Induction of Crassulacean Acid Metabolism in the Facultative Halophyte Mesembryanthemum crystallinum by Abscisic Acid

The facultative halophyte, Mesembryanthemum crystallinum, shifts its mode of carbon assimilation from the C₃ pathway to Crassulacean acid metabolism (CAM) in response to water stress. In this study, exogenously applied abscisic acid (ABA), at micromolar concentrations, could partially substitute for water stress in induction of CAM in this species. ABA at concentrations of 5 to 10 micromolar, when applied to leaves or to the roots in hydroponic culture or in soil, induced the expression of CAM within days (as indicated by the nocturnal accumulation of total titratable acidity and malate). After applying ABA there was also an increase in phosphoenolpyruvate carboxylase and NADP-malic enzyme activities. The degree and time course of induction by ABA were comparable to those induced by salt and water stress. Electrophoretic analyses of leaf soluble protein indicate that the increases in phosphoenolpyruvate carboxylase activity during the induction by ABA, salt, and water stress are due to an increase in the quantity of the enzyme protein. ABA may be a factor in the stress-induced expression of CAM in M. crystallinum, serving as a functional link between stress and biochemical adaptation.     

The occurrence of crassulacean acid metabolism in Cymbidium (Orchidaceae) and its ecological and evolutionary implications

Crassulacean acid metabolism (CAM) is one of the photosynthetic pathways regarded as adaptations to water stress in land plants. Little is known about correlations among the level of CAM activity, environment of habitat, life form, and phylogenetic relationship of a plant group from an evolutionary perspective. We examined these relationships in 18 species of Cymbidium (Orchidaceae) because the genus shows distinctive diversification of habitats and life forms. The photosynthetic type was classed into three categories, strong CAM, weak CAM, and C₃ on the basis of CAM activity. CAM expression in Cymbidium was confined to the epiphytic and lithophytic species. Especially, all of these species from tropical to subtropical rainforest exhibited CAM activity. On the other hand, the terrestrial species always exhibited C₃ metabolism irrespective of their varied habitats. Regarding the evolution of photosynthetic characters, weak CAM was the ancestral state in Cymbidium and strong CAM and C₃ metabolism occurred subsequently. The evolution of strong CAM likely enabled Cymbidium to extend to exposed sites in tropical lowland where marked water stress exists. Further, different levels of CAM activity characterized each species and such potential plasticity of CAM may realize the radiation of Cymbidium into sites with different environmental conditions.     

Leaf anatomy is not correlated to CAM function in a C3+CAM hybrid species,Yucca gloriosa

Background and AimsCrassulacean acid metabolism (CAM) is often considered to be a complex trait, requiring orchestration of leaf anatomy and physiology for optimal performance. However, the observation of trait correlations is based largely on comparisons between C₃ and strong CAM species, resulting in a lack of understanding as to how such traits evolve and the level of intraspecific variability for CAM and associated traits.MethodsTo understand intraspecific variation for traits underlying CAM and how these traits might assemble over evolutionary time, we conducted detailed time course physiological screens and measured aspects of leaf anatomy in 24 genotypes of a C₃+CAM hybrid species, Yucca gloriosa (Asparagaceae). Comparisons were made to Y. gloriosa’s progenitor species, Y. filamentosa (C₃) and Y. aloifolia (CAM).Key ResultsBased on gas exchange and measurement of leaf acids, Y. gloriosa appears to use both C₃ and CAM, and varies across genotypes in the degree to which CAM can be upregulated under drought stress. While correlations between leaf anatomy and physiology exist when testing across all three Yucca species, such correlations break down at the species level in Y. gloriosa.ConclusionsThe variation in CAM upregulation in Y. gloriosa is a result of its relatively recent hybrid origin. The lack of trait correlations between anatomy and physiology within Y. gloriosa indicate that the evolution of CAM, at least initially, can proceed through a wide combination of anatomical traits, and more favourable combinations are eventually selected for in strong CAM plants.     

Redox control of oxidative stress responses in the C3–CAM intermediate plant Mesembryanthemum crystallinum

Crassulacean acid metabolism (CAM) is named after the Crassulaceae family of succulent plants, in which this type of metabolism was first discovered at the beginning of the 19th century. In recent years, Mesembryanthemum crystallinum, a facultative halophyte and C₃–CAM intermediate plant, has become a favoured plant for studying stress response mechanisms during C₃–CAM shifts. Recent studies in this and related areas can provide a new model of how such mechanisms could operate for acclimation to high salinity or excess excitation energy. These include roles for photosynthetic electron transport chain components and reactive oxygen species. The diurnal rhythms of catalase, superoxide dismutase and some CAM-related enzyme activities are discussed in relation to the protective role of photorespiration during C₃–CAM transition. The role of excess excitation energy and redox events in the proximity of photosystem II (PSII) in regulation of ascorbate peroxidase (APX), superoxide dismutase (SOD): copper/zinc SOD (Cu/ZnSOD), iron SOD (FeSOD), and NAD(P)-malic enzyme gene expression are also discussed. We suggest a model in which the chloroplast plays a major role in regulation of acclimation to high salinity and/or excess exitation energy.     

