The role of carbonic anhydrase α-CA4 in the adaptive reactions of photosynthetic apparatus: the study with α-CA4 knockout plants

A Carbonic Anhydrase, ZmCA4, Contributes to Photosynthetic Efficiency and Modulates CO2 Signaling Gene Expression by Interacting with Aquaporin ZmPIP2;6 in Maize

Carbonic anhydrase (CA) catalyzes the reversible CO2 hydration reaction that produces bicarbonate for phosphoenolpyruvate carboxylase (PEPC). This is the initial step for transmitting the CO2 signal in C4 photosynthesis. However, it remains unknown whether the maize (Zea mays L.) CA gene, ZmCA4, plays a role in the maize photosynthesis process. In our study, we found that ZmCA4 was relatively highly expressed in leaves and localized in the chloroplast and the plasma membrane of mesophyll protoplasts. Knock-out of ZmCA4 reduced CA activity, while overexpression of ZmCA4 increased rubisco activity, as well as the quantum yield and relative electron transport rate in photosystem II. Overexpression of ZmCA4 enhanced maize yield-related traits. Moreover, ZmCA4 interacted with aquaporin ZmPIP2;6 in bimolecular fluorescence complementation and co-immunoprecipitation experiments. The double-knock-out mutant for ZmPIP2;6 and ZmCA4 genes showed reductions in its growth, CA and PEPC activities, assimilation rate and photosystem activity. RNA-Seq analysis revealed that the expression of other ZmCAs, ZmPIPs, as well as CO2 signaling pathway homologous genes, and photosynthetic-related genes was all altered in the double-knock-out mutant compared with the wild type. Altogether, our study's findings point to a critical role of ZmCA4 in determining photosynthetic capacity and modulating CO2 signaling regulation via its interaction with ZmPIP2;6, thus providing insight into the potential genetic value of ZmCA4 for maize yield improvement.

CAH3 from Chlamydomonas reinhardtii: Unique Carbonic Anhydrase of the Thylakoid Lumen

CAH3 is the only carbonic anhydrase (CA) present in the thylakoid lumen of the green algae Chlamydomonas reinhardtii. The monomer of the enzyme has a molecular weight of ~29.5 kDa with high CA activity. Through its dehydration activity, CAH3 can be involved either in the carbon-concentrating mechanism supplying CO2 for RuBisCO in the pyrenoid or in supporting the maximal photosynthetic activity of photosystem II (PSII) by accelerating the removal of protons from the active center of the water-oxidizing complex. Both proposed roles are considered in this review, together with a description of the enzymatic parameters of native and recombinant CAH3, the crystal structure of the protein, and the possible use of lumenal CA as a tool for increasing biomass production in higher plants. The identified involvement of lumenal CAH3 in the function of PSII is still unique among green algae and higher plants and can be used to understand the mechanism(s) of the functional interconnection between PSII and the proposed CA(s) of the thylakoid lumen in other organisms.

Features of Photosynthesis in Arabidopsis thaliana Plants with Knocked Out Gene of Alpha Carbonic Anhydrase 2

The knockout of the At2g28210 gene encoding α-carbonic anhydrase 2 (α-CA2) in Arabidopsis thaliana (Columbia) led to alterations in photosynthetic processes. The effective quantum yields of both photosystem II (PSII) and photosystem I (PSI) were higher in α-carbonic anhydrase 2 knockout plants (α-CA2-KO), and the reduction state of plastoquinone pool was lower than in wild type (WT). The electron transport rate in the isolated thylakoids measured with methyl viologen was higher in α-CA2-KO plants. The amounts of reaction centers of PSII and PSI were similar in WT and α-CA2-KO plants. The non-photochemical quenching of chlorophyll a fluorescence in α-CA2-KO leaves was lower at the beginning of illumination, but became slightly higher than in WT leaves when the steady state was achieved. The degree of state transitions in the leaves was lower in α-CA2-KO than in WT plants. Measurements of the electrochromic carotenoid absorbance shift (ECS) revealed that the light-dependent pH gradient (ΔpH) across the thylakoid membrane was lower in the leaves of α-CA2-KO plants than in WT plants. The starch content in α-CA2-KO leaves was lower than in WT plants. The expression levels of the genes encoding chloroplast CAs in α-CA2-KO changed noticeably, whereas the expression levels of genes of cytoplasmic CAs remained almost the same. It is proposed that α-CA2 may be situated in the chloroplasts.

