Direct uptake of HCO3- in the marine angiosperm Posidonia oceanica (L.) Delile driven by a plasma membrane H+ economy

Exploring the leaf regeneration cycles response of Zostera japonica to ocean acidification

Ocean acidification is one of the major global environmental problems facing humankind today, and it has far-reaching impacts on marine organisms and the entire marine ecosystem. Zostera japonica, an important supporting species of intertidal seagrass beds, exhibits high photosynthetic productivity and plays an important role in the carbon cycle of nearshore waters. However, little is known about the characteristics, processes, and mechanisms of its response to ocean acidification. In this study, we conducted a 120-day acidification experiment in Z. japonica; here, plants underwent four leaf regeneration cycles to reveal the response mechanism of Z. japonica to ocean acidification (OA). We found that acidification significantly affected the seedling stage of Z. japonica, impacting leaf regeneration cycles by altering physiological and molecular responses. In one leaf regeneration cycle, the short-term exposure to CO2 affected the seagrass parameters, such as the regulation of inorganic carbon uptake modes and the regulation of photosynthesis between the dark and light reactions, with the potential to affect the carbon sinks of the marine organisms. The long-term effects on the regulation of antioxidant enzymes and antioxidant metabolites, caused an improvement in the marine life adaptation to OA. In a comparison of the different leaf regeneration cycles, the response pattern of Z. japonica showed an offset of the acidification during the short cycles and an adaption to the acidification during the long cycles. This study revealed the response mechanism of Z. japonica to OA at different time scales and could provide a theoretical basis for accurately assessing the impact of OA on seagrass and the entire seagrass bed ecosystem.

A role for root carbonic anhydrase βCA4 in the bicarbonate tolerance of Arabidopsis thaliana

Carbonic anhydrases (CAs) are the main enzymes handling bicarbonate in the different cell compartments. This study analyses the expression of CAs in roots of Arabidopsis thaliana demes differing in tolerance to bicarbonate: the tolerant A1(C+) deme and the sensitive deme, T6(C-). Exposure to 10 mM NaCl caused a transient depolarization of the root cell membranes, and in contrast, the supply of 10 mM NaHCO3 caused hyperpolarization. This hyperpolarization was much stronger in A1(C+) than in T6(C-). Acetazolamide (AZ), a specific inhibitor of CAs, abolished the hyperpolarizing effect in A1(C+), indicating the implication of CAs in this fast membrane response. The time-dependent (3 to 72 h) expression profiles of 14 CAs in roots of A1(C+) and T6(C-) exposed to either control (0 mM NaHCO3, pH 5.9), or bicarbonate (10 mM NaHCO3,pH 8.3) conditions revealed a bicarbonate specific upregulation of BCA4.1 (from 3 to 12 h) in A1(C+). Contrastingly, in T6(C-) BCA4.1 was downregulated by NaHCO3. Exclusively in A1(C+), the enhanced expression of BCA4.1 under bicarbonate was parallelled by an increase of PIP1,3, SLAH1, SLAH3, AHA2, and FRO2 gene expression levels. Under HCO3 - exposure, a bca4 knockout mutant had a lower number of lateral roots, lower root diameters, and higher root lipid peroxidation than the WT. These results indicate that bicarbonate-induced root membrane hyperpolarization is the fast (minutes) initial signalling event in the tolerance response. This is followed by the specific upregulation of BCA4.1 and genes involved in H2O and CO2 transport, apoplast acidification, and iron acquisition.

Cooperation of an external carbonic anhydrase and HCO3- transporter supports underwater photosynthesis in submerged leaves of the amphibious plant Hygrophila difformis

BACKGROUND AND AIMS

HCO3- can be a major carbon resource for photosynthesis in underwater environments. Here we investigate the underlying mechanism of uptake and membrane transport of HCO3- in submerged leaves of Hygrophila difformis, a heterophyllous amphibious plant. To characterize these mechanisms, we evaluated the sensitivity of underwater photosynthesis to an external carbonic anhydrase (CA) inhibitor and an anion exchanger protein inhibitor, and we attempted to identify components of the mechanism of HCO3- utilization.

