Zulfira Rakhmankulova

Crassulacean Acid Metabolism (CAM) represents an alternative pathway of carbon fixation that enables plants to achieve high water-use efficiency and maintain competitive advantages under warm and arid climatic conditions. CAM plants are not only ecologically significant but also serve as important crops for food, feed, and bioenergy production, demonstrating exceptional resilience to climate change. Over the past decade, advances in systems biology, omics technologies, and bioinformatics have significantly deepened our understanding of CAM evolution, diel metabolic regulation, and environmentally induced CAM expression. In parallel, innovative genetic engineering approaches, including CRISPR/Cas-based techniques, have expanded the potential to introduce CAM traits into drought-sensitive major crops, improving their drought tolerance and productivity. In this review, we provide a comprehensive analysis of the molecular, metabolic, physiological, and ecological characteristics of CAM plants. We also highlight promising research directions, focusing on metabolic pathways, regulatory networks, and stress-responsive signaling systems, with the aim of predicting ecophysiological responses and evaluating the prospects for practical application of CAM traits in agriculture under global climate change.

This paper investigates the diversity of physiological responses of the C₂ halophyte Sedobassia sedoides to constant and fluctuating salinity. Experimental plants, grown from seeds of two populations naturally inhabiting areas with constant (P1) and fluctuating (P2) salinity, were subjected to different salinity regimes: constant salinity, alternating salinity and recovery periods, and a control condition with no salinity treatment. The effects of salinity, recovery and re-salinization on parameters of growth and water-salt balance, CO₂/H₂O gas exchange, efficiency of photosystem II (PSII) and activity of cyclic electron transport (CET) around photosystem I (PSI), proline content, activity of antioxidant enzymes, and expression levels of genes encoding components of PSI, PSII, and CET proteins were studied. P1 plants were found to produce more biomass and were more tolerant to constant salinity than P2 plants. However, P2 plants demonstrated greater tolerance to fluctuating salinity. Differences in mechanisms of adaptation to constant salinity between the populations were observed. Thus, changes in regulation of stomatal function and C₂ carbon concentrating mechanism (CCM) played a significant role in the adaptation of P1 plants, whereas in P2 plants proline accumulation and reactive oxygen species (ROS) scavenging were more important. The two studied populations differed in their recovery processes. After the recovery period, P1 plants demonstrated low stomatal conductance, whereas P2 plants restored control level of stomatal conductance. Apparently, P2 plants possess acclimation mechanisms, allowing them to open their stomata and resume CO₂ assimilation during substrate desalination. After re-salinization, P1 plants differed from plants grown under constant salinity solely in the expression of psaA, whereas P2 plants had higher expression of psaA and PnsB5, as well as higher levels of carbon dioxide compensatory point (Γ) and assimilation at high CO₂ concentrations than under constant salinity, which suggests different acclimation strategies between populations and a pronounced stress memory possessed by P2 plants.

Extreme weather events such as higher temperatures, droughts, and soil salinization are projected to increase as atmospheric CO₂ concentrations rise and climate change progresses. These factors have a negative impact on global food security, the water supply, and ecosystem productivity. The focus of this review is on modern concepts, comparative studies, and our data on the mechanisms of adaptation of halophytes and glycophytes with different types of photosynthetic metabolism (C₃, C₄) to the individual and combined effects of climatic factors. The analysis revealed that C₃ and C₄ species and C₄-NAD-ME and C₄-NADP-ME species differ in terms of stability and photosynthetic plasticity. Under drought conditions, both individually and in combination with other factors, C₄ halophytes demonstrate the advantages of efficient photosynthesis and salt tolerance. Halophytes with C₄-NADP-ME are characterized by uniquely high levels of plasticity and variability in photosynthetic metabolism. This is reflected in their ability to mitigate the negative effects of elevated temperatures and drought through the use of elevated CO₂ (eCO₂). The mitigating effect of eCO₂ on photosynthesis at elevated temperatures was not detected in halophytes, regardless of photosynthesis type. Halophytes possess an augmented capacity for heat tolerance. Integrating fundamental scientific knowledge with urgent practical needs will enable us to predict changes in ecosystems and create new, sustainable agricultural systems.

