Temperature effects on malic-acid efflux from the vacuoles and on the carboxylation pathways in crassulacean-acid-metabolism plants

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.

Vegetative traits can predict flowering quality in Phalaenopsis orchids despite large genotypic variation in response to light and temperature

Phalaenopsis is an economically important horticultural ornamental, but its growth is slow and costly. The vegetative cultivation phase is long and required to ensure sufficient plant size. This is needed to develop high quality flowering plants. We studied the effects of temperature (27 or 31 °C) and light intensity (60 or 140 μmol m^-2 s^-1) on plant growth and development during the vegetative cultivation phase in two experiments, with respectively 19 and 14 genotypes. Furthermore, the after-effects of treatments applied during vegetative growth on flowering traits were determined. Increasing light intensity in the vegetative phase accelerated both vegetative plant growth and development. Increasing temperature accelerated vegetative leaf appearance rate, but strongly reduced plant and root biomass accumulation when temperatures were too high. Flowering was greatly affected by treatments applied during vegetative growth, and increased light and temperature increased number of flower spikes, and number of flowers and buds. Genotypic variation was large in Phalaenopsis, especially in traits related to flowering, thus care is needed when generalising results based on a limited number of cultivars. Plant biomass and number of leaves during vegetative growth were positively correlated with flowering quality. These traits can be used as an early predictor for flowering capacity and quality of the final product. Additionally, this knowledge can be used to improve selection of new cultivars.

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.

Nonlinear dynamics of the CAM circadian rhythm in response to environmental forcing

Crassulacean acid metabolism (CAM) photosynthesis functions as an endogenous circadian rhythm coupled to external environmental forcings of energy and water availability. This paper explores the nonlinear dynamics of a new CAM photosynthesis model (Bartlett et al., 2014) and investigates the responses of CAM plant carbon assimilation to different combinations of environmental conditions. The CAM model (Bartlett et al., 2014) consists of a Calvin cycle typical of C3 plants coupled to an oscillator of the type employed in the Van der Pol and FitzHugh-Nagumo systems. This coupled system is a function of environmental variables including leaf temperature, leaf moisture potential, and irradiance. Here, we explore the qualitative response of the system and the expected carbon assimilation under constant and periodically forced environmental conditions. The model results show how the diurnal evolution of these variables entrains the CAM cycle with prevailing environmental conditions. While constant environmental conditions generate either steady-state or periodically oscillating responses in malic acid uptake and release, forcing the CAM system with periodic daily fluctuations in light exposure and leaf temperature results in quasi-periodicity and possible chaos for certain ranges of these variables. This analysis is a first step in quantifying changes in CAM plant productivity with variables such as the mean temperature, daily temperature range, irradiance, and leaf moisture potential. Results may also be used to inform model parametrization based on the observed fluctuating regime.

The "Kluge-Lüttge Kammer": a preliminary evaluation of an enclosed, Crassulacean acid metabolism (CAM) Mesocosm that allows separation of synchronized and desynchronized contributions of plants to whole system gas exchange

Crassulacean acid metabolism (CAM) is recognized as a photosynthetic adaptation of plants to arid habitats. This paper presents a proof-of-concept evaluation of partitioning net CO2 exchanges for soil and plants in an arid, exclusively CAM mesocosm, with soil depth and succulent plant biomass approximating that of natural Sonoran Desert ecosystems. We present the first evidence that an enclosed CAM-dominated soil and plant community exposed to a substantial day/night temperature difference (30/20 degrees C), exhibits a diel gas exchange pattern consisting of four consecutive phases with a distinct nocturnal CO2 uptake. These phases were modulated by plant assimilation and soil respiration processes. Day-time stomatal closure of the CAM cycle during phase III was used to eliminate aboveground photosynthetic assimilation and respiration and thereby to estimate belowground plant plus soil respiration. Rapid changes in temperature appeared to synchronize single plant gas exchange but individual plant gas exchange patterns were desynchronized at constant day/night temperatures (25 degrees C), masking the distinct mesocosm pattern. Overall, the mean carbon budget of this CAM model Sonoran Desert system was negative, releasing an average of 22.5 mmol CO2 m-2 d-1. The capacity for nocturnal CO2 assimilation in this exclusively CAM mesocosm was inadequate to recycle CO2 released by plant and soil respiration.

