Phenotypic changes in the fluidity of the tonoplast membrane of crassulacean-acid-metabolism plants in response to temperature and salinity stress

Effects of chilling on the photosynthetic performance of the CAM orchid Phalaenopsis

INTRODUCTION

Crassulacean acid metabolism (CAM) is one of the three main metabolic adaptations for CO₂ fixation found in plants. A striking feature for these plants is nocturnal carbon fixation and diurnal decarboxylation of malic acid to feed Rubisco with CO₂ behind closed stomata, thereby saving considerable amounts of water. Compared to the effects of high temperatures, drought, and light, much less information is available about the effects of chilling temperatures on CAM plants. In addition a lot of CAM ornamentals are grown in heated greenhouses, urging for a deeper understanding about the physiological responses to chilling in order to increase sustainability in the horticultural sector.

METHODS

The present study focuses on the impact of chilling temperatures (10°C) for 3 weeks on the photosynthetic performance of the obligate CAM orchid Phalaenopsis 'Edessa'. Detailed assessments of the light reactions were performed by analyzing chlorophyll a fluorescence induction (OJIP) parameters and the carbon fixation reactions by measuring diel leaf gas exchange and diel metabolite patterns.

RESULTS AND DISCUSSION

Results showed that chilling already affected the light reactions after 24h. Whilst the potential efficiency of photosystem II (PSII) (F_v/F_m) was not yet influenced, a massive decrease in the performance index (PI_abs) was noticed. This decrease did not depict an overall downregulation of PSII related energy fluxes since energy absorption and dissipation remained uninfluenced whilst the trapped energy and reduction flux were upregulated. This might point to the presence of short-term adaptation mechanisms to chilling stress. However, in the longer term the electron transport chain from PSII to PSI was affected, impacting both ATP and NADPH provision. To avoid over-excitation and photodamage plants showed a massive increase in thermal dissipation. These considerations are also in line with carbon fixation data showing initial signs of cold adaptation by achieving comparable Rubisco activity compared to unstressed plants but increasing daytime stomatal opening in order to capture a higher proportion of CO₂ during daytime. However, in accordance with the light reactions data, Rubisco activity declined and stomatal conductance and CO₂ uptake diminished to near zero levels after 3 weeks, indicating that plants were not successful in cold acclimation on the longer term.

Evolution of Crassulacean acid metabolism in response to the environment: past, present, and future

Crassulacean acid metabolism (CAM) is a mode of photosynthesis that evolved in response to decreasing CO2 levels in the atmosphere some 20 million years ago. An elevated ratio of O2 relative to CO2 caused many plants to face increasing stress from photorespiration, a process exacerbated for plants living under high temperatures or in water-limited environments. Today, our climate is again rapidly changing and plants' ability to cope with and adapt to these novel environments is critical for their success. This review focuses on CAM plant responses to abiotic stressors likely to dominate in our changing climate: increasing CO2 levels, increasing temperatures, and greater variability in drought. Empirical studies that have assessed CAM responses are reviewed, though notably these are concentrated in relatively few CAM lineages. Other aspects of CAM biology, including the effects of abiotic stress on the light reactions and the role of leaf succulence, are also considered in the context of climate change. Finally, more recent studies using genomic techniques are discussed to link physiological changes in CAM plants with the underlying molecular mechanism. Together, the body of work reviewed suggests that CAM plants will continue to thrive in certain environments under elevated CO2. However, how CO2 interacts with other environmental factors, how those interactions affect CAM plants, and whether all CAM plants will be equally affected remain outstanding questions regarding the evolution of CAM on a changing planet.