METHODS

We evaluated the effects of the external CA inhibitor and anion exchanger protein inhibitor on the NaHCO3 response of photosynthetic O2 evolution in submerged leaves of H. difformis. Furthermore, we performed a comparative transcriptomic analysis between terrestrial and submerged leaves.

KEY RESULTS

Photosynthesis in the submerged leaves was decreased by both the external CA inhibitor and anion exchanger protein inhibitor, but no additive effect was observed. Among upregulated genes in submerged leaves, two α-CAs, Hdα-CA1 and Hdα-CA2, and one β-carbonic anhydrase, Hdβ-CA1, were detected. Based on their putative amino acid sequences, the α-CAs are predicted to be localized in the apoplastic region. Recombinant Hdα-CA1 and Hdβ-CA1 showed dominant CO2 hydration activity over HCO3- dehydration activity.

CONCLUSIONS

We propose that the use of HCO3- for photosynthesis in submerged leaves of H. difformis is driven by the cooperation between an external CA, Hdα-CA1, and an unidentified HCO3- transporter.

The diversity and coevolution of Rubisco and CO2 concentrating mechanisms in marine macrophytes

The kinetic properties of Rubisco, the most important carbon-fixing enzyme, have been assessed in a small fraction of the estimated existing biodiversity of photosynthetic organisms. Until recently, one of the most significant gaps of knowledge in Rubisco kinetics was marine macrophytes, an ecologically relevant group including brown (Ochrophyta), red (Rhodophyta) and green (Chlorophyta) macroalgae and seagrasses (Streptophyta). These organisms express various Rubisco types and predominantly possess CO2 -concentrating mechanisms (CCMs), which facilitate the use of bicarbonate for photosynthesis. Since bicarbonate is the most abundant form of dissolved inorganic carbon in seawater, CCMs allow marine macrophytes to overcome the slow gas diffusion and low CO2 availability in this environment. The present review aims to compile and integrate recent findings on the biochemical diversity of Rubisco and CCMs in the main groups of marine macrophytes. The Rubisco kinetic data provided demonstrate a more relaxed relationship among catalytic parameters than previously reported, uncovering a variability in Rubisco catalysis that has been hidden by a bias in the literature towards terrestrial vascular plants. The compiled data indicate the existence of convergent evolution between Rubisco and biophysical CCMs across the polyphyletic groups of marine macrophytes and suggest a potential role for oxygen in shaping such relationship.

Seagrass genomes reveal ancient polyploidy and adaptations to the marine environment

We present chromosome-level genome assemblies from representative species of three independently evolved seagrass lineages: Posidonia oceanica, Cymodocea nodosa, Thalassia testudinum and Zostera marina. We also include a draft genome of Potamogeton acutifolius, belonging to a freshwater sister lineage to Zosteraceae. All seagrass species share an ancient whole-genome triplication, while additional whole-genome duplications were uncovered for C. nodosa, Z. marina and P. acutifolius. Comparative analysis of selected gene families suggests that the transition from submerged-freshwater to submerged-marine environments mainly involved fine-tuning of multiple processes (such as osmoregulation, salinity, light capture, carbon acquisition and temperature) that all had to happen in parallel, probably explaining why adaptation to a marine lifestyle has been exceedingly rare. Major gene losses related to stomata, volatiles, defence and lignification are probably a consequence of the return to the sea rather than the cause of it. These new genomes will accelerate functional studies and solutions, as continuing losses of the 'savannahs of the sea' are of major concern in times of climate change and loss of biodiversity.

Sensing and regulation of plant extracellular pH

In plants, pH determines nutrient acquisition and sensing, and triggers responses to osmotic stress, whereas pH homeostasis protects the cellular machinery. Extracellular pH (pHe) controls the chemistry and rheology of the cell wall to adjust its elasticity and regulate cell expansion in space and time. Plasma membrane (PM)-localized proton pumps, cell-wall components, and cell wall-remodeling enzymes jointly maintain pHe homeostasis. To adapt to their environment and modulate growth and development, plant cells must sense subtle changes in pHe caused by the environment or neighboring cells. Accumulating evidence indicates that PM-localized cell-surface peptide-receptor pairs sense pHe. We highlight recent advances in understanding how plants perceive and maintain pHe, and discuss future perspectives.