Temperature effect on entrainment, phase shifting, and amplitude of circadian clocks and its molecular bases

Effects of temperature and temperature changes on circadian clocks in cyanobacteria, unicellular algae, and plants, as well as fungi, arthropods, and vertebrates are reviewed. Periodic temperature with periods around 24 h even in the low range of 1-2 degrees C (strong Zeitgeber effect) can entrain all ectothermic (poikilothermic) organisms. This is also reflected by the phase shifts-recorded by phase response curves (PRCs)-that are elicited by step- or pulsewise changes in the temperature. The amount of phase shift (weak or strong type of PRC) depends on the amplitude of the temperature change and on its duration when applied as a pulse. Form and position of the PRC to temperature pulses are similar to those of the PRC to light pulses. A combined high/low temperature and light/dark cycle leads to a stabile phase and maximal amplitude of the circadian rhythm-when applied in phase (i.e., warm/light and cold/dark). When the two Zeitgeber cycles are phase-shifted against each other the phase of the circadian rhythm is determined by either Zeitgeber or by both, depending on the relative strength (amplitude) of both Zeitgeber signals and the sensitivity of the species/individual toward them. A phase jump of the circadian rhythm has been observed in several organisms at a certain phase relationship of the two Zeitgeber cycles. Ectothermic organisms show inter- and intraspecies plus seasonal variations in the temperature limits for the expression of the clock, either of the basic molecular mechanism, and/or the dependent variables. A step-down from higher temperatures or a step-up from lower temperatures to moderate temperatures often results in initiation of oscillations from phase positions that are about 180 degrees different. This may be explained by holding the clock at different phase positions (maximum or minimum of a clock component) or by significantly different levels of clock components at the higher or lower temperatures. Different permissive temperatures result in different circadian amplitudes, that usually show a species-specific optimum. In endothermic (homeothermic) organisms periodic temperature changes of about 24 h often cause entrainment, although with considerable individual differences, only if they are of rather high amplitudes (weak Zeitgeber effects). The same applies to the phase-shifting effects of temperature pulses. Isolated bird pineals and rat suprachiasmatic nuclei tissues on the other hand, respond to medium high temperature pulses and reveal PRCs similar to that of light signals. Therefore, one may speculate that the self-selected circadian rhythm of body temperature in reptiles or the endogenously controlled body temperature in homeotherms (some of which show temperature differences of more than 2 degrees C) may, in itself, serve as an internal entraining system. The so-called heterothermic mammals (undergoing low body temperature states in a daily or seasonal pattern) may be more sensitive to temperature changes. Effects of temperature elevation on the molecular clock mechanisms have been shown in Neurospora (induction of the frequency (FRQ) protein) and in Drosophila (degradation of the period (PER) and timeless (TIM) protein) and can explain observed phase shifts of rhythms in conidiation and locomotor activity, respectively. Temperature changes probably act directly on all processes of the clock mechanism some being more sensitive than the others. Temperature changes affect membrane properties, ion homeostasis, calcium influx, and other signal cascades (cAMP, cGMP, and the protein kinases A and C) (indirect effects) and may thus influence, in particular, protein phosphorylation processes of the clock mechanism. The temperature effects resemble to some degree those induced by light or by light-transducing neurons and their transmitters. In ectothermic vertebrates temperature changes significantly affect the melatonin rhythm, which in turn exerts entraining (phase shifting) functions.

PEP carboxylase kinase is a novel protein kinase controlled at the level of expression

Phosphoenolpyruvate (PEP) carboxylase plays a number of key roles in the central metabolism of higher plants. The enzyme is regulated by reversible phosphorylation in response to a range of signals in many different plant tissues. The data discussed here illustrate several novel features of this system. The phosphorylation state of PEP carboxylase is controlled largely by the activity of PEP carboxylase kinase. This enzyme comprises a protein kinase catalytic domain with no regulatory regions. In many systems it is controlled at the level of expression. In C₄ plants, expression of PEP carboxylase kinase is light-regulated and involves changes in cytosolic pH, InsP₃ and Ca²⁺ levels. Expression of PEP carboxylase kinase in CAM plants is regulated by a circadian oscillator, perhaps via metabolite control. Some plants contain multiple PEP carboxylase kinase genes, probably with different expression patterns and roles. A newly discovered PEP carboxylase kinase inhibitor protein might facilitate the net dephosphorylation of PEP carboxylase under conditions in which flux through this enzyme is not required.