Protection by light against heat stress in leaves of tropical crassulacean acid metabolism plants containing high acid levels

Heat tolerance of plants exhibiting crassulacean acid metabolism (CAM) was determined by exposing leaf sections to a range of temperatures both in the dark and the light, followed by measuring chlorophyll a fluorescence (Fv/Fm and F0) and assessing visible tissue damage. Three CAM species, Clusia rosea Jacq., Clusia pratensis Seem. and Agave angustifolia Haw., were studied. In acidified tissues sampled at the end of the night and exposed to elevated temperatures in the dark, the temperature that caused a 50% decline of Fv/Fm (T50), was remarkably low (40-43°C in leaves of C. rosea). Conversion of chlorophyll to pheophytin indicated irreversible tissue damage caused by malic acid released from the vacuoles. By contrast, when acidified leaves were illuminated during heat treatments, T50 was up to 50-51°C. In de-acidified samples taken at the end of the light period, T50 reached ∼54°C, irrespective of whether temperature treatments were done in the dark or light. Acclimation of A. angustifolia to elevated daytime temperatures resulted in a rise of T50 from ∼54° to ∼57°C. In the field, high tissue temperatures always occur during sun exposure. Measurements of the heat tolerance of CAM plants that use heat treatments of acidified tissue in the dark do not provide relevant information on heat tolerance in an ecological context. However, in the physiological context, such studies may provide important clues on vacuolar properties during the CAM cycle (i.e. on the temperature relationships of malic acid storage and malic acid release).

Temperature modulation of thermal tolerance of a CAM-tank bromeliad and the relationship with acid accumulation in different leaf regions

Physiological changes that increase plant performance during exposure to high temperatures may play an inverse role during exposure to low temperatures. The objective of this study was to test variations in photosystem II response to heat and cold stress in the leaves of a bromeliad with crassulacean acid metabolism submitted to high or low temperatures. Leaves were maintained under constant temperatures of 10 and 35°C and used to examine possible relationships among physiological responses to high and low temperatures and organic acid accumulation. We also tested if distinct parts of bromeliad leaves show differences in photosynthetic thermotolerance. The samples from leaves maintained at 35°C showed greater heat tolerance values, while those from leaves maintained at 10°C showed lower cold tolerance values. Our results identified a strong negative relationship between the organic acid accumulation and thermal tolerance of bromeliad leaves that largely explained the differences in thermal tolerance among groups. One of these differences occurred among regions of a single leaf, with the base showing critical heat values of up to 8°C higher than the top region, suggesting a possible partitioning of leaf response among its regions. Differences in thermal tolerance were also observed between sampling times, with higher values observed in the morning.

Seasonal variation in crassulacean acid metabolism by the aquatic isoetid Littorella uniflora

The seasonal temperature acclimation in crassulacean acid metabolism (CAM) and photosynthetic performance were investigated in the aquatic isoetid, Littorella uniflora. Plants were collected monthly from January to September, and CAM capacity and photosynthesis rates were measured at 5, 10, 15, and 20 °C. Seasonal acclimation was observed for CAM (Q(10) range: 0.6-1.8), and CAM was optimised close to ambient temperature throughout the season. Thus, in winter acclimated L. uniflora, the short-term response to raised temperature resulted in a decline in CAM capacity. Even though the ambient CAM increased from winter to spring/summer, CAM was present in cold acclimated plants, thus indicating an ecophysiological role for CAM even in winter. Similar to CAM, seasonal acclimation was observed in the light and carbon-saturated photosynthesis (Q(10) values ranged from 1.4 to 2.3), and the photosynthetic capacity was generally higher during the winter at all temperatures, indicating compensatory investments in the photosynthetic apparatus. Thus, L. uniflora displayed seasonal temperature acclimation with respect to both CAM and photosynthesis. The estimated in situ contribution of CAM to the carbon budget in L. uniflora was independent of season and varied from 23 to 46 %. A positive correlation between photosynthetic capacity and CAM capacity (both measured in the lab at temperature close to ambient temperature) was found, and the ratio of CAM activity to photosynthetic capacity was higher in summer compared with winter plants. Overall, the results from the present study support the suggested role of CAM as a carbon conserving mechanism of importance for survival in a carbon-limited habitat.