Transmembrane Transport of Bicarbonate by Anion Receptors

The development of synthetic anion transporters is motivated by their potential application as treatment for diseases that originate from deficient anion transport by natural proteins. Transport of bicarbonate is important for crucial biological functions such as respiration and digestion. Despite this biological relevance, bicarbonate transport has not been as widely studied as chloride transport. Herein we present an overview of the synthetic receptors that have been studied as bicarbonate transporters, together with the different assays used to perform transport studies in large unilamellar vesicles. We highlight the most active transporters and comment on the nature of the functional groups present in active and inactive compounds. We also address recent mechanistic studies that have revealed different processes that can lead to net transport of bicarbonate, as well as studies reported in cells and tissues, and comment on the key challenges for the further development of bicarbonate transporters.

The differential ability of two species of seagrass to use carbon dioxide and bicarbonate and their modelled response to rising concentrations of inorganic carbon

Seagrass meadows are one of the most productive ecosystems on the planet, but their photosynthesis rate may be limited by carbon dioxide but mitigated by exploiting the high concentration of bicarbonate in the ocean using different active processes. Seagrasses are declining worldwide at an accelerating rate because of numerous anthropogenic pressures. However, rising ocean concentrations of dissolved inorganic carbon, caused by increases in atmospheric carbon dioxide, may benefit seagrass photosynthesis. Here we compare the ability of two seagrass from the Mediterranean Sea, Posidonia oceanica (L.) Delile and Zostera marina L., to use carbon dioxide and bicarbonate at light saturation, and model how increasing concentrations of inorganic carbon affect their photosynthesis rate. pH-drift measurements confirmed that both species were able to use bicarbonate in addition to carbon dioxide, but that Z. marina was more effective than P. oceanica. Kinetic experiments showed that, compared to Z. marina, P. oceanica had a seven-fold higher affinity for carbon dioxide and a 1.6-fold higher affinity for bicarbonate. However, the maximal rate of bicarbonate uptake in Z. marina was 2.1-fold higher than in P. oceanica. In equilibrium with 410 ppm carbon dioxide in the atmosphere, the modelled rates of photosynthesis by Z. marina were slightly higher than P. oceanica, less carbon limited and depended on bicarbonate to a greater extent. This greater reliance by Z. marina is consistent with its less depleted ¹³C content compared to P. oceanica. Modelled photosynthesis suggests that both species would depend on bicarbonate alone at an atmospheric carbon dioxide partial pressure of 280 ppm. P. oceanica was projected to benefit more than Z. marina with increasing atmospheric carbon dioxide partial pressures, and at the highest carbon dioxide scenario of 1135 ppm, would have higher rates of photosynthesis and be more saturated by inorganic carbon than Z. marina. In both species, the proportional reliance on bicarbonate declined markedly as carbon dioxide concentrations increased and in P. oceanica carbon dioxide would become the major source of inorganic carbon.

Correlative adaptation between Rubisco and CO2-concentrating mechanisms in seagrasses

Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here we reveal a convergent kinetic adaptation of Rubisco in phylogenetically distant seagrass species that share catalytic efficiencies and CO₂ and O₂ affinities up to three times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish-water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO₂-concentrating mechanisms (CCMs), which probably evolved in response to the constant CO₂ limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C₄ plants. Our results thus demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses distantly related aquatic lineages.

