Diffusion Limitation and CO2 Concentrating Mechanisms in Bryophytes

Dehydration and rehydration differently affect photosynthesis and volatile monoterpenes in bryophytes with contrasting ecological traits

Bryophytes desiccate rapidly when relative humidity decreases. The capacity to withstand dehydration depends on several ecological and physiological factors. Volatile organic compounds (VOCs) may have a role in enhancing tolerance to desiccating bryophytes. However, the functions of VOCs in bryophytes have received little attention so far. We aimed to investigate the impact of a dehydration-rehydration treatment on primary carbon metabolism and volatile terpenes (VTs) in three bryophytes with contrasting ecological traits: Vessicularia dubyana, Porella platyphylla and Pleurochaete squarrosa. First, we confirmed the desiccation sensitivity gradient of the species. Under fully hydrated conditions, the photosynthetic rate (A) was inversely associated with stress tolerance, with a lower rate in more tolerant species. The partial recovery of A in P. platyphylla and P. squarrosa after rehydration confirmed the desiccation tolerance of these two species. On the other hand, A did not recover after rehydration in V. dubyana. Regarding VT, each species exhibited a distinct VT profile under optimum hydration, with the highest VT pool found in the more desiccation-sensitive species (V. dubyana). However, the observed species-specific VT pattern could be associated with the ecological habitat of each species. P. squarrosa, a moss of dry habitats, may synthesize mainly non-volatile secondary metabolites as stress-defensive compounds. On the other hand, V. dubyana, commonly found submerged, may need to invest photosynthetically assimilated carbon to synthesize a higher amount of VTs to cope with transient water stress occurrence. Further research on the functions of VTs in bryophytes is needed to deepen our understanding of their ecological significance.

Desiccation tolerance in bryophytes relates to elasticity but is independent of cell wall thickness and photosynthesis

Mosses have been found outliers of the trade-off between photosynthesis and bulk elastic modulus described for vascular plants. Hence, potential trade-offs among physical features of cell walls and desiccation tolerance, water relations, and photosynthesis were assessed in bryophytes and other poikilohydric species. Long-term desiccation tolerance was quantified after variable periods of desiccation/rehydration cycles. Water relations were analyzed by pressure-volume curves. Mesophyll conductance was estimated using both CO₂ curve-fitting and anatomical methods. Cell wall elasticity was the parameter that better correlated with the desiccation tolerance index for desiccation tolerant species and was antagonistic to higher absolute values of osmotic potential. Although high values of cell wall effective porosity were estimated compared with the values assumed for vascular plants, the desiccation tolerance index negatively correlated with the porosity in desiccation tolerant bryophytes. Neither cell wall thickness nor photosynthetic capacity were correlated with the desiccation tolerance index of the studied species. The existence of a potential evolutionary trade-off between cell wall thickness and desiccation tolerance is rejected. The photosynthetic capacity reported for bryophytes is independent of elasticity and desiccation tolerance. Furthermore, the role of cell wall thickness in limiting CO₂ conductance would be overestimated under a scenario of high cell wall porosity for most bryophytes.

Cell wall thickness and composition are related to photosynthesis in Antarctic mosses

Cell wall thickness (T_cw ) has been proposed as an important anatomical trait that could determine photosynthesis through land plants' phylogeny, bryophytes being the plant group presenting the thickest walls and the lowest photosynthetic rates. Also, it has recently been suggested that cell wall composition may have the potential to influence both thickness and mesophyll conductance (g_m ), representing a novel trait that could ultimately affect photosynthesis. However, only a few studies in spermatophytes have demonstrated this issue. In order to explore the role of cell wall composition in determining both T_cw and g_m in mosses, we tested six species grown under field conditions in Antarctica. We performed gas exchange and chlorophyll fluorescence measurements, an anatomical characterization, and a quantitative analysis of cell wall main composition (i.e., cellulose, hemicelluloses and pectins) in these six species. We found the photosynthetic rates to vary between the species, and they also presented differences in anatomical characteristics and in cell wall composition. Whilst g_m correlated negatively with T_cw and pectins content, a positive relationship between T_cw and pectins emerged, suggesting that pectins could contribute to determine cell wall porosity. Although our results do not allow us to provide conclusive statements, we suggest for the first time that cell wall composition-with pectins playing a key role-could strongly influence T_cw and g_m in Antarctic mosses, ultimately defining photosynthesis.

