Cytological insights into the desiccation biology of a model system: moss protonemata

Understanding desiccation tolerance and sex-specific responses in Bryum argenteum: insights from phenological phases and physiological analyses

BACKGROUND AND AIMS

Desiccation tolerance (DT) is crucial for survival in arid environments, where organisms develop strategies in reproduction, maintenance and defence to cope with water scarcity. Therefore, investigating the relationship between reproduction and DT is essential to understand the ecology and adaptive strategies of species. This study explores the connection between the development of male and female gametangia in the moss Bryum argenteum and the decrease in DT during the progression of phenological phases in gametangia and protonema.

METHODS

Samples collected from a dry tropical forest in Brazil were cultivated, cloned and subjected to desiccation. Subsequently, the physiological parameters of shoots and protonemata were analysed. Shoot and protonema regeneration were monitored for 28 d after the physiological analyses. Both phases were subjected to control and desiccation treatments.

KEY RESULTS

Significant effects of desiccation and sex on the physiological parameters and regeneration capacity of shoots and protonemata were found. Male shoots generally exhibited lower values of Fv/Fm (quantum efficiency of photosystem II) and ϕPSII (effective quantum yield of photosystem II), while females demonstrated higher values and better recovery after desiccation. Protonemata also showed variation in Fv/Fm over time and with sex, with no significant differences in ϕPSII between them. Desiccated male shoots had higher mortality rates and produced fewer new shoots. For females, the regeneration patterns varied between the desiccation-exposed groups and the control, with decreased shoot production, and some protonemata growing into filaments without forming shoots.

CONCLUSION

These findings improve our understanding of the ecological responses of bryophytes to desiccation stress and provide insights into their adaptive strategies in challenging environments, such as the possible rarity of males in dioicous moss populations.

Autophagy functions as a cytoprotective mechanism by regulating programmed cell death during desiccation in Syntrichia caninervis

Desiccation is a state of extreme water loss that is lethal to many plant species. Some desert plants have evolved unique strategies to cope with desiccation stress in their natural environment. Here we present the remarkable stress management mechanism of Syntrichia caninervis, a desert moss species which exhibits an 'A' category of desiccation tolerance. Our research demonstrated that desiccation stress triggers autophagy in S. caninervis while inhibiting Programmed Cell Death (PCD). Silencing of two autophagy-related genes, ATG6 and ATG2, in S. caninervis promoted PCD. Desiccation treatment accelerated cell death in ATG6 and ATG2 gene-silenced S. caninervis. Notably, trehalose was not detected during desiccation, and exogenous application of trehalose cannot activate autophagy. These results suggested that S. caninervis is independent of trehalose accumulation to triggered autophagy. Our results showed that autophagy function as prosurvival mechanism to enhance desiccation tolerance of S. caninervis. Our findings enrich the knowledge of the role of autophagy in plant stress response and may provide new insight into understanding of plant desiccation tolerance.

Stress-associated developmental reprogramming in moss protonemata by synthetic activation of the common symbiosis pathway

Symbioses between angiosperms and rhizobia or arbuscular mycorrhizal fungi are controlled through a conserved signaling pathway. Microbe-derived, chitin-based elicitors activate plant cell surface receptors and trigger nuclear calcium oscillations, which are decoded by a calcium/calmodulin-dependent protein kinase (CCaMK) and its target transcription factor interacting protein of DMI3 (IPD3). Genes encoding CCaMK and IPD3 have been lost in multiple non-mycorrhizal plant lineages yet retained among non-mycorrhizal mosses. Here, we demonstrated that the moss Physcomitrium is equipped with a bona fide CCaMK that can functionally complement a Medicago loss-of-function mutant. Conservation of regulatory phosphosites allowed us to generate predicted hyperactive forms of Physcomitrium CCaMK and IPD3. Overexpression of synthetically activated CCaMK or IPD3 in Physcomitrium led to abscisic acid (ABA) accumulation and ectopic development of brood cells, which are asexual propagules that facilitate escape from local abiotic stresses. We therefore propose a functional role for Physcomitrium CCaMK-IPD3 in stress-associated developmental reprogramming.

More than just lipid balls: quantitative analysis of plastoglobule attributes and their stress-related responses

Plastoglobules are ubiquitous under non-stress conditions and their morphology, closely related to their composition, changes differently depending on the specific stress that the plant undergoes. Plastoglobules are lipoprotein structures attached to thylakoid membranes, which participate in chloroplast metabolism and stress responses. Their structure contains a coating lipid monolayer and a hydrophobic core that differ in composition. Their function in chloroplasts has been studied focussing on their composition. However, we currently lack a comprehensive study that quantitatively evaluates the occurrence and morphology of plastoglobules. Following a literature search strategy, we quantified the main morphological attributes of plastoglobules from photosynthetic chloroplasts of more than 1000 TEM images published over the last 53 years, covering more than 100 taxa and 15 stress types. The analysis shows that plastoglobules under non-stress conditions are spherical, with an average diameter of 100-200 nm and cover less than 3% of the chloroplast cross-section area. This percentage rises under almost every type of stress, particularly in senescence. Interestingly, an apparent trade-off between increasing either the number or the diameter of plastoglobules governs this response. Our results show that plastoglobules are ubiquitous in chloroplasts of higher plants under non-stress conditions. Besides, provided the specific molecular composition of the core and coat of plastoglobules, we conclude that specific stress-related variation in plastoglobules attributes may allow inferring precise responses of the chloroplast metabolism.

Pseudocrossidium replicatum (Taylor) R.H. Zander is a fully desiccation-tolerant moss that expresses an inducible molecular mechanism in response to severe abiotic stress

KEY MESSAGE

The moss Pseudocrossidium replicatum is a desiccation-tolerant species that uses an inducible system to withstand severe abiotic stress in both protonemal and gametophore tissues. Desiccation tolerance (DT) is the ability of cells to recover from an air-dried state. Here, the moss Pseudocrossidium replicatum was identified as a fully desiccation-tolerant (FDT) species. Its gametophores rapidly lost more than 90% of their water content when exposed to a low-humidity atmosphere [23% relative humidity (RH)], but abscisic acid (ABA) pretreatment diminished the final water loss after equilibrium was reached. P. replicatum gametophores maintained good maximum photosystem II (PSII) efficiency (Fv/Fm) for up to two hours during slow dehydration; however, ABA pretreatment induced a faster decrease in the Fv/Fm. ABA also induced a faster recovery of the Fv/Fm after rehydration. Protein synthesis inhibitor treatment before dehydration hampered the recovery of the Fv/Fm when the gametophores were rehydrated after desiccation, suggesting the presence of an inducible protective mechanism that is activated in response to abiotic stress. This observation was also supported by accumulation of soluble sugars in gametophores exposed to ABA or NaCl. Exogenous ABA treatment delayed the germination of P. replicatum spores and induced morphological changes in protonemal cells that resembled brachycytes. Transcriptome analyses revealed the presence of an inducible molecular mechanism in P. replicatum protonemata that was activated in response to dehydration. This study is the first RNA-Seq study of the protonemal tissues of an FDT moss. Our results suggest that P. replicatum is an FDT moss equipped with an inducible molecular response that prepares this species for severe abiotic stress and that ABA plays an important role in this response.