Alkali salt stress causes fast leaf apoplastic alkalinization together with shifts in ion and metabolite composition and transcription of key genes during the early adaptive response of Vicia faba L

The mechanisms by which plants respond to alkali salt stress are still obscure, and the relevance of alkaline pH under combined alkali salt stress. Early stress responses can indicate mechanisms leading to damage and plant resistance. The apoplast contains essential determinants for plant growth, specifically early apoplastic pH fluctuations are induced by many stressors and hypothesized to be involved in stress signalling. Hence, this study aims to identify fast responses specific to alkaline pH and alkali salt stress by exposing the root of hydroponically grown Vicia faba L. plants to 150 min of either 50 mM NaHCO₃ (pH 9) treatment or alkaline pH 9 alone. Apoplastic pH was monitored in real-time by ratiometric fluorescence microscopy simultaneously with SWIR transmission-based measurements of leaf water content (LWC). Moreover, we examined the effect of these stresses on apoplastic, symplastic and xylem ion and metabolite composition together with transcriptions of certain stress-responsive genes. Physiological and transcriptional changes were observed in response to NaHCO₃ but not to alkaline pH alone. NaHCO₃ elicited a transient reduction in LWC, followed by a transient alkalinization of the apoplast and stomatal closure. Simultaneously, organic acids and sugars accumulated. Fast upregulation of stress-responsive genes showed the significance of gene regulation for early plant adaptation to alkali salt stress.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Alkalinity cycling and carbonate chemistry decoupling in seagrass mystify processes of acidification mitigation

The adverse conditions of acidification on sensitive marine organisms have led to the investigation of bioremediation methods as a way to abate local acidification. This phytoremediation, by macrophytes, is expected to reduce the severity of acidification in nearshore habitats on short timescales. Characterizing the efficacy of phytoremediation can be challenging as residence time, tidal mixing, freshwater input, and a limited capacity to fully constrain the carbonate system can lead to erroneous conclusions. Here, we present in situ observations of carbonate chemistry relationships to seagrass habitats by comparing dense (DG), patchy (PG), and no grass (NG) Zostera marina pools in the high intertidal experiencing intermittent flooding. High-frequency measurements of pH, alkalinity (TA), and total-CO₂ elucidate extreme diel cyclicity in all parameters. The DG pool displayed frequent decoupling between pH and aragonite saturation state (Ω_arg) suggesting pH-based inferences of acidification remediation by seagrass can be misinterpreted as pH and Ω_arg can be independent stressors for some bivalves. Estimates show the DG pool had an integrated ΔTA of 550 μmol kg⁻¹ over a 12 h period, which is ~ 60% > the PG and NG pools. We conclude habitats with mixed photosynthesizers (i.e., PG pool) result in less decoupling between pH and Ω_arg.

Transition From Proto-Kranz-Type Photosynthesis to HCO3 - Use Photosynthesis in the Amphibious Plant Hygrophila polysperma

Hygrophila polysperma is a heterophyllous amphibious plant. The growth of H. polysperma in submerged conditions is challenging due to the low CO2 environment, increased resistance to gas diffusion, and bicarbonate ion (HCO3 -) being the dominant dissolved inorganic carbon source. The submerged leaves of H. polysperma have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that H. polysperma acclimates to the submerged condition by using HCO3 - for photosynthesis. H. polysperma transports HCO3 - into the leaf by a DIDS-sensitive HCO3 - transporter and converted to CO2 by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that H. polysperma is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that H. polysperma performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO3 - use photosynthesis in the submerged environments.

Short-Term Response of Cytosolic N O 3 - to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells

The concentration of CO₂ in the atmosphere has increased over the past 200 years and is expected to continue rising in the next 50 years at a rate of 3 ppm·year^-1. This increase has led to a decrease in seawater pH that has changed inorganic carbon chemical speciation, increasing the dissolved HC O 3 - . Posidonia oceanica is a marine angiosperm that uses HC O 3 - as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HC O 3 - is the diminution of cytosolic Cl^- (Cl^-c) in mesophyll leaf cells due to the efflux through anion channels and, probably, to intracellular compartmentalization. Since anion channels are also permeable to N O 3 - we hypothesize that high HC O 3 - , or even CO₂, would also promote a decrease of cytosolic N O 3 - ( N O 3 - c ). In this work we have used N O 3 - - and Cl^--selective microelectrodes for the continuous monitoring of the cytosolic concentration of both anions in P. oceanica leaf cells. Under light conditions, mesophyll leaf cells showed a N O 3 - c of 5.7 ± 0.2 mM, which rose up to 7.2 ± 0.6 mM after 30 min in the dark. The enrichment of natural seawater (NSW) with 3 mM NaHCO₃ caused both a N O 3 - c decrease of 1 ± 0.04 mM and a Cl c - decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO₂ also produced a diminution of the N O 3 - c , but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon ( HC O 3 - or CO₂) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells. In the presence of 0.1 mM ethoxyzolamide, the plasma membrane-permeable carbonic anhydrase inhibitor, the CO₂-induced cytosolic N O 3 - diminution was much lower (0.1 ± 0.08 mM), pointing to HC O 3 - as the inorganic carbon species that causes the cytosolic N O 3 - leak. The incubation of P. oceanica leaf pieces in 3 mM HC O 3 - -enriched NSW triggered a short-term external N O 3 - net concentration increase consistent with the N O 3 - c leak. As a consequence, the cytosolic N O 3 - diminution induced in high inorganic carbon could result in both the decrease of metabolic N flux and the concomitant biomass N impoverishment in P. oceanica and, probably, in other aquatic plants.

Regional and Microenvironmental Scale Characterization of the Zostera muelleri Seagrass Microbiome

Seagrasses are globally distributed marine plants that represent an extremely valuable component of coastal ecosystems. Like terrestrial plants, seagrass productivity and health are likely to be strongly governed by the structure and function of the seagrass microbiome, which will be distributed across a number of discrete microenvironments within the plant, including the phyllosphere, the endosphere and the rhizosphere, all different in physical and chemical conditions. Here we examined patterns in the composition of the microbiome of the seagrass Zostera muelleri, within six plant-associated microenvironments sampled across four different coastal locations in New South Wales, Australia. Amplicon sequencing approaches were used to characterize the diversity and composition of bacterial, microalgal, and fungal microbiomes and ultimately identify "core microbiome" members that were conserved across sampling microenvironments. Discrete populations of bacteria, microalgae and fungi were observed within specific seagrass microenvironments, including the leaves and roots and rhizomes, with "core" taxa found to persist within these microenvironments across geographically disparate sampling sites. Bacterial, microalgal and fungal community profiles were most strongly governed by intrinsic features of the different seagrass microenvironments, whereby microscale differences in community composition were greater than the differences observed between sampling regions. However, our results showed differing strengths of microbial preferences at the plant scale, since this microenvironmental variability was more pronounced for bacteria than it was for microalgae and fungi, suggesting more specific interactions between the bacterial consortia and the seagrass host, and potentially implying a highly specialized coupling between seagrass and bacterial metabolism and ecology. Due to their persistence within a given seagrass microenvironment, across geographically discrete sampling locations, we propose that the identified "core" microbiome members likely play key roles in seagrass physiology as well as the ecology and biogeochemistry of seagrass habitats.

Photosynthetic acclimation of terrestrial and submerged leaves in the amphibious plant Hygrophila difformis

