblages of crassulacean acid metabolism (CAM) plants in Big Bend National Park, Texas. The elevational distribution of the CAM photosynthetic pathway community importance is bimodal; while the pathway is most dominant in low elevation desert scrub communities, it is also important in high elevation woodlands. The principal members of these groups (A gave lecheguilla at low and Opuntia engelmannii at high elevations) were examined in both the laboratory and the field to describe differences in gas exchange patterns between the two assemblages. Primary production potential, expressed as CO2 assimilation capacity and diurnal acidification, and water economy, expressed as transpiration and transpiration ratio (TR), were examined. Low elevation CAM species have lower CO2 assimilation potential but have more efficient water utilization and conservation mechanisms, while high elevation CAM species have higher CO2 assimilation potential but less efficient water conservation. These differences are discussed in light of interspecific competitive ability with plants using alternate photosynthetic pathways (C3 and C4), macroclimatic differences along the elevational gradient and basic distributional patterns.