Plant growth and productivity are predicted to be affected by rising CO₂ concentrations, drought and temperature stress. The C₃ crop model in a changing climate is Chenopodium quinoa Willd—a protein-rich pseudohalphyte (Amaranthaceae). Morphophysiological, biochemical and molecular genetic studies were performed on quinoa grown at ambient (400 ppm, aCO₂) and elevated (800 ppm, eCO₂) CO₂ concentrations, drought (D) and/or high temperature (eT) treatments. Among the single factors, drought caused the greatest stress response, inducing disturbances in the light and dark photosynthesis reactions (PSII, apparent photosynthesis) and increasing oxidative stress (MDA). Futhermore, compensation mechanisms played an important protective role against eT or eCO₂. The disruption of the PSII function was accompanied by the activation of the expression of PGR5, a gene of PSI cyclic electron transport (CET). Wherein under these conditions, the constant Rubisco content was maintained due to an increase in its biosynthesis, which was confirmed by the activation of rbcL gene expression. In addition, the combined stress treatments D+eT and eCO₂+D+eT caused the greatest negative effect, as measured by increased oxidative stress, decreased water use efficiency, and the functioning of protective mechanisms, such as photorespiration and the activity of antioxidant enzymes. Furthermore, decreased PSII efficiency and increased non-photochemical quenching (NPQ) were not accompanied by the activation of protective mechanisms involving PSI CET. In summary, results show that the greatest stress experienced by C. quinoa plants was caused by drought and the combined stresses D+eT and eCO₂+D+eT. Thus, drought consistently played a decisive role, leading to increased oxidative stress and a decrease in defense mechanism effectiveness.

The effect of elevated CO₂ (eCO₂) on changes in the morphophysiological, biochemical, and molecular genetic parameters of the C₄-NADP halophyte Kochia prostrata (L.) Schrad. under short-term water deficit (WD) and/or elevated temperatures (eT) was studied. The changes in the parameters of growth, water–salt balance, CO₂/H₂O gas exchange, PSII efficiency, PSI cyclic electron transport (CET) activity, photosynthetic gene expression, key carboxylation enzyme content, and antioxidative system activity were studied. Plants reacted more negatively to water deficit than to other individual factors (eCO₂ or eT). The expression of genes encoding components of PSI (psaA, psaB), ribulose-1,5-bisphosphate carboxylase/oxygenase Rubisco (rbcL), and pyruvate phosphate dikinase (PPDK) was downregulated, and a decrease was observed in the content of the photosynthetic enzymes Rubisco and phosphoenolpyruvate carboxylase (PEPС) and fresh biomass, PSII efficiency, apparent photosynthesis, and transpiration rates, indicating a stomatal and metabolic limitation of photosynthesis associated with insufficient functioning of the C₄ carbon-concentrating mechanism. eT did not significantly affect the growth parameters and water–salt balance of K. prostrata. Decreased apparent photosynthesis intensity at eT was mainly associated with stomatal limitation (decreased transpiration). The most severe stress was caused by eT + WD. A decrease in psaA, psaB, rbcL, and PPDK expression, PSII efficiency and PSI CET activity, photosynthesis and transpiration intensity, fresh biomass and water content and an increase in proline accumulation and oxidative stress indices were observed. The effect of eCO₂ mitigated the negative effect of individual and combined water deficit and temperature stress on CO₂/H₂O gas exchanges (apparent photosynthesis, transpiration) but enhanced their negative effect on PSII functioning. Additionally, under eT + WD, eCO₂ contributed to a significant increase in proline content and the activation of antioxidant defense, with the participation of catalase, phenolic compounds, and PSI CET. Overall, eCO₂ affected the mechanisms of homeostatic water balance and antioxidant defense and the ratio of light and dark reactions of photosynthesis during the adaptation of K. prostrata to drought and/or eT compared with ambient CO₂ conditions.