Hygrophila difformis, a heterophyllous amphibious plant, develops serrated or dissected leaves when grown in terrestrial or submerged conditions, respectively. In this study, we tested whether submerged leaves and ethylene-induced leaves of the heterophyllous, amphibious plant H. difformis have improved photosynthetic ability under submerged conditions. Also, we investigated how this amphibious plant photosynthesizes underwater and whether a HCO3 - transport system is present. We have analysed leaf morphology, measured underwater photosynthetic rates and HCO3 - affinity in H. difformis to determine if there are differences in acclimation ability dependent on growth conditions: terrestrial, submerged, terrestrial treated with ethylene and submerged treated with an ethylene inhibitor. Moreover, we measured time courses for changes in leaf anatomical characteristics and underwater photosynthesis in terrestrial leaves after submersion. Compared with the leaves of terrestrially grown plants, leaf thickness of submerged plants was significantly thinner. The stomatal density on the abaxial surface of submerged leaves was also reduced, and submerged plants had a significantly higher O2 evolution rate. When the leaves of terrestrially grown plants were treated with ethylene, their leaf morphology and underwater photosynthesis increased to levels comparable to those of submerged leaves. Underwater photosynthesis of terrestrial leaves was significantly higher by 5 days after submersion. In contrast, leaf morphology did not change after submergence. Submerged leaves and submerged terrestrial leaves were able to use bicarbonate but submerged terrestrial leaves had an intermediate ability to use HCO3 - that was between terrestrial leaves and submerged leaves. Ethoxyzolamide, an inhibitor of intracellular carbonic anhydrase, significantly inhibited underwater photosynthesis in submerged leaves. This amphibious plant acclimates to the submerged condition by changing leaf morphology and inducing a HCO3 - utilizing system, two processes that are regulated by ethylene.

Na⁺-Dependent High-Affinity Nitrate, Phosphate and Amino Acids Transport in Leaf Cells of the Seagrass Posidonia oceanica (L.) Delile

Posidonia oceanica (L.) Delile is a seagrass, the only group of vascular plants to colonize the marine environment. Seawater is an extreme yet stable environment characterized by high salinity, alkaline pH and low availability of essential nutrients, such as nitrate and phosphate. Classical depletion experiments, membrane potential and cytosolic sodium measurements were used to characterize the high-affinity NO₃^-, Pi and amino acids uptake mechanisms in this species. Net uptake rates of both NO₃^- and Pi were reduced by more than 70% in the absence of Na⁺. Micromolar concentrations of NO₃^- depolarized mesophyll leaf cells plasma membrane. Depolarizations showed saturation kinetics (Km = 8.7 ± 1 μM NO₃^-), which were not observed in the absence of Na⁺. NO₃^- induced depolarizations at increasing Na⁺ also showed saturation kinetics (Km = 7.2 ± 2 mM Na⁺). Cytosolic Na⁺ measured in P. oceanica leaf cells (17 ± 2 mM Na⁺) increased by 0.4 ± 0.2 mM Na⁺ upon the addition of 100 μM NO₃^-. Na⁺-dependence was also observed for high-affinity l-ala and l-cys uptake and high-affinity Pi transport. All together, these results strongly suggest that NO₃^-, amino acids and Pi uptake in P. oceanica leaf cells are mediated by high-affinity Na⁺-dependent transport systems. This mechanism seems to be a key step in the process of adaptation of seagrasses to the marine environment.

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO₃− may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO₂-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C₄ and Crassulacean Acid Metabolism (CAM). The presence of HCO₃− in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO₃− in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO₃− transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO₃− in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO₂/HCO₃− in stomatal guard cells. Plant responses to excess soil HCO₃− is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO₃− uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO₃− tolerance in crop plants.

The pH of the Apoplast: Dynamic Factor with Functional Impact Under Stress

The apoplast is an interconnected compartment with a thin water-film that alkalinizes under stress. This systemic pH increase may be a secondary effect without functional implications, arising from ion movements or proton-pump regulations. On the other hand, there are increasing indications that it is part of a mechanism to withstand stress. Regardless of this controversy, alkalinization of the apoplast has received little attention. The apoplastic pH (pH_apo) increases not only during plant-pathogen interactions but also in response to salinity or drought. Not much is known about the mechanisms that cause the leaf apoplast to alkalinize, nor whether, and if so, how functional impact is conveyed. Controversial explanations have been given, and the unusual complexity of pH_apo regulation is considered as the primary reason behind this lack of knowledge. A gathering of scattered information revealed that changes in pH_apo convey functionality by regulating stomatal aperture via the effects exerted on abscisic acid. Moreover, apoplastic alkalinization may regulate growth under stress, whereas this needs to be verified. In this review, a comprehensive survey about several physiological mechanisms that alkalize the apoplast under stress is given, and the suitability of apoplastic alkalinization as transducing element for the transmission of sensory information is discussed.