How plants grow under gravity conditions besides 1 g: perspectives from hypergravity and space experiments that employ bryophytes as a model organism

Plants have evolved and grown under the selection pressure of gravitational force at 1 g on Earth. In response to this selection pressure, plants have acquired gravitropism to sense gravity and change their growth direction. In addition, plants also adjust their morphogenesis in response to different gravitational forces in a phenomenon known as gravity resistance. However, the gravity resistance phenomenon in plants is poorly understood due to the prevalence of 1 g gravitational force on Earth: not only it is difficult to culture plants at gravity > 1 g(hypergravity) for a long period of time but it is also impossible to create a < 1 genvironment (μg, micro g) on Earth without specialized facilities. Despite these technical challenges, it is important to understand how plants grow in different gravity conditions in order to understand land plant adaptation to the 1 g environment or for outer space exploration. To address this, we have developed a centrifugal device for a prolonged duration of plant culture in hypergravity conditions, and a project to grow plants under the μg environment in the International Space Station is also underway. Our plant material of choice is Physcomitrium (Physcomitrella) patens, one of the pioneer plants on land and a model bryophyte often used in plant biology. In this review, we summarize our latest findings regarding P. patens growth response to hypergravity, with reference to our on-going "Space moss" project. In our ground-based hypergravity experiments, we analyzed the morphological and physiological changes and found unexpected increments of chloroplast size and photosynthesis rate, which might underlie the enhancement of growth and increase in the number of gametophores and rhizoids. We further discussed our approaches at the cellular level and compare the gravity resistance in mosses and that in angiosperms. Finally, we highlight the advantages and perspectives from the space experiments and conclude that research with bryophytes is beneficial to comprehensively and precisely understand gravitational responses in plants.

Cell wall thickness and composition are involved in photosynthetic limitation

The key role of cell walls in setting mesophyll conductance to CO2 (gm) and, consequently, photosynthesis is reviewed. First, the theoretical properties of cell walls that can affect gm are presented. Then, we focus on cell wall thickness (Tcw) reviewing empirical evidence showing that Tcw varies strongly among species and phylogenetic groups in a way that correlates with gm and photosynthesis; that is, the thicker the mesophyll cell walls, the lower the gm and photosynthesis. Potential interplays of gm, Tcw, dehydration tolerance, and hydraulic properties of leaves are also discussed. Dynamic variations of Tcw in response to the environment and their implications in the regulation of photosynthesis are discussed, and recent evidence suggesting an influence of cell wall composition on gm is presented. We then propose a hypothetical mechanism for the influence of cell walls on photosynthesis, combining the effects of thickness and composition, particularly pectins. Finally, we discuss the prospects for using biotechnology for enhancing photosynthesis by altering cell wall-related genes.

Anthoceros genomes illuminate the origin of land plants and the unique biology of hornworts

Hornworts comprise a bryophyte lineage that diverged from other extant land plants >400 million years ago and bears unique biological features, including a distinct sporophyte architecture, cyanobacterial symbiosis and a pyrenoid-based carbon-concentrating mechanism (CCM). Here, we provide three high-quality genomes of Anthoceros hornworts. Phylogenomic analyses place hornworts as a sister clade to liverworts plus mosses with high support. The Anthoceros genomes lack repeat-dense centromeres as well as whole-genome duplication, and contain a limited transcription factor repertoire. Several genes involved in angiosperm meristem and stomatal function are conserved in Anthoceros and upregulated during sporophyte development, suggesting possible homologies at the genetic level. We identified candidate genes involved in cyanobacterial symbiosis and found that LCIB, a Chlamydomonas CCM gene, is present in hornworts but absent in other plant lineages, implying a possible conserved role in CCM function. We anticipate that these hornwort genomes will serve as essential references for future hornwort research and comparative studies across land plants.