Action of 2,6-Dichloro-1,4-benzoquinone on the O2-Evolving Activity of Photosystem II in Chlamydomonas reinhardtii Cells with and without Cell Wall: Inhibitory Effect of Its Oxidized Form

Chlamydomonas reinhardtii is a widely used object in studies on green algae concerning both photosynthesis aspects and possible biotechnological approaches. The measurement of the maximum O2 evolution by photosystem II (PSII) in living algal cells in the presence of artificial acceptors is one of the commonly used methods for determining the photosynthetic apparatus state or its change as compared to a control, parent strain, etc., because PSII is the most sensitive component of the thylakoid membrane. The present study shows the need to use low concentrations of 2,6-dichloro-1,4-benzoquinone (DCBQ) paired with potassium ferricyanide (FeCy) for achieving the maximum O2 evolution rate, while a DCBQ concentration above certain threshold results in strong suppression of O2 evolution. The required DCBQ concentration depends on the presence of the cell wall and should be exactly ~0.1 mM or in the range of 0.2–0.4 mM for cells with and without a cell wall, respectively. The inhibition effect is caused, probably, by a higher content of DCBQ in the oxidized form inside cells; this depends on the presence of the cell wall, which influences the efficiency of DCBQ diffusion into and out of the cell, where it is maintained by FeCy in the oxidized state. The possible mechanism of DCBQ inhibition action is discussed.

Role of C4 photosynthetic enzyme isoforms in C3 plants and their potential applications in improving agronomic traits in crops

As compared to C3, C4 plants have higher photosynthetic rates and better tolerance to high temperature and drought. These traits are highly beneficial in the current scenario of global warming. Interestingly, all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), the decarboxylating enzymes NAD/NADP-malic enzyme (NAD/NADP-ME), and phosphoenolpyruvate carboxykinase (PEPCK), and finally pyruvate orthophosphate dikinase (PPDK) catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation. Consistent with this view differential expression pattern of these non-photosynthetic C3 isoforms has been observed in different tissues across the plant developmental stages, such as germination, grain filling, and leaf senescence. Also abundance of these C3 isoforms is increased considerably in response to environmental fluctuations particularly during abiotic stress. Here we review the vital roles played by C3 isoforms of C4 enzymes and the probable mechanisms by which they help plants in acclimation to adverse growth conditions. Further, their potential applications to increase the agronomic trait value of C3 crops is discussed.

The Role of Carbonic Anhydrase αCA4 in Photosynthetic Reactions in Arabidopsis thaliana Studied, Using the Cas9 and T-DNA Induced Mutations in Its Gene

An homozygous mutant line of Arabidopsis thaliana with a knocked out At4g20990 gene encoding thylakoid carbonic anhydrase αCA4 was created using a CRISPR/Cas9 genome editing system. The effects of the mutation were compared with those in two mutant lines obtained by the T-DNA insertion method. In αCA4 knockouts of all three lines, non-photochemical quenching of chlorophyll a fluorescence was lower than in the wild type (WT) plants due to a decrease in its energy-dependent component. The αCA4 knockout also affected the level of expression of the genes encoding all proteins of the PSII light harvesting antennae, the genes encoding cytoplasmic and thylakoid CAs and the genes induced by plant immune signals. The production level of starch synthesis during the light period, as well as the level of its utilization during the darkness, were significantly higher in these mutants than in WT plants. These data confirm that the previously observed differences between insertional mutants and WT plants were not the result of the negative effects of T-DNA insertion transgenesis but the results of αCA4 gene knockout. Overall, the data indicate the involvement of αCA4 in the photosynthetic reactions in the thylakoid membrane, in particular in processes associated with the protection of higher plants' photosynthetic apparatus from photoinhibition.