The regulation of phosphoenolpyruvate carboxylase in CAM plants

Phosphoenolpyruvate carboxylase catalyses the primary assimilation of CO(2) in Crassulacean acid metabolism plants. It is activated by phosphorylation, and this plays a major role in setting the day-night pattern of metabolism in these plants. The key factor that controls the phosphorylation state of phosphoenolpyruvate carboxylase is the activity of phosphoenolpyruvate carboxylase kinase. Recent work on Crassulacean acid metabolism plants has established this enzyme as a novel protein kinase and has provided new insights into the regulation of protein phosphorylation. Phosphoenolpyruvate carboxylase kinase is controlled by synthesis and degradation in response to a circadian oscillator. The circadian control of phosphoenolpyruvate carboxylase kinase can be overridden by changes in metabolite levels. The primary effect of the circadian oscillator in this system may be at the level of the tonoplast, and changes in kinase expression may be secondary to circadian changes in the concentration of a metabolite, perhaps cytosolic malate.

Metabolite Control Overrides Circadian Regulation of Phosphoenolpyruvate Carboxylase Kinase and CO(2) Fixation in Crassulacean Acid Metabolism

Phosphoenolpyruvate carboxylase (PEPc) catalyzes the primary fixation of CO(2) in Crassulacean acid metabolism plants. Flux through the enzyme is regulated by reversible phosphorylation. PEPc kinase is controlled by changes in the level of its translatable mRNA in response to a circadian rhythm. The physiological significance of changes in the levels of PEPc-kinase-translatable mRNA and the involvement of metabolites in control of the kinase was investigated by subjecting Kalanchoë daigremontiana leaves to anaerobic conditions at night to modulate the magnitude of malate accumulation, or to a rise in temperature at night to increase the efflux of malate from vacuole to cytosol. Changes in CO(2) fixation and PEPc kinase activity reflected those in kinase mRNA. The highest rates of CO(2) fixation and levels of kinase mRNA were observed in leaves subjected to anaerobic treatment for the first half of the night and then transferred to ambient air. In leaves subjected to anaerobic treatment overnight and transferred to ambient air at the start of the day, PEPc-kinase-translatable mRNA and activity, the phosphorylation state of PEPc, and fixation of atmospheric CO(2) were significantly higher than those for control leaves for the first 3 h of the light period. A nighttime temperature increase from 19 degrees C to 27 degrees C led to a rapid reduction in kinase mRNA and activity; however, this was not observed in leaves in which malate accumulation had been prevented by anaerobic treatment. These data are consistent with the hypothesis that a high concentration of malate reduces both kinase mRNA and the accumulation of the kinase itself.

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

Electron paramagnetic resonance-spectroscopic studies on spin-labeled purified tonoplast membranes showed that in the obligate crassulacean-acid-metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. the fluidity of the tonoplast decreased during acclimation to higher temperatures. This phenotypic change in tonoplast fluidity was paralleled by a decrease in the mobilization of malic acid from the vacuoles during CAM in the light. The shift from the C3 to the CAM mode of photosynthesis in the facultative CAM plant Mesembryanthemum crystallinum L. also led to a decrease in the fluidity of the tonoplast membrane. The results are consistent with the hypothesis that the ability to store malic acid during CAM in the vacuoles depends largely on the actual fluidity of the tonoplast membrane.