The adaptation of plants to combined stresses requires unique responses capable of overcoming both the negative effects of each individual stress and their combination. Here, we studied the C₃-C₄ (C₂) halophyte Sedobassia sedoides in response to elevated temperature (35 °C) and salinity (300 mM NaCl) as well as their combined effect. The responses we studied included changes in water–salt balance, light and dark photosynthetic reactions, the expression of photosynthetic genes, the activity of malate dehydrogenase complex enzymes, and the antioxidant system. Salt treatment led to altered water–salt balance, improved water use efficiency, and an increase in the abundance of key enzymes involved in intermediate C₃-C₄ photosynthesis (i.e., Rubisco and glycine decarboxylase). We also observed a possible increase in the activity of the C₂ carbon-concentrating mechanism (CCM), which allowed plants to maintain high photosynthesis intensity and biomass accumulation. Elevated temperatures caused an imbalance in the dark and light reactions of photosynthesis, leading to stromal overreduction and the excessive generation of reactive oxygen species (ROS). In response, S. sedoides significantly activated a metabolic pathway for removing excess NADPH, the malate valve, which is catalyzed by NADP-MDH, without observable activation of the antioxidant system. The combined action of these two factors caused the activation of antioxidant defenses (i.e., increased activity of SOD and POX and upregulation of FDI), which led to a decrease in oxidative stress and helped restore the photosynthetic energy balance. Overall, improved PSII functioning and increased activity of PSI cyclic electron transport (CET) and C₂ CCM led to an increase in the photosynthesis intensity of S. sedoides under the combined effect of salinity and elevated temperature relative to high temperature alone.

Biochemical, physiological, and anatomical features of salt tolerance of Sedum dendroideum Moc. & Sessé ex DC species with obligate CAM photosynthesis were studied at moderate (300 mM NaCl) and high (500 mM NaCl) salinity. Differences in the response of plants to different levels of salinity were revealed. Under moderate salinity, an increase in photosynthetic CAM metabolism was observed, which was accompanied by a significant increase in night-time acidification of cell sap in tissues, the content of PEP carboxylase and proline, and activation of cyclic electron transport around PS I and superoxide dismutase activity. Significant anatomical changes in mesophyll cells of leaf and stem were revealed: compression of vascular bundles, granularity of the cytoplasm, plasmolysis of leaf mesophyll parenchyma cells and their displacement towards the vascular bundle. A number of parameters changed depending on the level of salinity and these changings were more pronounced at high salinity. Significant reduction in leaf biomass, accumulation of sodium ions, increased plasmolysis of leaf cells, and an increase in the volume of water-bearing parenchyma cells of the stem, accompanied by the destruction of their cell walls (lysis) was observed at high salinity. Oxidative stress was observed only under high salinity but was mild, probably due to a significant increase in peroxidase activity, the content of fatty acid esters and phenolic compounds, namely flavans. Thus, an increase in the photosynthetic CAM pathway was observed only under moderate salinity. Under high salinity, the decisive factor in the adaptation process was the activation of the antioxidant system. Implementation of S. dendroideum at different levels of salinity and various adaptation mechanisms, as well as regulation by external factors of the degree of manifestation of the obligate CAM pathway, is discussed.

The thermal stability of plants with different types of photosynthesis Chenopodium quinoa Willd. (C3) and Amaranthus retroflexus L. (C4-NAD) to short-term elevated temperature (35°С, eT) at ambient (400 ppm, aCO2) and elevated (800 ppm, eCO2) CO2 concentration was investigated. Growth parameters, water, proline, and MDA content, CO2/H2О gas exchange parameters, the functioning of PS I and PS II, and the content of key photosynthetic (Rubisco, PEPC) and photorespiratory (GDC) enzymes were analyzed. Under control conditions, C4-type plants, compared with the C3-type, show higher values of dry biomass growth, intensity of visible photosynthesis, transpiration, and PS I activity and lower proline content. The photosynthetic and stomatal apparatus of both types was sensitive to eT, which manifested itself in a decrease in the intensity of apparent photosynthesis and transpiration. In addition, suppression of light reactions (PS II) and intensity of photorespiration (according to GDC) was observed in the C3-species and an increase in the content of proline in the C4-species. Under eCO2 conditions, the C3-species showed a decrease in the intensity of photorespiration, while oxidative stress (twofold increase in the content of MDA) was accompanied by reduced intensity of apparent photosynthesis, transpiration, and increased intensity of dark mitochondrial respiration in the C4-species. A softening effect of eCO2 on thermal stability data for C3- and C4-plants was not established. With the combined action of eCO2 and eT, both types exhibited oxidative stress, reduced efficiency of PS II and apparent photosynthesis, and activation of dark respiration. However, differences were also observed: oxidative stress was accompanied by a decrease in the increase in dry biomass and water content in tissues, as well as suppression of photorespiration, in the C3-species, while there was a decrease in the intensity of transpiration and an increase in the content of PEPC in the C4-species. Reduced WUE with combined action of eCO2 and eT in plants of the C4-species was less significant than the C3-species. The different response of quinoa plants (C3) and amaranth (C4) on the combined effect of climatic factors of elevated temperature and CO2 concentration is discussed.