Phytohormone biosynthesis and signaling pathways of mosses

Most known phytohormones regulate moss development. We present a comprehensive view of the synthesis and signaling pathways for the most investigated of these compounds in mosses, focusing on the model Physcomitrium patens. The last 50 years of research have shown that most of the known phytohormones are synthesized by the model moss Physcomitrium patens (formerly Physcomitrella patens) and regulate its development, in interaction with responses to biotic and abiotic stresses. Biosynthesis and signaling pathways are best described in P. patens for the three classical hormones auxins, cytokinins and abscisic acid. Furthermore, their roles in almost all steps of development, from early filament growth to gametophore development and sexual reproduction, have been the focus of much research effort over the years. Evidence of hormonal roles exist for ethylene and for CLE signaling peptides, as well as for salicylic acid, although their possible effects on development remain unclear. Production of brassinosteroids by P. patens is still debated, and modes of action for these compounds are even less known. Gibberellin biosynthesis and signaling may have been lost in P. patens, while gibberellin precursors such as ent-kaurene derivatives could be used as signals in a yet to discover pathway. As for jasmonic acid, it is not used per se as a hormone in P. patens, but its precursor OPDA appears to play a corresponding role in defense against abiotic stress. We have tried to gather a comprehensive view of the biosynthesis and signaling pathways for all these compounds in mosses, without forgetting strigolactones, the last class of plant hormones to be reported. Study of the strigolactone response in P. patens points to a novel signaling compound, the KAI2-ligand, which was likely employed as a hormone prior to land plant emergence.

How dry is dry? Molecular mobility in relation to thallus water content in a lichen

Lichens can withstand extreme desiccation to water contents of ≤ 0.1 g H2O g-1 DW, and in the desiccated state are among the most extremotolerant organisms known. Desiccation-tolerant life-forms such as seeds, mosses and lichens survive 'vitrification', that is the transition of their cytoplasm to a 'glassy' state, which causes metabolism to cease. However, our understanding of the mechanisms of desiccation tolerance is hindered by poor knowledge of what reactions occur in the desiccated state. Using Flavoparmelia caperata as a model lichen, we determined at what water contents vitrification occurred upon desiccation. Molecular mobility was assessed by dynamic mechanical thermal analysis, and the de- and re-epoxidation of the xanthophyll cycle pigments (measured by HPLC) was used as a proxy to assess enzyme activity. At 20 °C vitrification occurred between 0.12-0.08 g H2O g-1 DW and enzymes were active in a 'rubbery' state (0.17 g H2O g-1 DW) but not in a glassy state (0.03 g H2O g-1 DW). Therefore, desiccated tissues may appear to be 'dry' in the conventional sense, but subtle differences in water content will have substantial consequences on the types of (bio)chemical reactions that can occur, with downstream effects on longevity in the desiccated state.

Strategies of desiccation tolerance vary across life phases in the moss Syntrichia caninervis

PREMISE

Desiccation tolerance (DT) is a widespread phenomenon among land plants, and variable ecological strategies for DT are likely to exist. Using Syntrichia caninervis, a dryland moss and model system used in DT studies, we hypothesized that DT is lowest in juvenile (protonemal) tissues, highest in asexual reproductive propagules (gemmae), and intermediate in adults (shoots). We tested the long-standing hypothesis of an inherent constitutive strategy of DT in this species.

METHODS

Plants were rapidly dried to levels of equilibrating relative humidity (RHeq) ranging from 0 to 93%. Postrehydration recovery was assessed using chlorophyll fluorescence, regeneration rates, and visual tissue damage. For each life phase, we estimated the minimum rate of drying (RoD_min ) at RHeq = 42% that did not elicit damage 24 h postrehydration.

RESULTS

DT strategy varied with life phase, with adult shoots having the lowest RoD_min (10-25 min), followed by gemmae (3-10 h) and protonema (14-20 h). Adult shoots exhibited no detectable damage 24 h postrehydration following a rapid-dry only at the highest RHeq used (93%), but when dried to lower RHs the response declined to <50% of control fluorescence values. Notably, immediately following rehydration (0 h postrehydration), shoots were damaged below control levels of fluorescence regardless of the RHeq, thus implicating damage.

CONCLUSIONS

Life phases of the moss S. caninervis had a range of strategies from near constitutive (adult shoots) to demonstrably inducible (protonema). A new response variable for assessing degree of DT is introduced as the minimum rate of drying from which full recovery occurs.

The evolving role of abscisic acid in cell function and plant development over geological time

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the function of key genes in the ABA signalling pathway of angiosperms has revealed that this signal transduction pathway is ancient, yet considerable change in the physiological roles of this hormone have occurred over geological time. With recent advances in our capacity to characterise gene function in non-angiosperms we are on the cusp of revealing the origins of this critical hormonal signalling pathway in plants, and understanding how a simple hormone may have shaped land plant diversity, ecology and adaptation over the past 500 million years.

A Chloroplast COR413 Protein From Physcomitrella patens Is Required for Growth Regulation Under High Light and ABA Responses

COR413 genes belong to a poorly characterized group of plant-specific cold-regulated genes initially identified as part of the transcriptional activation machinery of plants during cold acclimation. They encode multispanning transmembrane proteins predicted to target the plasma membrane or the chloroplast inner membrane. Despite being ubiquitous throughout the plant kingdom, little is known about their biological function. In this study, we used reverse genetics to investigate the relevance of a predicted chloroplast localized COR413 protein (PpCOR413im) from the moss Physcomitrella patens in developmental and abiotic stress responses. Expression of PpCOR413im was strongly induced by abscisic acid (ABA) and by various environmental stimuli, including low temperature, hyperosmosis, salinity and high light. In vivo subcellular localization of PpCOR413im-GFP fusion protein revealed that this protein is localized in chloroplasts, confirming the in silico predictions. Loss-of-function mutants of PpCOR413im exhibited growth and developmental alterations such as growth retardation, reduced caulonema formation and hypersensitivity to ABA. Mutants also displayed altered photochemistry under various abiotic stresses, including dehydration and low temperature, and exhibited a dramatic growth inhibition upon exposure to high light. Disruption of PpCOR413im also caused altered chloroplast ultrastructure, increased ROS accumulation, and enhanced starch and sucrose levels under high light or after ABA treatment. In addition, loss of PpCOR413im affected both nuclear and chloroplast gene expression in response to ABA and high light, suggesting a role for this gene downstream of ABA in the regulation of growth and environmental stress responses. Developmental alterations exhibited by PpCOR413im knockout mutants had remarkable similarities to those exhibited by hxk1, a mutant lacking a major chloroplastic hexokinase, an enzyme involved in energy homeostasis. Based on these findings, we propose that PpCOR413im is involved in coordinating energy metabolism with ABA-mediated growth and developmental responses.

Ultrastructural and biochemical analyses reveal cell wall remodelling in lichen-forming microalgae submitted to cyclic desiccation-rehydration

BACKGROUND AND AIMS

One of the most distinctive features of desiccation-tolerant plants is their high cell wall (CW) flexibility. Most lichen microalgae can tolerate drastic dehydration-rehydration (D/R) conditions; however, their mechanisms of D/R tolerance are scarcely understood. We tested the hypothesis that D/R-tolerant microalgae would have flexible CWs due to species-specific CW ultrastructure and biochemical composition, which could be remodelled by exposure to cyclic D/R.

METHODS

Two lichen microalgae, Trebouxia sp. TR9 (TR9, adapted to rapid D/R cycles) and Coccomyxa simplex (Csol, adapted to seasonal dry periods) were exposed to no or four cycles of desiccation [25-30 % RH (TR9) or 55-60 % RH (Csol)] and 16 h of rehydration (100 % RH). Low-temperature SEM, environmental SEM and freeze-substitution TEM were employed to visualize structural alterations induced by D/R. In addition, CWs were extracted and sequentially fractionated with hot water and KOH, and the gel permeation profile of polysaccharides was analysed in each fraction. The glycosyl composition and linkage of the main polysaccharides of each CW fraction were analysed by GC-MS.