The hornwort genome and early land plant evolution

Hornworts, liverworts and mosses are three early diverging clades of land plants, and together comprise the bryophytes. Here, we report the draft genome sequence of the hornwort Anthoceros angustus. Phylogenomic inferences confirm the monophyly of bryophytes, with hornworts sister to liverworts and mosses. The simple morphology of hornworts correlates with low genetic redundancy in plant body plan, while the basic transcriptional regulation toolkit for plant development has already been established in this early land plant lineage. Although the Anthoceros genome is small and characterized by minimal redundancy, expansions are observed in gene families related to RNA editing, UV protection and desiccation tolerance. The genome of A. angustus bears the signatures of horizontally transferred genes from bacteria and fungi, in particular of genes operating in stress-response and metabolic pathways. Our study provides insight into the unique features of hornworts and their molecular adaptations to live on land.

Photosynthesis and photosynthetic efficiencies along the terrestrial plant's phylogeny: lessons for improving crop photosynthesis

Photosynthesis is the basis of all life on Earth. Surprisingly, until very recently, data on photosynthesis, photosynthetic efficiencies, and photosynthesis limitations in terrestrial land plants other than spermatophytes were very scarce. Here we provide an updated data compilation showing that maximum photosynthesis rates (expressed either on an area or dry mass basis) progressively scale along the land plant's phylogeny, from lowest values in bryophytes to largest in angiosperms. Unexpectedly, both photosynthetic water (WUE) and nitrogen (PNUE) use efficiencies also scale positively through the phylogeny, for which it has been commonly reported that these two efficiencies tend to trade-off between them when comparing different genotypes or a single species subject to different environmental conditions. After providing experimental evidence that these observed trends are mostly due to an increased mesophyll conductance to CO₂ - associated with specific anatomical changes - along the phylogeny, we discuss how these findings on a large phylogenetic scale can provide useful information to address potential photosynthetic improvements in crops in the near future.

Anatomical constraints to nonstomatal diffusion conductance and photosynthesis in lycophytes and bryophytes

Photosynthesis in bryophytes and lycophytes has received less attention than terrestrial plant groups. In particular, few studies have addressed the nonstomatal diffusion conductance to CO₂ g_nsd of these plant groups. Their lower photosynthetic rate per leaf mass area at any given nitrogen concentration compared with vascular plants suggested a stronger limitation by CO₂ diffusion. We hypothesized that bryophyte and lycophyte photosynthesis is largely limited by low g_nsd . Here, we studied CO₂ diffusion inside the photosynthetic tissues and its relationships with photosynthesis and anatomical parameters in bryophyte and lycophyte species in Antarctica, Australia, Estonia, Hawaii and Spain. On average, lycophytes and, specially, bryophytes had the lowest photosynthetic rates and nonstomatal diffusion conductance reported for terrestrial plants. These low values are related to their very thick cell walls and their low exposure of chloroplasts to cell perimeter. We conclude that the reason why bryophytes lie at the lower end of the leaf economics spectrum is their strong nonstomatal diffusion conductance limitation to photosynthesis, which is driven by their specific anatomical characteristics.

The Hidden Face of Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) fixes atmospheric CO₂ into organic compounds and is composed of eight copies each of a large subunit (RbcL) and a small subunit (RbcS). Recent reports have revealed unusual RbcS, which are expressed in particular tissues and confer higher catalytic rate, lesser affinity for CO₂, and a more acidic profile of the activity versus pH. The resulting Rubisco was proposed to be adapted to a high CO₂ environment and recycle CO₂ generated by the metabolism. These RbcS belong to a cluster named T (for trichome), phylogenetically distant from cluster M, which gathers well-characterized RbcS expressed in mesophyll or bundle-sheath tissues. Cluster T is largely represented in different plant phyla, including pteridophytes and bryophytes, indicating an ancient origin.