The thermal stability of plants with different types of photosynthesis Chenopodium quinoa Willd. (C₃) and Amaranthus retroflexus L. (C₄-NAD) to short-term elevated temperature (35°С, eT) at ambient (400 ppm, aCO₂) and elevated (800 ppm, eCO₂) CO₂ concentration was investigated. Growth parameters, water, proline, and MDA content, CO₂/H₂О gas exchange parameters, the functioning of PS I and PS II, and the content of key photosynthetic (Rubisco, PEPC) and photorespiratory (GDC) enzymes were analyzed. Under control conditions, C₄-type plants, compared with the C₃-type, show higher values of dry biomass growth, intensity of visible photosynthesis, transpiration, and PS I activity and lower proline content. The photosynthetic and stomatal apparatus of both types was sensitive to eT, which manifested itself in a decrease in the intensity of apparent photosynthesis and transpiration. In addition, suppression of light reactions (PS II) and intensity of photorespiration (according to GDC) was observed in the C₃-species and an increase in the content of proline in the C₄-species. Under eCO₂ conditions, the C₃-species showed a decrease in the intensity of photorespiration, while oxidative stress (twofold increase in the content of MDA) was accompanied by reduced intensity of apparent photosynthesis, transpiration, and increased intensity of dark mitochondrial respiration in the C₄-species. A softening effect of eCO₂ on thermal stability data for C₃- and C₄-plants was not established. With the combined action of eCO₂ and eT, both types exhibited oxidative stress, reduced efficiency of PS II and apparent photosynthesis, and activation of dark respiration. However, differences were also observed: oxidative stress was accompanied by a decrease in the increase in dry biomass and water content in tissues, as well as suppression of photorespiration, in the C₃-species, while there was a decrease in the intensity of transpiration and an increase in the content of PEPC in the C₄-species. Reduced WUE with combined action of eCO₂ and eT in plants of the C₄-species was less significant than the C₃-species. The different response of quinoa plants (C₃) and amaranth (C₄) on the combined effect of climatic factors of elevated temperature and CO₂ concentration is discussed.

Research into organization and peculiarities of carbon-concentrating mechanism (CCM) functioning in plants with intermediate C₃–С₄ (C₂) photosynthesis type characterized by resistance to arid climate is of paramount importance given the global climate changes. In this study two populations (P1 and P2) of С₃–С₄ Sedobassia sedoides (Chenopodiaceae) different in productivity were investigated under PEG-induced osmotic stress. As a result of our studies, a less productive P1 population was classified as the Type II of С₂ photosynthesis, since there was a relatively low efficiency of cyclic electron transport around PSI, and high glycine decarboxylase (GDC) content. On the contrary, P2 population was more productive and based on type of photosynthesis was classified as С₄-like plants. It exhibited a higher efficiency of cyclic electron transport around PSI and lower GDC content. Plants in each population also demonstrated heterogeneity in terms of the degree of dehydration under osmotic stress and fell into two groups: stress-tolerant (ST) and not stress-tolerant (NST) plants. NST plants from both populations showed a significant increase in proline content and a decrease of maximum quantum yield of PSII (F_v/F_m) evidencing a severe osmotic stress. P1 and P2 plants showed differences in mechanisms ensuring drought tolerance, more pronounced in NST plants. NST plants from P1 showed a decrease in K⁺/Na⁺ ratio, an increase in cyclic electron flow around PSI and a decrease in efficiency of PSII (due to NPQ), presence of oxidative stress (an increase in catalase and SOD activity), increase in content of C₄ CCM enzymes PEPС and NAD-Me. NST plants from P2 retained considerable dry biomass and shoot length (which were formed before stress), demonstrated a decline in Na⁺ accumulation, an increase in K⁺/Na⁺ ratio, and a decrease in activity of cyclic electron flow around PSI and PSII efficiency (due to NPQ), absence of oxidative stress, a decrease in Rubisco content and content of C₄ CCM enzymes PEPC and NAD-Me. The complexity of determining the specific mechanisms of biochemical response of plants to stress is discussed, as well as the high stress-induced plasticity of this type of photosynthesis, i.e. the possibility of switch from one type of C₃–C₄ photosynthesis to another under drought, either towards enhancing С₄ characteristics or towards their weakening.