KEY RESULTS

All ultrastructural analyses consistently showed that desiccation caused progressive cell shrinkage and deformation in both microalgae, which could be rapidly reversed when water availability increased. Notably, the plasma membrane of TR9 and Csol remained in close contact with the deformed CW. Exposure to D/R strongly altered the size distribution of TR9 hot-water-soluble polysaccharides, composed mainly of a β-3-linked rhamnogalactofuranan and Csol KOH-soluble β-glucans.

CONCLUSIONS

Cyclic D/R induces biochemical remodelling of the CW that could increase CW flexibility, allowing regulated shrinkage and expansion of D/R-tolerant microalgae.

Acclimation and endogenous abscisic acid in the moss Physcomitrella patens during acquisition of desiccation tolerance

The moss Physcomitrella patens has been used as a model organism to study the induction of desiccation tolerance (DT), but links between dehydration rate, the accumulation of endogenous abscisic acid (ABA) and DT remain unclear. In this study, we show that prolonged acclimation of P. patens at 89% relative humidity (RH) [-16 MPa] can induce tolerance of desiccation at 33% RH (-153 MPa) in both protonema and gametophore stages. During acclimation, significant endogenous ABA accumulation occurred after 1 day in gametophores and after 2 days in protonemata. Physcomitrella patens expressing the ABA-inducible EARLY METHIONINE promoter fused to a cyan fluorescent protein (CFP) reporter gene revealed a mostly uniform distribution of the CFP increasing throughout the tissues during acclimation. DT was measured by day 6 of acclimation in gametophores, but not until 9 days of acclimation for protonemata. These results suggest that endogenous ABA accumulating when moss cells experience moderate water loss requires sufficient time to induce the changes that permit cells to survive more severe desiccation. These results provide insight for ongoing studies of how acclimation induces metabolic changes to enable DT in P. patens.

Cold Acclimation Improves the Desiccation Stress Resilience of Polar Strains of Klebsormidium (Streptophyta)

Biological soil crusts (BSCs) are complex communities of autotrophic, heterotrophic, and saprotrophic (micro)organisms. In the polar regions, these biocrust communities have essential ecological functions such as primary production, nitrogen fixation, and ecosystem engineering while coping with extreme environmental conditions (temperature, desiccation, and irradiation). The microalga Klebsormidium is commonly found in BSCs all across the globe. The ecophysiological resilience of various Klebsormidium species to desiccation and other stresses has been studied intensively. Here we present the results of transcriptomic analyses of two different Klebsormidium species, K. dissectum and K. flaccidum, isolated from Antarctic and Arctic BSCs. We performed desiccation stress experiments at two different temperatures mimicking fluctuations associated with global change. Cultures grown on agar plates were desiccated on membrane filters at 10% relative air humidity until the photosynthetic activity as reflected in the effective quantum yield of photosystem II [Y(II)] ceased. For both species, the response to dehydration was much faster at the higher temperature. At the transcriptome level both species responded more strongly to the desiccation stress at the higher temperature suggesting that adaptation to cold conditions enhanced the resilience of both algae to desiccation stress. Interestingly, the two different species responded differently to the applied desiccation stress with respect to the number as well as function of genes showing differential gene expression. The portion of differentially expressed genes shared between both taxa was surprisingly low indicating that both Klebsormidium species adapted independently to the harsh conditions of Antarctica and the Arctic, respectively. Overall, our results indicate that environmental acclimation has a great impact on gene expression and the response to desiccation stress in Klebsormidium.

Unraveling metabolic mechanisms behind chloroplast desiccation tolerance: Chlorophyllous fern spore as a new promising unicellular model

Fern spores are unicellular structures produced by the sporophyte generation that give rise to the haploid gametophyte. When released from the sporangium, spores are desiccation tolerant (DT) in the royal fern (Osmunda regalis) and contain fully developed chloroplasts. As a consequence, this type of spores is called chlorophyllous spores (CS). Upon transfer to germination conditions, CS initiate a process of imbibition that suppresses DT in 72 h, before the germination starts. In parallel to such change in DT, thylakoids undergo a profound remodelling in composition and function. Firstly, sustained quenching of chlorophyll fluorescence is relaxed, giving rise to photochemically active CS, while lipid composition shifts from that of a resting structure to a metabolically active cell. Basically trigalactolipids decreased in favour of monogalactolipids, with a parallel desaturation of fatty acids. Storage lipids such as triacylglycerol were quickly depleted. These results highlight the importance of the structure of thylakoids lipid as a key to protect membrane integrity during desiccation, together with the saturation of fatty acids and the constitutive chlorophyll quenching to prevent oxidative damage. The CS used here, in which the same cell shifts from DT to sensitive strategy in 72 h, reveal their potential as unicellular models for future studies on DT.

On the origins of osmotically driven stomatal movements

Contents Summary 84 I. Introduction 84 II. Stomatal form and biomechanics 85 III. Stomatal function 86 IV. Evolution of guard cell ion channels 87 V. Conclusions 88 Acknowledgements 88 Author contributions 88 References 88 SUMMARY: Stomatal pores with apertures that can be adjusted by changes in guard cell turgor have facilitated plant success in dry environments. We explore their evolutionary origins, considering recent findings from bryophytes. Unlike vascular plant stomata, which close to prevent water loss, bryophyte stomata become locked open to promote spore desiccation. We find that the families of ion channels, known to control stomatal movements in angiosperms, are ancient and represented across extant land plants. However, although angiosperm guard cells express specific ion channel genes, none appear specifically expressed in stomata-bearing moss tissues. Given the evolutionary shift in stomatal function from promotion to prevention of water loss, we postulate that ion channels adopted guard cell-specific functions after the divergence of bryophytes.

The Xanthomonas effector XopL uncovers the role of microtubules in stromule extension and dynamics in Nicotiana benthamiana

Xanthomonas campestris pv. vesicatoria type III-secreted effectors were screened for candidates influencing plant cell processes relevant to the formation and maintenance of stromules in Nicotiana benthamiana lower leaf epidermis. Transient expression of XopL, a unique type of E3 ubiquitin ligase, led to a nearly complete elimination of stromules and the relocation of plastids to the nucleus. Further characterization of XopL revealed that the E3 ligase activity is essential for the two plastid phenotypes. In contrast to the XopL wild type, a mutant XopL lacking E3 ligase activity specifically localized to microtubules. Interestingly, mutant XopL-labeled filaments frequently aligned with stromules, suggesting an important, yet unexplored, microtubule-stromule relationship. High time-resolution movies confirmed that microtubules provide a scaffold for stromule movement and contribute to stromule shape. Taken together, this study has defined two populations of stromules: microtubule-dependent stromules, which were found to move slower and persist longer, and microtubule-independent stromules, which move faster and are transient. Our results provide the basis for a new model of stromule dynamics including interactions with both actin and microtubules.