Identification of rhythmic subsystems in the circadian cycle of crassulacean acid metabolism under thermoperiodic perturbations

Leaves of the Crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier de la Bâthie show overt circadian rhythms in net CO2 uptake, leaf conductance to water and intercellular CO2 concentration, which are entrained by periodic temperature cycles. To probe their sensitivity to thermoperiodic perturbations, intact leaves were exposed to continuous light intensity and temperature cycles with a period of 16 h, applying a set of different baseline temperatures and thermodriver amplitudes. All three overt rhythms were analyzed with respect to their frequency spectra and their phase relations with the thermodriver. For most stimulation protocols, stomatal conductance and net CO2 change were fully or partially entrained by the temperature pulses, while the internal CO2 concentration remained dominated by oscillations in the circadian range. Prolonged time series recorded for up to 22 d in continuous light underline the robustness of these circadian oscillations. This suggests that the overt circadian rhythm of net CO2 uptake in CAM results from the interaction of two coupled original systems: (i) an endogenous cycle of CO2 fixation in the mesophyll, showing very robust periodic activity, and (ii) stomatal movements that respond to environmental stimuli independently of rhythmic processes in the mesophyll, and thus modulate the gas exchange amplitude.

Changes in physical properties of vacuolar membrane during transformation of protein bodies into vacuoles in germinating pumpkin seeds

Changes in membrane molecular dynamics associated with the transformation of protein body membranes into vacuolar membranes during pumpkin seed germination, were monitored by EPR-spin probe technique. Using highly purified membrane preparations as well as 5-SASL and 16-SASL spin labels, parameters like general membrane lipid fluidity, order parameter, semicone angle, rotational correlation times tau 2B and tau 2C, ratio of immobilized to mobile lipids were determined and the activation energy for rotational diffusion of 16-SASL was calculated. Analysis of these parameters at different temperatures indicated a more rigid nature of protein body membrane comparing to vacuolar membrane, as a result of a more restricted motional freedom of lipids. These differences are discussed in terms of protein composition and various functional specialization of both types of membranes.

A salinity-induced gene from the halophyte M. crystallinum encodes a glycolytic enzyme, cofactor-independent phosphoglyceromutase

In the facultative halophyte Mesembryanthemum crystallinum (ice plant), salinity stress triggers significant changes in gene expression, including increased expression of mRNAs encoding enzymes involved with osmotic adaptation to water stress and the crassulacean acid metabolism (CAM) photosynthetic pathway. To investigate adaptive stress responses in the ice plant at the molecular level, we generated a subtracted cDNA library from stressed plants and identified mRNAs that increase in expression upon salt stress. One full-length cDNA clone was found to encode cofactor-independent phosphoglyceromutase (PGM), an enzyme involved in glycolysis and gluconeogenesis. Pgm1 expression increased in leaves of plants exposed to either saline or drought conditions, whereas levels of the mRNA remained unchanged in roots of hydroponically grown plants. Pgm1 mRNA was also induced in response to treatment with either abscisic acid or cytokinin. Transcription run-on experiments confirmed that Pgm1 mRNA accumulation in leaves was due primarily to increased transcription rates. Immunoblot analysis indicated that Pgm1 mRNA accumulation was accompanied by a modest but reproductible increase in the level of PGM protein. The isolation of a salinity-induced gene encoding a basic enzyme of glycolysis and gluconeogenesis indicates that adaptation to salt stress in the ice plant involves adjustments in fundamental pathways of carbon metabolism and that these adjustments are controlled at the level of gene expression. We propose that the leaf-specific expression of Pgm1 contributes to the maintenance of efficient carbon flux through glycolysis/gluconeogenesis in conjunction with the stress-induced shift to CAM photosynthesis.

The role of crassulacean acid metabolism (CAM) in the adaptation of plants to salinity

Two case studies are presented illustrating how the behaviour of plants using crassulacean acid metabolism (CAM) provides adaptation to salinity. Perennial cacti having constitutive CAM show adaptation at the whole-plant level, engaging regulation of stomata, internal CO₂ -recycling and root physiology with salt exclusion. They are stress avoiders. Annual plants such as Mesembryanthemum crystallinum, with inducible CAM, are salt includers. They are stress-tolerant and show reactions at an array of levels: (i) regulation of turgor and gas exchange at the whole-plant level; (ii) metabolic adjustments at the cellular level; (iii) adapptive transport proteins at the membrane level and also (iv) at the macromolecular level; and (v) inductive changes at the gene expression level of the enzyme complement for metabolism (in particular involving glycolysis and malic-acid synthesis with phosphoenolpyruvate carboxylase (PEPC) as the key enzyme, and gluconeogenesis (with pyruvate-phosphate dikinase (PPDK) as a key enzyme) and membrane transport (in particular involving the tonoplast ATPase).