Effect of CO2 Content in Air on the Activity of Carbonic Anhydrases in Cytoplasm, Chloroplasts, and Mitochondria and the Expression Level of Carbonic Anhydrase Genes of the α- and β-Families in Arabidopsis thaliana Leaves

The carbonic anhydrase (CA) activities of the preparations of cytoplasm, mitochondria, chloroplast stroma, and chloroplast thylakoids, as well as the expression levels of genes encoding αCA1, αCA2, αCA4, βCA1, βCA2, βCA3, βCA4, βCA5, and βCA6, were measured in the leaves of Arabidopsis thaliana plants, acclimated to different CO₂ content in the air: low (150 ppm, lCO₂), normal (450 ppm, nCO₂), and high (1200 ppm, hCO₂). To evaluate the photosynthetic apparatus operation, the carbon assimilation and chlorophyll a fluorescence were measured under the same conditions. It was found that the CA activities of the preparations of cytoplasm, chloroplast stroma, and chloroplast thylakoids measured after two weeks of acclimation were higher, the lower CO₂ concentration in the air. That was preceded by an increase in the expression levels of genes encoding the cytoplasmic form of βCA1, and other cytoplasmic CAs, βCA2, βCA3, and βCA4, as well as of the chloroplast CAs, βCA5, and the stromal forms of βCA1 in a short-term range 1–2 days after the beginning of the acclimation. The dependence on the CO₂ content in the air was most noticeable for the CA activity of the preparations of the stroma; it was two orders higher in lCO₂ plants than in hCO₂ plants. The CA activity of thylakoid membranes from lCO₂ plants was higher than that in nCO₂ and hCO₂ plants; however, in these plants, a significant increase in the expression levels of the genes encoding αCA2 and αCA4 located in thylakoid membranes was not observed. The CA activity of mitochondria and the expression level of the mitochondrial βCA6 gene did not depend on the content of carbon dioxide. Taken together, the data implied that in the higher plants, the supply of inorganic carbon to carboxylation sites is carried out with the cooperative functioning of CAs located in the cytoplasm and CAs located in the chloroplasts.

Loss of carbonic anhydrase in the thylakoid lumen causes unusual moderate-light-induced rearrangement of the chloroplast in Chlamydomonas reinhardtii as a way of photosystem II photoprotection

Chlamydomonas reinhardtii cells have a single large cup-shaped chloroplast that can lose lobes under high light to prevent photodamage of the photosynthetic apparatus, including photosystem II (PSII). Here, under moderate light treatment, the development of the unusual morphology of the chloroplast is shown for mutant cia3, which is deficient in carbonic anhydrase (EC 4.2.1.1) CAH3 in the thylakoid lumen, while such light intensity is harmless for wild type (WT) cells for hours. Cia3 cells had more activated PSII photoprotective mechanisms and showed a tendency to shift in the balance of the PSII damage-repair cycle, whereas PSII retained the same photosynthetic efficiency as in the WT. These findings allow speculation about the unique PSII photoprotection strategy by rearranging the chloroplast in the absence of CAH3. CAH3, in turn, is suggested to be an important participant of the C. reinhardtii photosynthetic apparatus operation, functioning in close connection with PSII.

Unsolved Problems of Carbonic Anhydrases Functioning in Photosynthetic Cells of Higher C3 Plants

The review presents current data on carbonic anhydrases found in various compartments of photosynthetic cells of higher plants. The available data on expression of genes some of carbonic anhydrases and its dependence on environmental factors and plant age are considered. The existing hypotheses on the functions of carbonic anhydrases of plasma membrane, cytoplasm, as well as of stroma and thylakoids of chloroplast, first of all, the hypothesis on participation of these enzymes in supplying carbon dioxide molecules to ribulose-bisphosphate carboxylase (Rubisco) are analyzed. Difficulties of establishing physiological role of the plant cell carbonic anhydrase are discussed in detail.