The evolution of the stomatal apparatus: intercellular spaces and sporophyte water relations in bryophytes-two ignored dimensions

Cryo-scanning electron microscopy shows that nascent intercellular spaces (ICSs) in bryophytes are liquid-filled, whereas these are gas-filled from the outset in tracheophytes except in the gametophytes of Lycopodiales. ICSs are absent in moss gametophytes and remain liquid-filled in hornwort gametophytes and in both generations in liverworts. Liquid is replaced by gas following stomatal opening in hornworts and is ubiquitous in moss sporophytes even in astomate taxa. New data on moss water relations and sporophyte weights indicate that the latter are homiohydric while X-ray microanalysis reveals an absence of potassium pumps in the stomatal apparatus. The distribution of ICSs in bryophytes is strongly indicative of very ancient multiple origins. Inherent in this scenario is either the dual or triple evolution of stomata. The absence, in mosses, of any relationship between increases in sporophyte biomass and stomata numbers and absences, suggests that CO2 entry through the stomata, possible only after fluid replacement by gas in the ICSs, makes but a minor contribution to sporophyte nutrition. Save for a single claim of active regulation of aperture dimensions in mosses, all other functional and structural data point to the sporophyte desiccation, leading to spore discharge, as the primeval role of the stomatal apparatus.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Desiccation tolerance in Physcomitrella patens: Rate of dehydration and the involvement of endogenous abscisic acid (ABA)

The moss Physcomitrella patens, a model system for basal land plants, tolerates several abiotic stresses, including dehydration. We previously reported that Physcomitrella patens survives equilibrium dehydration to -13 MPa in a closed system at 91% RH. Tolerance of desiccation to water potentials below -100 MPa was only achieved by pretreatment with exogenous abscisic acid (ABA). We report here that gametophores, but not protonemata, can survive desiccation below -100 MPa after a gradual drying regime in an open system, without exogenous ABA. In contrast, faster equilibrium drying at 90% RH for 3-5 days did not induce desiccation tolerance in either tissue. Endogenous ABA accumulated in protonemata and gametophores under both drying regimes, so did not correlate directly with desiccation tolerance. Gametophores of a Ppabi3a/b/c triple knock out transgenic line also survived the gradual dehydration regime, despite impaired ABA signaling. Our results suggest that the initial drying rate, and not the amount of endogenous ABA, may be critical in the acquisition of desiccation tolerance. Results from this work will provide insight into ongoing studies to uncover the role of ABA in the dehydration response and the underlying mechanisms of desiccation tolerance in this bryophyte.

The terrestrial green macroalga Prasiola calophylla (Trebouxiophyceae, Chlorophyta): ecophysiological performance under water-limiting conditions

The phylogenetic placement of Prasiola calophylla, from an anthropogenic habitat previously shown to contain a novel UV sunscreen compound, was confirmed by analysis of its rbcL gene. This alga has the capacity to tolerate strong water-limiting conditions. The photosynthetic performance and ultrastructural changes under desiccation and osmotic stress were investigated. Freshly harvested thalli showed an effective quantum yield of PSII [Y(II)] of 0.52 ± 0.06 that decreased to ∼60% of the initial value at 3000 mM sorbitol, and 4000 mM sorbitol led to a complete loss of Y(II). The Y(II) of thalli exposed to controlled desiccating conditions at 60% relative humidity (RH) ceased within 240 min, whereas zero values were reached after 120 min at 20% RH. All investigated samples completely recovered Y(II) within ∼100 min after rehydration. Relative electron transport rates (rETR) were temperature dependent, increasing from 5, 10, to 25 °C but strongly declining at 45 °C. Transmission electron microscopy of samples desiccated for 2.5 h showed an electron dense appearance of the entire cytoplasm when compared to control samples. Thylakoid membranes were still visible in desiccated cells, corroborating the ability to recover. Control and desiccated cells contained numerous storage lipids and starch grains, providing reserves. Overall, P. calophylla showed a high capacity to cope with water-limiting conditions on a physiological and structural basis. A lipophilic outer layer of the cell walls might contribute to reduce water evaporation in this poikilohydric organism.

F-actin reorganization upon de- and rehydration in the aeroterrestrial green alga Klebsormidium crenulatum

Filamentous actin (F-actin) is a dynamic network involved in many cellular processes like cell division and cytoplasmic streaming. While many studies have addressed the involvement of F-actin in different cellular processes in cultured cells, little is known on the reactions to environmental stress scenarios, where this system might have essential regulatory functions. We investigated here the de- and rehydration kinetics of breakdown and reassembly of F-actin in the streptophyte green alga Klebsormidium crenulatum. Measurements of the chlorophyll fluorescence (effective quantum yield of photosystem II [ΔF/Fm']) via pulse amplitude modulation were performed as a measure for dehydration induced shut down of physiological activity, which ceased after 141±15min at ∼84% RH. We hypothesized that there is a link between this physiological parameter and the status of the F-actin system. Indeed, 20min of dehydration (ΔF/Fm'=0) leads to a breakdown of the fine cortical F-actin network as visualized by Atto 488 phalloidin staining, and dot-like structures remained. Already 10min after rehydration a beginning reassembly of F-actin is observed, after 25min the F-actin network appeared similar to untreated controls, indicating a full recovery. These results demonstrate the fast kinetics of F-actin dis- and reassembly likely contributing to cellular reorganization upon rehydration.

Dehydration-responsive features of Atrichum undulatum

Drought is an increasingly important limitation on plant productivity worldwide. Understanding the mechanisms of drought tolerance in plants can lead to new strategies for developing drought-tolerant crops. Many moss species are able to survive desiccation-a more severe state of dehydration than drought. Research into the mechanisms and evolution of desiccation tolerance in basal land plants is of particular significance to both biology and agriculture. In this study, we conducted morphological, cytological, and physiological analyses of gametophytes of the highly desiccation-tolerant bryophyte Atrichum undulatum (Hedw.) P. Beauv during dehydration and rehydration. Our results suggested that the mechanisms underlying the dehydration-recovery cycle in A. undulatum gametophytes include maintenance of membrane stability, cellular structure protection, prevention of reactive oxygen species (ROS) generation, elimination of ROS, protection against ROS-induced damage, and repair of ROS-induced damage. Our data also indicate that this dehydration-recovery cycle consists not only of the physical removal and addition of water, but also involves a highly organized series of cytological, physiological, and biochemical changes. These attributes are similar to those reported for other drought- and desiccation-tolerant plant species. Our findings provide major insights into the mechanisms of dehydration-tolerance in the moss A. undulatum.

Moss antheridia are desiccation tolerant: Rehydration dynamics influence sperm release in Bryum argenteum

PREMISE OF THE STUDY

Free-living sperm of mosses are known to be partially desiccation tolerant. We hypothesized that mature moss antheridia should also tolerate desiccation and that rehydration to partial turgor (prehydration) or rehydration to full turgor (rehydration) before immersion in water is required for full recovery from any damaging effects of prior desiccation.

METHODS

Bryum argenteum (silvery-thread moss) was grown in continuous culture for several months, produced mature perigonia (clusters of antheridia), and these were subjected to a slow rate of drying (∼36 h from full turgor to desiccation) and equilibration with 50% relative humidity. Perigonia were prehydrated (exposed to a saturated atmosphere) or rehydrated (planted upright in saturated media) for 0, 45, 90, 135, 180, and 1440 min, then immersed in sterile water. Time to first sperm mass release, number of antheridia releasing sperm masses, and the integrity of the first sperm mass released were assessed.

KEY RESULTS

Rehydration of dried antheridia for at least 3 h before immersion in water resulted in antheridia functioning similar to control undried antheridia. Compared with rehydration, prehydration was not effective in the recovery of antheridia from desiccation.

CONCLUSIONS

For the first time, moss antheridia are shown to be fully desiccation tolerant at a functional level, capable of releasing fully functional sperm following a slow drying event provided the antheridia are allowed to rehydrate at least 3 h before immersion in water.

Distribution drivers and physiological responses in geothermal bryophyte communities

PREMISE OF STUDY

Our ability to explain community structure rests on our ability to define the importance of ecological niches, including realized ecological niches, in shaping communities, but few studies of plant distributions have combined predictive models with physiological measures.

METHODS

Using field surveys and statistical modeling, we predicted distribution drivers in geothermal bryophyte (moss) communities of Lassen Volcanic National Park (California, USA). In the laboratory, we used drying and rewetting experiments to test whether the strong species-specific effects of relative humidity on distributions predicted by the models were correlated with physiological characters.