A few enlarged chloroplasts are less efficient in photosynthesis than a large population of small chloroplasts in Arabidopsis thaliana

The photosynthetic, biochemical, and anatomical traits of accumulation and replication of chloroplasts (arc) mutants of Arabidopsis thaliana were investigated to study the effects of chloroplast size and number on photosynthesis. Chloroplasts were found to be significantly larger, and the chloroplast surface area exposed to intercellular air spaces (S c) significantly lower in the mutants than in their wild-types. The decreased S c and increase cytoplasm thickness in the mutants resulted in a lower mesophyll conductance (g m) and a consequently lower chloroplast CO2 concentration (C c). There were no significant differences between the mutants and their wild-types in maximal carboxylation rate (V cmax), maximal electron transport (J cmax), and leaf soluble proteins. Leaf nitrogen (N) and Rubisco content were similar in both Wassilewskija (Ws) wild-type (Ws-WT) and the Ws mutant (arc 8), whereas they were slightly higher in Columbia (Col) wild-type (Col-WT) than the Col mutant (arc 12). The photosynthetic rate (A) and photosynthetic N use efficiency (PNUE) were significantly lower in the mutants than their wild-types. The mutants showed similar A/C c responses as their wild-type counterparts, but A at given C c was higher in Col and its mutant than in Ws and its mutant. From these results, we conclude that decreases in g m and C c are crucial to the reduction in A in arc mutants.

A hypergravity environment increases chloroplast size, photosynthesis, and plant growth in the moss Physcomitrella patens

The physiological and anatomical responses of bryophytes to altered gravity conditions will provide crucial information for estimating how plant physiological traits have evolved to adapt to significant increases in the effects of gravity in land plant history. We quantified changes in plant growth and photosynthesis in the model plant of mosses, Physcomitrella patens, grown under a hypergravity environment for 25 days or 8 weeks using a custom-built centrifuge equipped with a lighting system. This is the first study to examine the response of bryophytes to hypergravity conditions. Canopy-based plant growth was significantly increased at 10×g, and was strongly affected by increases in plant numbers. Rhizoid lengths for individual gametophores were significantly increased at 10×g. Chloroplast diameters (major axis) and thicknesses (minor axis) in the leaves of P. patens were also increased at 10×g. The area-based photosynthesis rate of P. patens was also enhanced at 10×g. Increases in shoot numbers and chloroplast sizes may elevate the area-based photosynthesis rate under hypergravity conditions. We observed a decrease in leaf cell wall thickness under hypergravity conditions, which is in contrast to previous findings obtained using angiosperms. Since mosses including P. patens live in dense populations, an increase in canopy-based plant numbers may be effective to enhance the toughness of the population, and, thus, represents an effective adaptation strategy to a hypergravity environment for P. patens.

Evolution of photorespiration from cyanobacteria to land plants, considering protein phylogenies and acquisition of carbon concentrating mechanisms

Photorespiration and oxygenic photosynthesis are intimately linked processes. It has been shown that under the present day atmospheric conditions cyanobacteria and all eukaryotic phototrophs need functional photorespiration to grow autotrophically. The question arises as to when this essential partnership evolved, i.e. can we assume a coevolution of both processes from the beginning or did photorespiration evolve later to compensate for the generation of 2-phosphoglycolate (2PG) due to Rubisco's oxygenase reaction? This question is mainly discussed here using phylogenetic analysis of proteins involved in the 2PG metabolism and the acquisition of different carbon concentrating mechanisms (CCMs). The phylogenies revealed that the enzymes involved in the photorespiration of vascular plants have diverse origins, with some proteins acquired from cyanobacteria as ancestors of the chloroplasts and others from heterotrophic bacteria as ancestors of mitochondria in the plant cell. Only phosphoglycolate phosphatase was found to originate from Archaea. Notably glaucophyte algae, the earliest branching lineage of Archaeplastida, contain more photorespiratory enzymes of cyanobacterial origin than other algal lineages or land plants indicating a larger initial contribution of cyanobacterial-derived proteins to eukaryotic photorespiration. The acquisition of CCMs is discussed as a proxy for assessing the timing of periods when photorespiratory activity may have been enhanced. The existence of CCMs also had marked influence on the structure and function of photorespiration. Here, we discuss evidence for an early and continuous coevolution of photorespiration, CCMs and photosynthesis starting from cyanobacteria via algae, to land plants.