Involvement of Carbonic Anhydrase CAH3 in the Structural and Functional Stabilization of the Water-Oxidizing Complex of Photosystem II from Chlamydomonas reinhardtii

The involvement of carbonic anhydrases (CA) and CA activity in the functioning of photosystem II (PSII) has been studied for a long time and has been shown in many works. However, so far only for CAH3 from Chlamydomonas reinhardtii there is evidence for its association with the donor side of PSII, where the CA activity of CAH3 can influence the functioning of the water-oxidizing complex (WOC). Our results suggest that CAH3 is also involved in the organization of the native structure of WOC independently of its CA activity. It was shown that in PSII preparations from wild type (WT) the high O2-evolving activity of WOC was observed up to 100 mM NaCl in the medium and practically did not decrease with increasing incubation time with NaCl. At the same time, the WOC function in PSII preparations from CAH3-deficient mutant cia3 is significantly inhibited already at NaCl concentrations above 35 mM, reaching 50% at 100 mM NaCl and increased incubation time. It is suggested that the absence of CAH3 in PSII from cia3 causes disruption of the native structure of WOC, allowing more pronounced conformational changes of its proteins and, consequently, suppression of the WOC active center function, when the ionic strength of the medium is increased. The results of Western blot analysis indicate a more difficult removal of PsbP protein from PSII of cia3 at higher NaCl concentrations, apparently due to the changes in the intermolecular interactions between proteins of WOC in the absence of CAH3. At the same time, the values of the maximum quantum yield of PSII did not practically differ between preparations from WT and cia3, indicating no effect of CAH3 on the photoinduced electron transfer in the reaction center of PSII. The obtained results indicate the involvement of the CAH3 protein in the native organization of the WOC and, as a consequence, in the stabilization of its functional state in PSII from C. reinhardtii.

Advances in understanding the physiological role and locations of carbonic anhydrases in C3 plant cells

The review describes the structures of plant carbonic anhydrases (CAs), enzymes catalyzing the interconversion of inorganic carbon forms and belonging to different families, as well as the interaction of inhibitors and activators of CA activity with the active sites of CAs in representatives of these families. We outline the data that shed light on the location of CAs in green cells of C3 plants, algae and angiosperms, with the emphasis on the recently obtained data. The proven and proposed functions of CAs in these organisms are listed. The possibility of the involvement of several chloroplast CAs in acceleration of the conversion of bicarbonate to CO₂ and in supply of CO₂ for fixation by Rubisco is particularly considered. Special attention is paid to CAs in various parts of thylakoids and to discussion about current knowledge of their possible physiological roles. The review states that, despite the significant progress in application of the mutants with suppressed CAs synthesis, the approach based on the use of the inhibitors of CA activity in some cases remains quite effective. Combination of these two approaches, namely determining the effect of CA activity inhibitors in plants with certain knocked-out CA genes, turns out to be very useful for understanding the functions of other CAs.

Stress-Related Changes in the Expression and Activity of Plant Carbonic Anhydrases

The data on stress-related changes in the expression and activity of plant carbonic anhydrases (CAs) suggest that they are generally upregulated at moderate stress severity. This indicates probable involvement of CAs in adaptation to drought, high salinity, heat, high light, C_i deficit, and excess bicarbonate. The changes in CA levels under cold stress are less studied and generally represented by the downregulation of CAs excepting βCA2. Excess Cd²⁺ and deficit of Zn²⁺ specifically reduce CA activity and reduce its synthesis. Probable roles of βCAs in stress adaptation include stomatal closure, ROS scavenging and partial compensation for decreased mesophyll CO₂ conductance. βCAs play contrasting roles in pathogen responses, interacting with phytohormone signaling networks. Their role can be either negative or positive, probably depending on the host-pathogen system, pathogen initial titer, and levels of ·NO and ROS. It is still not clear why CAs are suppressed under severe stress levels. It should be noted, that the role of βCAs in the facilitation of CO₂ diffusion and their involvement in redox signaling or ROS detoxication are potentially antagonistic, as they are inactivated by oxidation or nitrosylation. Interestingly, some chloroplastic βCAs may be relocated to the cytoplasm under stress conditions, but the physiological meaning of this effect remains to be studied.

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.