KEY RESULTS

We found that the three most common bryophytes in geothermal communities were significantly affected by three distinct distribution drivers: temperature, light, and relative humidity. Aulacomnium palustre, whose distribution is significantly affected by relative humidity according to our model, and which occurs in high-humidity sites, showed extreme signs of stress after drying and never recovered optimal values of PSII efficiency after rewetting. Campylopus introflexus, whose distribution is not affected by humidity according to our model, was able to maintain optimal values of PSII efficiency for 48 hr at 50% water loss and recovered optimal values of PSII efficiency after rewetting.

CONCLUSIONS

Our results suggest that species-specific environmental stressors tightly constrain the ecological niches of geothermal bryophytes. Tests of tolerance to drying in two bryophyte species corresponded with model predictions of the comparative importance of relative humidity as distribution drivers for these species.

Embryos of a moss can be hardened to desiccation tolerance: effects of rate of drying on the timeline of recovery and dehardening in Aloina ambigua (Pottiaceae)

BACKGROUND AND AIMS

Embryonic sporophytes of the moss Aloina ambigua are inducibly desiccation tolerant (DT). Hardening to DT describes a condition of temporary tolerance to a rapid-drying event conferred by a previous slow-drying event. This paper aimed to determine whether sporophytic embryos of a moss can be hardened to DT, to assess how the rate of desiccation influences the post-rehydration dynamics of recovery, hardening and dehardening, and to determine the minimum rate of drying for embryos and shoots.

METHODS

Embryos were exposed to a range of drying rates using wetted filter paper in enclosed Petri dishes, monitoring relative humidity (RH) inside the dish and equilibrating tissues with 50% RH. Rehydrated embryos and shoots were subjected to a rapid-drying event at intervals, allowing assessments of recovery, hardening and dehardening times.

KEY RESULTS

The minimum rate of slow drying for embryonic survival was ∼3·5 h and for shoots ∼9 h. Hardening to DT was dependent upon the prior rate of drying. When the rate of drying was extended to 22 h, embryonic hardening was strong (>50% survival) with survival directly proportional to the post-rehydration interval preceding rapid drying. The recovery time (repair/reassembly) was so short as to be undetectable in embryos and shoots desiccated gradually; however, embryos dried in <3·5 h exhibited a lag time in development of ∼4 d, consistent with recovery. Dehardening resulted in embryos incapable of surviving a rapid-drying event.

CONCLUSIONS

The ability of moss embryos to harden to DT and the influence of prior rate of drying on the dynamics of hardening are shown for the first time. The minimum rate of drying is introduced as a new metric for assessing ecological DT, defined as the minimum duration at sub-turgor during a drying event in which upon rehydration the plant organ of interest survives relatively undamaged from the desiccating event.

Developing sporophytes transition from an inducible to a constitutive ecological strategy of desiccation tolerance in the moss Aloina ambigua: effects of desiccation on fitness

BACKGROUND AND AIMS

Two ecological strategies of desiccation tolerance exist in plants, constitutive and inducible. Because of difficulties in culturing sporophytes, very little is known about desiccation tolerance in this generation and how desiccation affects sexual fitness.

METHODS

Cultured sporophytes and vegetative shoots from a single genotype of the moss Aloina ambigua raised in the laboratory were tested for their strategy of desiccation tolerance by desiccating the shoot-sporophyte complex and vegetative shoots at different intensities, and comparing outcomes with those of undried shoot-sporophyte complexes and vegetative shoots. By using a dehardened clonal line, the effects of field, age and genetic variance among plants were removed.

KEY RESULTS

The gametophyte and embryonic sporophyte were found to employ a predominantly inducible strategy of desiccation tolerance, while the post-embryonic sporophyte was found to employ a moderately constitutive strategy of desiccation tolerance. Further, desiccation reduced sporophyte fitness, as measured by sporophyte mass, seta length and capsule size. However, the effects of desiccation on sporophyte fitness were reduced if the stress occurred during embryonic development as opposed to postembryonic desiccation.

CONCLUSIONS

The effects of desiccation on dehardened sporophytes of a bryophyte are shown for the first time. The transition from one desiccation tolerance strategy to the other in a single structure or generation is shown for only the second time in plants and for the first time in bryophytes. Finding degrees of inducible strategies of desiccation tolerance in different life phases prompts the formulation of a continuum hypothesis of ecological desiccation tolerance in mosses, where desiccation tolerance is not an either/or phenomenon, but varies in degree along a gradient of ecological inducibility.

Physiological history may mask the inherent inducible desiccation tolerance strategy of the desert moss Crossidium crassinerve

Shoots of bryophytes collected in the desiccated state from the field are likely to be hardened to desiccation tolerance (DT) to varying degrees. To account for this, most studies on DT include a relatively short deacclimation period. However, no study has experimentally determined the appropriate deacclimation time for any bryophyte species. Our purposes are to (i) determine if 'field effects' are biologically relevant to DT studies and how long a deacclimation period is required to remove them; and (ii) utilise field versus cultured shoot responses within the context of a deacclimation period to elucidate the ecological strategy of DT. Our hypothesis (based on an extensive literature on DT) is that a deacclimation period from 24 to 72 h should be sufficient to eliminate historical stress effects on the physiology of the shoots and allow an accurate determination of the inherent ecological DT strategy (constitutive or inducible). We determined, however, using chlorophyll fluorescence and visual estimates of shoot damage, that field-collected shoots of the desert moss Crossidium crassinerve required an experimental deacclimation period of >7 days before field effects were removed, and revealed an ecological DT strategy of inducible DT. If the deacclimation period was <6 days, the shoot response conformed to an ecological strategy of constitutive protection. Thus the presence of field effects can obscure the ecological strategy of desiccation tolerance exhibited by the species, and this translates into a need to re-evaluate previous mechanistic and ecological studies of desiccation tolerance in plants.

Desiccation tolerance of Sphagnum revisited: a puzzle resolved

As ecosystem engineers, Sphagnum mosses control their surroundings through water retention, acidification and peat accumulation. Because water retention avoids desiccation, sphagna are generally intolerant to drought; however, the literature on Sphagnum desiccation tolerance (DT) provides puzzling results, indicating the inducible nature of their DT. To test this, various Sphagnum species and other mesic bryophytes were hardened to drought by (i) slow drying; (ii) ABA application and (iii) chilling or frost. DT tolerance was assessed as recovery of chlorophyll fluorescence parameters after severe desiccation. We monitored the seasonal course of DT in bog bryophytes. Under laboratory conditions, following initial de-hardening, untreated Sphagnum shoots lacked DT; however, DT was induced by all hardening treatments except chilling, notably by slow drying, and in Sphagnum species of the section Cuspidata. In the field, sphagna in hollows and lawns developed DT several times during the growing season, responding to reduced precipitation and a lowered water table. Hummock and aquatic species developed DT only in late autumn, probably as a response to frost. Sphagnum protonemata failed to develop DT; hence, desiccation may limit Sphagnum establishment in drier habitats with suitable substrate chemistry. Desiccation avoiders among sphagna form compact hummocks or live submerged; thus, they do not develop DT in the field, lacking the initial desiccation experience, which is frequent in hollow and lawn habitats. We confirmed the morpho-physiological trade-off: in contrast to typical hollow sphagna, hummock species invest more resources in water retention (desiccation avoidance), while they have a lower ability to develop physiological DT.

The rate of drying determines the extent of desiccation tolerance in Physcomitrella patens

The effect of differential drying rates on desiccation tolerance in Physcomitrella patens (Hedw.) Bruch & Schimp. is examined. In order to provide more evidence as to the status of desiccation tolerance in P. patens, a system was designed that allowed alteration of the rate of water loss within a specific relative humidity. An artificial substrate consisting of layers of wetted filter paper was used to slow the drying process to as long as 284h, a significant increase over the commonly used method of exposure (saturated salt solution). By slowing the rate of drying, survival rates and chlorophyll fluorescence parameters improved, and tissue regeneration time was faster. These results indicate a trend where the capacity for desiccation tolerance increases with slower drying, and reveal a much stronger capacity for desiccation tolerance in P. patens than was previously known.

Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms

Although most green algae typically occur in aquatic ecosystems, many species also live partly or permanently under aeroterrestrial conditions, where the cells are exposed to the atmosphere and hence regularly experience dehydration. The ability of algal cells to survive in an air-dried state is termed desiccation tolerance. The mechanisms involved in desiccation tolerance of green algae are still poorly understood, and hence the aim of this review is to summarize recent findings on the effects of desiccation and osmotic water loss. Starting from structural changes, physiological, and biochemical consequences of desiccation will be addressed in different green-algal lineages. The available data clearly indicate a range of strategies, which are rather different in streptophycean and non-streptophycean green algae. While members of the Trebouxiophyceae exhibit effective water loss-prevention mechanisms based on the biosynthesis and accumulation of particular organic osmolytes such as polyols, these compounds are so far not reported in representatives of the Streptophyta. In members of the Streptophyta such as Klebsormidium, the most striking observation is the appearance of cross-walls in desiccated samples, which are strongly undulating, suggesting a high degree of mechanical flexibility. This aids in maintaining structural integrity in the dried state and allows the cell to maintain turgor pressure for a prolonged period of time during the dehydration process. Physiological strategies in aeroterrestrial green algae generally include a rapid reduction of photosynthesis during desiccation, but also a rather quick recovery after rewetting, whereas aquatic species are sensitive to drying. The underlying mechanisms such as the affected molecular components of the photosynthetic machinery are poorly understood in green algae. Therefore, modern approaches based on transcriptomics, proteomics, and/or metabolomics are urgently needed to better understand the molecular mechanisms involved in desiccation-stress physiology of these organisms. The very limited existing information is described in the present review.

The desert moss Pterygoneurum lamellatum (Pottiaceae) exhibits an inducible ecological strategy of desiccation tolerance: effects of rate of drying on shoot damage and regeneration

PREMISE OF THE STUDY

Bryophytes include clades that incorporate constitutive desiccation tolerance, especially terrestrial species. Here we test the hypothesis that the opposing ecological strategy of desiccation tolerance, inducibility, is present in a desert moss, and address this hypothesis by varying rates of drying in a laboratory study. Desiccation tolerance is arguably the most important evolutionary innovation relevant to the colonization of land by plants; increased understanding of the ecological drivers of this trait will eventually illuminate the responsible mechanisms and ultimately open doors to the potential for the application of this trait in cultivated plants.

METHODS

Plants were cloned, grown in continuous culture (dehardened) for several months, and subjected to rates of drying (drying times) ranging from 30 min to 53 h, rehydrated and tested for recovery using chlorophyll fluorescence, leaf damage, and regeneration of protonema and shoots.

KEY RESULTS

Rate of drying significantly affected all recovery responses, with very rapid drying rates severely damaging the entire shoot except the shoot apex and resulting in slower growth rates, fewer regenerative shoots produced, and a compromised photosynthetic system as inferred from fluorescence parameters.

CONCLUSIONS

For the first time, a desert moss is shown to exhibit an ecological strategy of desiccation tolerance that is inducible, challenging the assumption that arid-land bryophytes rely exclusively on constitutive protection. Results indicate that previous considerations defining a slow-dry event in bryophytes need reevaluation, and that the ecological strategy of inducible desiccation tolerance is probably more common than currently understood among terrestrial bryophytes.

The response of Asterochloris erici (Ahmadjian) Skaloud et Peksa to desiccation: a proteomic approach

The study of desiccation tolerance of lichens, and of their chlorobionts in particular, has frequently focused on the antioxidant system that protects the cell against photo-oxidative stress during dehydration/rehydration cycles. In this study, we used proteomic and transcript analyses to assess the changes associated with desiccation in the isolated phycobiont Asterochloris erici. Algae were dried either slowly (5-6 h) or rapidly (<60 min), and rehydrated after 24 h in the desiccated state. To identify proteins that accumulated during the drying or rehydration processes, we employed two-dimensional (2D) difference gel electrophoresis (DIGE) coupled with individual protein identification using trypsin digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Proteomic analyses revealed that desiccation caused an increase in relative abundance of only 11-13 proteins, regardless of drying rate, involved in glycolysis, cellular protection, cytoskeleton, cell cycle, and targeting and degradation. Transcripts of five Hsp90 and two β-tubulin genes accumulated primarily at the end of the dehydration process. In addition, transmission electron microscopy (TEM) images indicate that ultrastructural cell injuries, perhaps resulting from physical or mechanical stress rather than metabolic damage, were more intense after rapid dehydration. This occurred with no major change in the proteome. These results suggest that desiccation tolerance of A. erici is achieved by constitutive mechanisms.

Evidence for the absence of enzymatic reactions in the glassy state. A case study of xanthophyll cycle pigments in the desiccation-tolerant moss Syntrichia ruralis

Desiccation-tolerant plants are able to withstand dehydration and resume normal metabolic functions upon rehydration. These plants can be dehydrated until their cytoplasm enters a 'glassy state' in which molecular mobility is severely reduced. In desiccation-tolerant seeds, longevity can be enhanced by drying and lowering storage temperature. In these conditions, they still deteriorate slowly, but it is not known if deteriorative processes include enzyme activity. The storage stability of photosynthetic organisms is less studied, and no reports are available on the glassy state in photosynthetic tissues. Here, the desiccation-tolerant moss Syntrichia ruralis was dehydrated at either 75% or <5% relative humidity, resulting in slow (SD) or rapid desiccation (RD), respectively, and different residual water content of the desiccated tissues. The molecular mobility within dry mosses was assessed through dynamic mechanical thermal analysis, showing that at room temperature only rapidly desiccated samples entered the glassy state, whereas slowly desiccated samples were in a 'rubbery' state. Violaxanthin cycle activity, accumulation of plastoglobules, and reorganization of thylakoids were observed upon SD, but not upon RD. Violaxanthin cycle activity critically depends on the activity of violaxanthin de-epoxidase (VDE). Hence, it is proposed that enzymatic activity occurred in the rubbery state (after SD), and that in the glassy state (after RD) no VDE activity was possible. Furthermore, evidence is provided that zeaxanthin has some role in recovery apparently independent of its role in non-photochemical quenching of chlorophyll fluorescence.

Abiotic stress-induced oscillations in steady-state transcript levels of Group 3 LEA protein genes in the moss, Physcomitrella patens

The moss, Physcomitrella patens is a non-seed land plant belonging to early diverging lineages of land plants following colonization of land in the Ordovician period in Earth's history. Evidence suggests that mosses can be highly tolerant of abiotic stress. We showed previously that dehydration stress and abscisic acid treatments induced oscillations in steady-state levels of LEA (Late Embryogenesis Abundant) protein transcripts, and that removal of ABA resulted in rapid attenuation of oscillatory increases in transcript levels. Here, we show that other abiotic stresses like salt and osmotic stresses also induced oscillations in steady-state transcript levels and that the amplitudes of the oscillatory increases in steady-state transcript levels are reflective of the severity of the abiotic stress treatment. Together, our results suggest that oscillatory increases in transcript levels in response to abiotic stresses may be a general phenomenon in P. patens and that temporally dynamic increases in steady-state transcript levels may be important for adaptation to life in constantly fluctuating environmental conditions.

Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms

Although most green algae typically occur in aquatic ecosystems, many species also live partly or permanently under aeroterrestrial conditions, where the cells are exposed to the atmosphere and hence regularly experience dehydration. The ability of algal cells to survive in an air-dried state is termed desiccation tolerance. The mechanisms involved in desiccation tolerance of green algae are still poorly understood, and hence the aim of this review is to summarize recent findings on the effects of desiccation and osmotic water loss. Starting from structural changes, physiological, and biochemical consequences of desiccation will be addressed in different green-algal lineages. The available data clearly indicate a range of strategies, which are rather different in streptophycean and non-streptophycean green algae. While members of the Trebouxiophyceae exhibit effective water loss-prevention mechanisms based on the biosynthesis and accumulation of particular organic osmolytes such as polyols, these compounds are so far not reported in representatives of the Streptophyta. In members of the Streptophyta such as Klebsormidium, the most striking observation is the appearance of cross-walls in desiccated samples, which are strongly undulating, suggesting a high degree of mechanical flexibility. This aids in maintaining structural integrity in the dried state and allows the cell to maintain turgor pressure for a prolonged period of time during the dehydration process. Physiological strategies in aeroterrestrial green algae generally include a rapid reduction of photosynthesis during desiccation, but also a rather quick recovery after rewetting, whereas aquatic species are sensitive to drying. The underlying mechanisms such as the affected molecular components of the photosynthetic machinery are poorly understood in green algae. Therefore, modern approaches based on transcriptomics, proteomics, and/or metabolomics are urgently needed to better understand the molecular mechanisms involved in desiccation-stress physiology of these organisms. The very limited existing information is described in the present review.

Osmotic stress in Arctic and Antarctic strains of the green alga Zygnema (Zygnematales, Streptophyta): effects on photosynthesis and ultrastructure

The osmotic potential and effects of plasmolysis on photosynthetic oxygen evolution and chlorophyll fluorescence were studied in two Arctic Zygnema sp. (strain B, strain G) and two Antarctic Zygnema sp. (strain E, strain D). Antarctic strain D was newly characterized by rbcL sequence analysis in the present study. The two Antarctic strains, D and E, are most closely related and may represent different isolates of the same species, in contrast, strain B and G are separate lineages. Incipient plasmolysis in the cells was determined by light microscopy after incubating cells in sorbitol solutions ranging between 200 mM and 1000 mM sorbitol for 3, 6 and 24 h. In Zygnema strain B and G incipient plasmolysis occurred at ∼600 mM sorbitol solution (720 mOsmol kg⁻¹, ψ = −1.67 MPa) and in strains D and E at ∼300 mM (318 mOsmol kg⁻¹, ψ = −0.8 MPa) sorbitol solution. Hechtian strands were visualized in all plasmolysed cells, which is particularly interesting, as these cells lack pores or plasmodesmata. Ultrastructural changes upon osmotic stress were a retraction of the condensed cytoplasm from the cell walls, damages to chloroplast and mitochondrial membranes, increasing numbers of plastoglobules in the chloroplasts and membrane enclosed particles in the extraplasmatic space. Maximum photosynthetic rates (P_max) in light saturated range were between 145.5 μmol O₂ h⁻¹ mg⁻¹ Chl a in Zygnema G and 752.9 μmol O₂ h⁻¹ mg⁻¹ Chl a in Zygnema E. After incubation in 800 mM sorbitol for 3 h P_max decreased to the following percentage of the initial values: B: 16.3%, D: 16.8%, E: 26.1% and G: 35.0%. Osmotic stress (800 mM sorbitol) decreased maximum photochemical quantum yield of photosystem II (F_v/F_m) when compared to controls. Maximum values of relative electron transport rates of photosystem II (rETR_max) decreased after incubation in 400 mM sorbitol in Zygnema D and E, while they decreased in Zygnema B and G only after incubation in 800 mM sorbitol. The kinetics of the rETR curves were similar for the Arctic strains Zygnema B and G, but distinct from the Antarctic strains Zygnema D and E, which were similar when compared with each other. This suggests that the investigated Arctic Zygnema sp. strains might be better adapted to tolerate osmotic water stress than the investigated strains from the Antarctic.

Plasmolysis effects and osmotic potential of two phylogenetically distinct alpine strains of Klebsormidium (Streptophyta)

The osmotic potential and effects of plasmolysis were investigated in two different Klebsormidium strains from alpine habitats by incubation in 300-2,000 (3,000) mM sorbitol. Several members of this genus were previously found to tolerate desiccation in the vegetative state yet information was lacking on the osmotic potentials of these algae. The strains were morphologically determined as Klebsormidium crenulatum and Klebsormidium nitens. These species belong to distinct clades, as verified by phylogenetic analysis of the rbcL gene. K. crenulatum is part of to the K. crenulatum/mucosum ('F' clade) and K. nitens of the 'E2' clade. Plasmolysis occurred in K. crenulatum at 800 mM sorbitol (961 mOsmol kg(-1), Ψ = -2.09 MPa) and in K. nitens at 600 mM sorbitol (720 mOsmol kg(-1), Ψ = -1.67 MPa). These are extraordinarily high osmotic values (very negative osmotic potentials) compared with values reported for other green algae. In K. crenulatum, the maximum photosynthetic rate (Pmax) in the light-saturated range was 116 μmol O(2) h(-1) mg(-1) chl a. Incubation in 1,000 mM sorbitol decreased Pmax to 74.1% of the initial value, whereas 2,000 mM sorbitol (Ψ = -5.87 MPa) lead to an almost complete loss of oxygen production. In K. nitens, Pmax was 91 μmol O(2) h(-1) mg(-1) chl a under control conditions and incubation in 800 mM sorbitol did not decrease Pmax, 2,000 mM sorbitol decreased Pmax only to about 62.6% of the initial value whereas 3,000 mM sorbitol stopped oxygen evolution. This indicated a broader amplitude for photosynthesis in the examined strain of K. nitens. Control samples and samples plasmolysed for 3 h in 800 mM sorbitol (K. nitens), 1,000 mM sorbitol (K. crenulatum), or 2,000 mM sorbitol were investigated by transmission electron microscopy after chemical or high-pressure freeze fixation. In cells undergoing plasmolysis the protoplasts were retracted from the cell wall, the cytoplasm appeared dense, vacuoles were small and fragmented, and the cytoplasm was filled with ribosomes. Thin cytoplasmic strands were connected to the cell wall; 2,000 mM sorbitol increased the effect. The content of soluble carbohydrates in these two strains was investigated by HPLC, as this is one known mechanism for cells to maintain high osmotic pressure of the cytosol. Both Klebsormidium species contained diverse soluble carbohydrates, including a dominant mixed peak of unidentified oligosaccharides, and more minor amounts of raffinose, sucrose, glucose, xylose, galactose, mannose, inositol, fructose, glycerol, mannitol, and sorbitol. The total content of soluble carbohydrates was approximately 1.2% of the dry weight, indicating that this is not a major factor contributing to the high osmotic potential in these strains of Klebsormidium.

Differential contribution of individual dehydrin genes from Physcomitrella patens to salt and osmotic stress tolerance

The moss Physcomitrella patens can withstand extreme environmental conditions including drought and salt stress. Tolerance to dehydration in mosses is thought to rely on efficient limitation of stress-induced cell damage and repair of cell injury upon stress relief. Dehydrin proteins (DHNs) are part of a conserved cell protecting mechanism in plants although their role in stress tolerance is not well understood. Four DHNs and two DHN-like proteins were identified in the predicted proteome of P. patens. Expression of PpDHNA and PpDHNB was induced by salt and osmotic stress and controlled by abscisic acid. Subcellular localization of the encoded proteins suggested that these dehydrins are localized in cytosol and accumulate near membranes during stress. Comparative analysis of dhnA and dhnB targeted knockout mutants of P. patens revealed that both genes play a role in cellular protection during salt and osmotic stress, although PpDHNA has a higher contribution to stress tolerance. Overexpression of PpDHNA and PpDHNB genes in transgenic Arabidopsis improved rosette and root growth in stress conditions, although PpDHNA was more efficient in this role. These results suggest that specific DHNs contribute considerably to the high stress tolerance of mosses and offer novel tools for genetic engineering stress tolerance of higher plants.