Tansley Review No. 37 Circadian rhythms: their origin and control

This article reviews the circadian rhythm of carbon dioxide metabolism in leaves of the Crassulacean plant Bryophyllum (Kalanchoë) fedtsckenkoi which persists both in continuous darkness and a CO₂ -free atmosphere, and in continuous light and normal air. Under both conditions the rhythm is due to the periodic activity of the enzyme phosphoenolpyruvate carboxylase (PEPc). The physiological characteristics of the rhythm are described in detail and, from these characteristics, hypotheses are advanced to account for both the generation of the rhythm and the regulation of its phase and period by environmental factors. The periodic activity of PEPc is ascribed to the periodic accumulation of an allosteric inhibitor, malate, in the cytoplasm and its subsequent removal either to the vacuole in continuous darkness, or by metabolism in continuous light. Also involved in the generation of the rhythm is a periodic change in the sensitivity of PEPc to malate inhibition due to the periodic phosphorylation and dephosphorylation of PEPc which changes its K₁ by a factor of 10 from 30 to 0.3 mM and vice versa. This periodic phosphorylation of PEPc is apparently achieved by the periodic synthesis and breakdown of a PEPc kinase which phosphorylates the enzyme on a serine residue; dephosphorylation is achieved by a type 2A phosphatase which shows no rhythmic variation. The induction of phase shifts in the rhythm in continuous darkness and CO₂ -free air has been explained in terms of light and high-temperature activated gates or channels in the tonoplast which, when open, allow malate to diffuse between the vacuole and cytoplasm. For the rhythm in continuous light and normal air phase, control by environmental signals can be attributed to changes in the malate levels in critical cell compartments, or in particular cell populations such as the stomatal guard cells, due to regulation of the malate synthesizing enzyme system involving PEPc, and malic enzyme which is responsible for malate metabolism. The role of the stomata in the generation of the rhythm is also discussed. The biochemical events which appear to give rise to the well-studied circadian rhythms in leaf movement in Samanea and Albizza, in luminescence in Gonyaulax polyedra and in the synthesis of the chlorophyll a/b binding protein are also reviewed in an attempt to identify similarities between these events and those involved in the Bryophyllum rhythm. Finally, the somewhat similar nature of the genes apparently responsible for circadian rhythmicity in Neurospora and Drosophila are discussed, and suggestions made for utilizing anti-sense nucleic acid technology in the further elucidation of the critical biochemical events involved in the basic, temperature-compensated circadian oscillator in living organisms. CONTENTS Summary 347 I. Introduction 348 II. Occurrence of circadian rhythms 348 III. Physiological characteristics of circadian rhythms 349 IV. Biochemical and molecular events involved in the circadian rhythm in Bryophyllum leaves 362 V. Biochemical and molecular events involved in the origin and control of circadian rhythmicity in other organisms 366 VI. Genetic studies 370 VII. Conclusion 371 References 372.

Phase resetting of the circadian rhythm of carbon dioxide assimilation inBryophyllum leaves in relation to their malate content following brief exposure to high and low temperatures, darkness and 5% carbon dioxide

Leaves ofBryophyllum fedtschenkoi show a persistent circadian rhythm in CO2 assimilation when kept in continuous illumination and normal air at 15°C. The induction of phase shifts in this rhythm by exposing the leaves for four hours at different times in the circadian cycle to 40° C, 2° C, darkness and 5% CO2 have been investigated. Exposure to high temperature has no effect on the phase at the apex of the peak but is effective at all other times in the cycle, whereas exposure to low temperature, darkness or 5% CO2 is without effect between the peaks and induces a phase shift at all other times. The next peak of the rhythm occurs 17 h after a 40° C treatment and 7-10 h after a 2° C, dark or 5% CO2 treatment regardless of their position in the cycle. When these treatments are given at times in the cycle when they induce maximum phase shifts, they cause no change in the gross malate status of the leaf. The gross malate content of the leaf in continuous light and normal air at 15% shows a heavily damped circadian oscillation which virtually disappears by the time of the third cycle, but the CO2 assimilation rhythm persists for many days. The generation of the rhythm, and the control of its phase by environmental factors are discussed in terms of mechanisms that involve the synthesis and metabolism of malate in specific localised pools in the cytoplasm of the leaf cells.

Period and phase control by temperature in the circadian rhythm of carbon dioxide fixation in illuminated leaves of Bryophyllum fedtschenkoi

The rhythm of CO2 assimilation exhibited by leaves of Bryophyllum fedtschenkoi maintained in light and normal air occurs only at constant ambient temperatures between 10°C and 30°C. Over this range the period increases linearly with increasing temperature from the extremely low value of 15.7 h to 23.3 h, but shows a considerable degree of temperature compensation. Outside the range 10°C-30°C the rhythm is inhibited but re-starts on changing the temperature to 15°C. Prolonged exposure of leaves to high (40°C) and low (2°C) temperature inhibits the rhythm by driving the basic oscillator to fixed phase points in the cycle which differ by 180°, and which have been characterised in terms of the malate status of the leaf cells. At both temperatures loss of the circadian rhythm of CO2 assimilation is due to the inhibition of phosphoenolpyruvate carboxylase (PEPCase) activity, but the inhibition is apparently achieved in different ways at 40°C and 2°C. High temperature appears to inhibit directly PEPCase activity, but not the activity of the enzymes responsible for the breakdown of malate, with the result that the leaf acquires a low malate status. In contrast, low temperature does not directly inhibit PEPCase activity, but does inhibit enzymes responsible for malate breakdown, so that the malate level in the leaf increases to a high value and PEPCase is eventually allosterically inhibited. The different malate status of leaves held at these two temperatures accounts for the phases of the rhythms being reversed on returning the leaves to 15°C. After exposure to high temperature, CO2 fixation by PEPCase activity can begin immediately, whereas after exposure to low temperature, the large amount of malate accumulated in the leaves has to be decarboxylated before CO2 fixation can begin.