The photosynthetic capacity in 35 ferns and fern allies: mesophyll CO2 diffusion as a key trait

Ferns and fern allies have low photosynthetic rates compared with seed plants. Their photosynthesis is thought to be limited principally by physical CO2 diffusion from the atmosphere to chloroplasts. The aim of this study was to understand the reasons for low photosynthesis in species of ferns and fern allies (Lycopodiopsida and Polypodiopsida). We performed a comprehensive assessment of the foliar gas-exchange and mesophyll structural traits involved in photosynthetic function for 35 species of ferns and fern allies. Additionally, the leaf economics spectrum (the interrelationships between photosynthetic capacity and leaf/frond traits such as leaf dry mass per unit area or nitrogen content) was tested. Low mesophyll conductance to CO2 was the main cause for low photosynthesis in ferns and fern allies, which, in turn, was associated with thick cell walls and reduced chloroplast distribution towards intercellular mesophyll air spaces. Generally, the leaf economics spectrum in ferns follows a trend similar to that in seed plants. Nevertheless, ferns and allies had less nitrogen per unit DW than seed plants (i.e. the same slope but a different intercept) and lower photosynthesis rates per leaf mass area and per unit of nitrogen.

Higher photosynthetic capacity and different functional trait scaling relationships in erect bryophytes compared with prostrate species

Ecophysiological studies of bryophytes have generally been conducted at the shoot or canopy scale. However, their growth forms are diverse, and knowledge of whether bryophytes with different shoot structures have different functional trait levels and scaling relationships is limited. We collected 27 bryophyte species and categorised them into two groups based on their growth forms: erect and prostrate species. Twenty-one morphological, nutrient and photosynthetic traits were quantified. Trait levels and bivariate trait scaling relationships across species were compared between the two groups. The two groups had similar mean values for shoot mass per area (SMA), light saturation point and mass-based nitrogen (N(mass)) and phosphorus concentrations. Erect bryophytes possessed higher values for mass-based chlorophyll concentration (Chl(mass)), light-saturated assimilation rate (A(mass)) and photosynthetic nitrogen/phosphorus use efficiency. N(mass), Chl(mass) and A(mass) were positively related, and these traits were negatively associated with SMA. Furthermore, the slope of the regression of N(mass) versus Chl(mass) was steeper for erect bryophytes than that for prostrate bryophytes, whereas this pattern was reversed for the relationship between Chl(mass) and A(mass). In conclusion, erect bryophytes possess higher photosynthetic capacities than prostrate species. Furthermore, erect bryophytes invest more nitrogen in chloroplast pigments to improve their light-harvesting ability, while the structure of prostrate species permits more efficient light capture. This study confirms the effect of growth form on the functional trait levels and scaling relationships of bryophytes. It also suggests that bryophytes could be good models for investigating the carbon economy and nutrient allocation of plants at the shoot rather than the leaf scale.

The arc mutants of Arabidopsis with fewer large chloroplasts have a lower mesophyll conductance

Photosynthetic cells of most land plant lineages have numerous small chloroplasts even though most algae, and even the early diverging land plant group the hornworts, tend to have one or a few large chloroplasts. One constraint that small chloroplasts could improve is the resistance to CO2 diffusion from the atmosphere to the chloroplast stroma. We examined the mesophyll conductance (inverse of the diffusion resistance) of mutant Arabidopsis thaliana plants with one or only a few large chloroplasts per cell. The accumulation and replication of chloroplasts (arc) mutants of A. thaliana were studied by model fitting to gas exchange data and (13)CO2 discrimination during carbon fixation. The two methods generally agreed, but the value of the CO2 compensation point of Rubisco (Γ *) used in the model had a large impact on the estimated photosynthetic parameters, including mesophyll conductance. We found that having only a few large chloroplasts per cell resulted in a 25-50 % reduction in the mesophyll conductance at ambient CO2.