Cell organelles from crassulacean acid metabolism (CAM) plants : II. Compartmentation of enzymes of the crassulacean acid metabolism

Laying the Foundation for Crassulacean Acid Metabolism (CAM) Biodesign: Expression of the C4 Metabolism Cycle Genes of CAM in Arabidopsis

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that exploits a temporal CO2 pump with nocturnal CO2 uptake and concentration to reduce photorespiration, improve water-use efficiency (WUE), and optimize the adaptability of plants to hotter and drier climates. Introducing the CAM photosynthetic machinery into C3 (or C4) photosynthesis plants (CAM Biodesign) represents a potentially breakthrough strategy for improving WUE while maintaining high productivity. To optimize the success of CAM Biodesign approaches, the functional analysis of individual C4 metabolism cycle genes is necessary to identify the essential genes for robust CAM pathway introduction. Here, we isolated and analyzed the subcellular localizations of 13 enzymes and regulatory proteins of the C4 metabolism cycle of CAM from the common ice plant in stably transformed Arabidopsis thaliana. Six components of the carboxylation module were analyzed including beta-carbonic anhydrase (McBCA2), phosphoenolpyruvate carboxylase (McPEPC1), phosphoenolpyruvate carboxylase kinase (McPPCK1), NAD-dependent malate dehydrogenase (McNAD-MDH1, McNAD-MDH2), and NADP-dependent malate dehydrogenase (McNADP-MDH1). In addition, seven components of the decarboxylation module were analyzed including NAD-dependent malic enzyme (McNAD-ME1, McNAD-ME2), NADP-dependent malic enzyme (McNADP-ME1, NADP-ME2), pyruvate, orthophosphate dikinase (McPPDK), pyruvate, orthophosphate dikinase-regulatory protein (McPPDK-RP), and phosphoenolpyruvate carboxykinase (McPEPCK). Ectopic overexpression of most C4-metabolism cycle components resulted in increased rosette diameter, leaf area, and leaf fresh weight of A. thaliana except for McNADP-MDH1, McPPDK-RP, and McPEPCK. Overexpression of most carboxylation module components resulted in increased stomatal conductance and dawn/dusk titratable acidity (TA) as an indirect measure of organic acid (mainly malate) accumulation in A. thaliana. In contrast, overexpression of the decarboxylating malic enzymes reduced stomatal conductance and TA. This comprehensive study provides fundamental insights into the relative functional contributions of each of the individual components of the core C4-metabolism cycle of CAM and represents a critical first step in laying the foundation for CAM Biodesign.

Mitochondrial respiration in ME-CAM, PEPCK-CAM, and C₃ succulents: comparative operation of the cytochrome, alternative, and rotenone-resistant pathways

Mitochondria are important in the function and control of Crassulacean acid metabolism (CAM) during organic acid accumulation at night and acid decarboxylation in the day. In plants of the malic enzyme-(ME) type and the phosphoenolpyruvate carboxykinase- (PEPCK) type, mitochondria may exert their role in the control of the diurnal rhythm of malic and citric acids to a differential degree. In plants of both CAM types, the oxidative capacity of mitochondria, as well as the activity of CAM-linked mitochondrial enzymes, and of the alternative and the rotenone-resistant pathways of substrate oxidation were compared. Furthermore, a C₃ succulent was included, as well as both C₃ and CAM forms of Mesembryanthemum crystallinum during a salt-induced C₃-to-CAM shift. Mitochondria of PEPCK-type CAM plants exhibited a lower activity of malate oxidation, ratio of malate to succinate oxidation, and activity of mitochondrial NAD-ME. With the exception of Kalanchoë daigremontiana, leaf mitochondria of all other CAM species were highly sensitive to cyanide (80-100%), irrespective of the oxidant used. This indicates that the alternative oxidase is not of general importance in CAM. By contrast, rotenone-insensitive substrate oxidation was very high (50-90%) in all CAM species. This is the first comparison of the rotenone-insensitive pathway of respiration in plants with different CAM-types. The results of this study confirm that mitochondria are involved in the control of CAM to different degrees in the two CAM types, and they highlight the multiple roles of mitochondria in CAM.

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.

A comparative immunocytochemical localization study of phosphoenolpyruvate carboxylase in leaves of higher plants

The intracellular localization of phosphoenolpyruvate (PEP) carboxylase in plants belonging to the C4, Crassulacean acid metabolism (CAM) and C3 types was invetigated using an immunocytochemical method with an immune serum raised against the sorghum leaf enzyme. The plants studied were sorghum, maize (C4 type), kalanchoe (CAM type), french bean, and spinach (C3 type). In the green leaves of C4 plants, it was shown that the carboxylase was located in the mesophyll and stomatic cells, being largely cytosolic in the mesophyll cells. Similarly, in CAM plants, the enzyme was found mainly outside the chloroplasts. In contrast, in C3 plants, the PEP carboxylase appeared to be distributed between the cytosol and the chloroplasts of foliar parenchyma. Examination of sections from etiolated leaves showed fluorescence emission from etioplasts and cytosol for the parenchyma of french bean as well as for the bundle sheath and mesophyll of sorghum leaves. This data indicated that during the greening process photoregulation and evolution of PEP carboxylase is dependent on the tissue and on the metabolic type of the plant considered.

On the cellular localization of phosphoenolpyruvate carboxylase in Sorghum leaves

The localization of phosphoenol pyruvate carboxylase (EC 4.1.1.3.1.) in the leaf cells of Sorghum vulgare was investigated by using three techniques: the conventional aqueous and non aqueous methods gave conflicting results; the immunocytochemical techniques clearly showed that the enzyme is predominantly located in the cytoplasm of mesophyll cells.