Dehydration stress-induced oscillations in LEA protein transcripts involves abscisic acid in the moss, Physcomitrella patens

• Physcomitrella patens is a bryophyte belonging to early diverging lineages of land plants following colonization of land in the Ordovician period. Mosses are typically found in refugial habitats and can experience rapidly fluctuating environmental conditions. The acquisition of dehydration tolerance by bryophytes is of fundamental importance as they lack water-conducting tissues and are generally one cell layer thick. • Here, we show that dehydration induced oscillations in the steady-state transcript abundances of two group 3 late embryogenesis abundant (LEA) protein genes in P. patens protonemata, and that the amplitudes of these oscillations are reflective of the severity of dehydration stress. • Dehydration stress also induced elevations in the concentrations of abscisic acid (ABA), and ABA alone can also induce dosage-dependent oscillatory increases in the steady-state abundance of LEA protein transcripts. Additionally, removal of ABA resulted in rapid attenuation of these oscillatory increases. • Our data demonstrate that dehydration stress-regulated expression of LEA protein genes is temporally dynamic and highlight the importance of oscillations as a robust mechanism for optimal responses. Our results suggest that dehydration stress-induced oscillations in the steady-state abundance of LEA protein transcripts may constitute an important cellular strategy for adaptation to life in a constantly changing environment.

Plastid stromules are induced by stress treatments acting through abscisic acid

Stromules are highly dynamic stroma-filled tubules that extend from the surface of all plastid types in all multi-cellular plants examined to date. The stromule frequency (percentage of plastids with stromules) has generally been regarded as characteristic of the cell and tissue type. However, the present study shows that various stress treatments, including drought and salt stress, are able to induce stromule formation in the epidermal cells of tobacco hypocotyls and the root hairs of wheat seedlings. Application of abscisic acid (ABA) to tobacco and wheat seedlings induced stromule formation very effectively, and application of abamine, a specific inhibitor of ABA synthesis, prevented stromule induction by mannitol. Stromule induction by ABA was dependent on cytosolic protein synthesis, but not plastid protein synthesis. Stromules were more abundant in dark-grown seedlings than in light-grown seedlings, and the stromule frequency was increased by transfer of light-grown seedlings to the dark and decreased by illumination of dark-grown seedlings. Stromule formation was sensitive to red and far-red light, but not to blue light. Stromules were induced by treatment with ACC (1-aminocyclopropane-1-carboxylic acid), the first committed ethylene precursor, and by treatment with methyl jasmonate, but disappeared upon treatment of seedlings with salicylate. These observations indicate that abiotic, and most probably biotic, stresses are able to induce the formation of stromules in tobacco and wheat seedlings.

Physiological consequences of desiccation in the aquatic bryophyte Fontinalis antipyretica

The moss Fontinalis antipyretica, an aquatic bryophyte previously described as desiccation-intolerant, is known to survive intermittent desiccation events in Mediterranean rivers. To better understand the mechanisms of desiccation tolerance in this species and to reconcile the apparently conflicting evidence between desiccation tolerance classifications and field observations, gross photosynthesis and chlorophyll a fluorescence were measured in field-desiccated bryophyte tips and in bryophyte tips subjected in the laboratory to slow, fast, and very fast drying followed by either a short (30 min) or prolonged (5 days) recovery. Our results show, for the first time, that the metabolic response of F. antipyretica to desiccation, both under field and laboratory conditions, is consistent with a desiccation-tolerance pattern; however, drying must proceed slowly for the bryophyte to regain its pre-desiccation state following rehydration. In addition, the extent of dehydration was found to influence metabolism whereas the drying rate determined the degree of recovery. Photosystem II (PSII) regulation and structural maintenance may be part of the induced desiccation tolerance mechanism allowing this moss to recover from slow drying. The decrease in the photochemical quenching coefficient (qP) immediately following rehydration may serve to alleviate the effects of excess energy on photosystem I (PSI), while low-level non-photochemical quenching (NPQ) would allow an energy shift enabling recovery subsequent to extended periods of desiccation. The findings were confirmed in field-desiccated samples, whose behavior was similar to that of samples slowly dried in the laboratory.

DESICCATION STRESS CAUSES STRUCTURAL AND ULTRASTRUCTURAL ALTERATIONS IN THE AEROTERRESTRIAL GREEN ALGA KLEBSORMIDIUM CRENULATUM (KLEBSORMIDIOPHYCEAE, STREPTOPHYTA) ISOLATED FROM AN ALPINE SOIL CRUST1

Klebsormidium crenulatum (Kütz.) Lokhorst (Klebsormidiophyceae, Streptophyta) isolated from an alpine soil in Tyrol, Austria, was experimentally exposed to desiccation under various relative air humidities (RH 5, 75, and >95%, ambient air 55%-60%). The effects on the structure and ultrastructure of K. crenulatum after 1, 4, or 7 d of desiccation at 5, 75, and >95% RH were investigated. The cross walls were deformed to an undulated shape, and the cell diameter was reduced to ∼60% of the control. Regardless of the RH applied, in all cases the cytoplasm appeared denser compared to that of liquid-culture-grown cells. Electron-dense particles with diameters of 0.4 μm-0.8 μm were observed in the cytoplasm, likely representing lipid droplets. The chloroplasts of desiccated samples contained a large number of plastoglobules. The number and appearance of mitochondria were not visibly altered, as also verified by 3,3' dihexyloxacarbocyanine iodine (DIOC₆ ) staining. The amphiphilic styryl dye FM 1-43 resulted in staining of the plasma membrane in cells from liquid culture. In 7 d desiccated samples, a marked fluorescence is seen in ∼40%-50% of the cells, which were dead. Actin microfilaments (MFs) were drastically disrupted after desiccation; only dotlike actin batches remained. These results demonstrate that flexibility of the cell walls and maintenance of the key organelles play a key role in the tolerance of desiccation stress in K. crenulatum.

Reorganization of photosystem II is involved in the rapid photosynthetic recovery of desert moss Syntrichia caninervis upon rehydration

The moss Syntrichia caninervis (S. caninervis) is one of the dominant species in biological soil crusts of deserts. It has long been the focus of scientific research because of its ecological value. Moreover, S. caninervis has a special significance in biogenesis research because it is characterized by its fast restoration of photosynthesis upon onset of rehydration of the desiccated organism. In order to study the mechanisms of rapid photosynthetic recovery in mosses upon rewatering, we investigated the kinetics of the recovery process of photosynthetic activity in photosystem (PS) II, with an indirect assessment of the photochemical processes based on chlorophyll (Chl) fluorescence measurements. Our results showed that recovery can be divided into two phases. The fast initial phase, completed within 3 min, was characterized by a quick increase in maximal quantum efficiency of PSII (F(v)/F(m)). Over 50% of the PSII activities, including excitation energy transfer, oxygen evolution, charge separation, and electron transport, recovered within 0.5 min after rehydration. The second, slow phase was dominated by the increase of plastoquinone (PQ) reduction and the equilibrium of the energy transport from the inner antenna to the reaction center (RC) of PSII. Analysis of the recovery process in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) revealed that blocking the electron transport from Q(A) to Q(B) did not hamper Chl synthesis or Chl organization in thylakoid membranes under light conditions. A de novo chloroplast protein synthesis was not necessary for the initial recovery of photochemical activity in PSII. In conclusion, the moss's ability for rapid recovery upon rehydration is related to Chl synthesis, quick structural reorganization of PSII, and fast restoration of PSII activity without de novo chloroplast protein synthesis.