C3 photosynthesis in the gametophyte of the epiphytic CAM fern Pyrrosia longifolia (Polypodiaceae)

Sporophytes of some epiphytic species in the fern genus Pyrrosia exhibit Crassulacean acid metabolism (CAM), generally considered to be a derived physiological response to xeric habitats. Because these species alternate between independent sporophytic and gametophytic generations yet only the sporophyte has been characterized physiologically, experiments were conducted to determine the photosynthetic pathways present in mature sporophytes, immature sporophytes, and gametophytes of Pyrrosia longifolia. Diurnal CO₂ exchange and malic acid fluctuations demonstrated that although the mature sporophytes exhibited CAM, only C₃ photosynthesis occurred in the gametophytes and young sporophytes. Consideration of the above results and those from previous studies, as well as the life cycle of ferns, indicates that the induction of CAM probably occurs at a certain developmental stage of the sporophyte and/or following exposure to stress. Elucidation of the precise mechanisms underlying this C₃-CAM transition awaits further research.     

Ancestry and adaptive radiation of Bacteroidetes as assessed by comparative genomics

To date, the phylum Bacteroidetes comprises more than 1,500 described species with diverse ecological roles. However, there is little understanding of archetypal Bacteroidetes traits at a genomic level. In this study, a representative set of 89 Bacteroidetes genomes was compiled, and pairwise reciprocal best-match gene comparisons and gene syntenies were used to identify common traits that allowed Bacteroidetes evolution and adaptive radiation to be traced. The type IX secretion system (T9SS) was highly conserved among all studied Bacteroidetes. Class-level comparisons furthermore suggested that the ACIII-caa₃COX super-complex evolved in the ancestral aerobic bacteroidetal lineage, and was secondarily lost in extant anaerobic Bacteroidetes. Another Bacteroidetes-specific respiratory chain adaptation was the sodium-pumping Nqr complex I that replaced the ancestral proton-pumping complex I in marine species. T9SS plays a role in gliding motility and the acquisition of complex macro-molecular organic compounds, and the ACIII-caa₃COX super-complex allows effective control of electron flux during respiration. This combination likely provided ancestral Bacteroidetes with a decisive competitive advantage to effectively scavenge, uptake and degrade complex organic molecules, and therefore has played a pivotal role in the successful adaptive radiation of the phylum.     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Overexpression of Arabidopsis ABF3 gene confers enhanced tolerance to drought and heat stress in creeping bentgrass

The Arabidopsis, abscisic acid responsive element-binding factor 3, ABF3 is known to play an important role in stress responses via regulating the expression of stress-responsive genes. In this study, we introduced pCAMBIA3301 vector harboring the ABF3 gene into creeping bentgrass (Agrostis stolonifera) through Agrobacterium-mediated transformation in order to develop a stress-tolerant variety of turfgrass. After transformation, putative transgenic plants were selected using the herbicide resistance assay. Genomic integration of the transgene was confirmed by genomic PCR and Southern blot analysis, and gene expression was validated by northern blot analysis. Under drought-stressed condition, the transgenic plants overexpressing ABF3 displayed significantly enhanced drought tolerance with higher water content and slower water loss rate than the control plants. Furthermore, the stomata of the ABF3 transgenic plants closed more than those of wild-type creeping bentgrass plants, under both non-stressed and ABA treatment conditions. In addition, the transgenic plants showed enhanced tolerance to heat stress. These results suggest that the overexpression of the ABF3 gene in creeping bentgrass might enhance survival in water-limiting and high temperature environments through increased stomatal closure and reduced water losses.     

Arabidopsis basic leucine zipper proteins that mediate stress-responsive abscisic acid signaling

The phytohormone abscisic acid (ABA) plays an essential role in adaptive stress responses. The hormone regulates, among others, the expression of numerous stress-responsive genes. From various promoter analyses, ABA-responsive elements (ABREs) have been determined and a number of ABRE binding factors have been isolated, although their in vivo roles are not known. Here we report that the ABRE binding factors ABF3 and ABF4 function in ABA signaling. The constitutive overexpression of ABF3 or ABF4 in Arabidopsis resulted in ABA hypersensitivity and other ABA-associated phenotypes. In addition, the transgenic plants exhibited reduced transpiration and enhanced drought tolerance. At the molecular level, altered expression of ABA/stress-regulated genes was observed. Furthermore, the temporal and spatial expression patterns of ABF3 and ABF4 were consistent with their suggested roles. Thus, our results provide strong in vivo evidence that ABF3 and ABF4 mediate stress-responsive ABA signaling.     

Is Sedum acre L. a CAM plant?

Summary Sedum acre L. collected from its natural stands south of Darmstadt (Germany) showed ¹³C values typical for C₃ plants. This suggests that in situ at the natural stand CO₂ was fixed mainly via the C₃ mode of photosynthesis rather than via the CAM mode. However, experimental water stress shifts the CO₂ exchange pattern from the C₃ type to CAM type. Simultaneously, a diurnal rhythm of malic acid oscillation, typical for CAM, and increase of PEP-carboxylase and malic enzyme activities developed. Hence, Sedum acre is obviously to be classified as a facultative CAM plant. Because of the temperature characteristics of CO₂ exchange in Sedum acre, in situ CO₂ should be harvested from the atmosphere mainly during the seasons where water stress situations capable of inducing CAM are unlikely to occur.     

Comparison of CAM expression,photochemistry and antioxidant responses in Sedum album and Portulaca oleracea under combined stress

Previous studies of crassulacean acid metabolism (CAM) pathway during stress have been directed at individual drought and salinity stress, here, we studied the effects of a combination of drought and salt on CAM expression, chlorophyll fluorescence and antioxidant parameters in the C₃-CAM facultative Sedum album and C₄-CAM facultative Portulaca oleracea plants. While salinity alone was not able to induce functional CAM expression in P. oleracea leaves, we showed that salinity induced low level of nocturnal acid accumulation in S. album species. After 20 d of exposure to the combination of simultaneous salt and drought stress, P. oleracea plants exhibited more resistance to photoinhibition as compared to S. album plants. The decrease of maximum quantum yield (F_v/F_m) in S. album leaves under combined stress was in parallel with the largest suppression of CAM expression of >50%, probably displaying the withdrawal of functional CAM back to C₃ pathway. However, under drought treatment alone, S. album plants exhibited higher photosynthetic flexibility, which was associated with the up-regulation of antioxidant enzymes activities and maintenance of glutathione (GSH) pool, and consequently higher photochemical functioning. The levels of nitric oxide (NO) correlated well with CAM expression, which was observed only in S. album, suggesting that NO acts in a different way in C₃ and C₄ species during CAM induction. Additionally, in both species, over the course of CAM induction, the changes in CAM expression parameters exhibited a similar pattern to that of antioxidant capacity and photochemical functioning parameters.     

Variable Photosynthetic Metabolism in Leaves and Stems of Cissus quadrangularis L

By measuring titratable acidity, gas exchange parameters, mesophyll succulence, and ¹³C/¹²C ratios, we have shown that Cissus quadrangularis L. has C₃-like leaves and stems with Crassulacean acid metabolism (CAM). In addition, the nonsucculent leaves show the diurnal fluctuations in organic acids termed recycling despite the fact that all CO₂ uptake and stomatal opening occurs during the day. Young succulent stems have more C₃ photosynthesis than older stems, but both have characteristics of CAM. The genus Cissus will be a fruitful group to study the physiology, ecology, and evolution of C₃ and CAM since species occur that exhibit characteristics of both photosynthetic pathways.     

Changes of the chlorophyll fluorescence induction kinetics of C3 and CAM plants during day/night cycles

The induction kinetics of the 680 nm chlorophyll fluorescence were measured on attached leaves of Kalanchoë daigremontiana R. Hamet et Perr. (CAM plant), Sedum telephium L. and Sedum spectabile Bor. (C₃ plant in spring, CAM plant in summer) and Raphanus sativus L. (C₃ plant) at three different times during a 12/12 h day/night cycle. During the fluorescence transient the fluorescence intensity at the O, P and T-level (f_O, f_max, f_st,) was different for the plant species tested; this may be due to their different leaf structure, pigment composition and organization of their photosystems. The kinetics of the fluorescence induction depended on the time of preillumination or dark adaptation during the light/dark cycle but not on the type of primary CO₂ fixation mechanism (C₃ and CAM). For dark adapted leaves measured either at the end of the dark phase or after dark adaptation of plants taken from the light phase a higher P-level fluorescence, a higher variable fluorescence (P-O) and a larger complementary area were found than for leaves of plants taken directly from the light phase. This indicates the presence of largely oxidized photosystem 2 acceptor pools during darkness. During the light phase the fluorescence decline after the P-level was faster than during the dark phase; from this we conclude that the light adaptation of the photosynthetic apparatus (state 1state 2 transition, pH) during the induction period proceeded faster in plants taken from the light phase than in plants taken from the dark phase.Abbreviations C₃ plant plant with primary CO₂ fixation on ribulose-1,5-bis-phosphate (Calvin-Benson cycle) - CAM Crassulacean Acid Metabolism