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

Ageing kinetics of fern chlorophyllous spores during dry storage is determined by its antioxidant potential and likely induced by photosynthetic machinery

Ageing in dry chlorophyllous propagules is leaded by photooxidation through the photosynthetic machinery, but why species differ in longevity and the ageing mechanisms of when light and oxygen are absent are unknown. We hypothesize that the cellular antioxidant capacity is key for the inter- and intra-specific differences in the ageing process. We have tested this hypothesis in chlorophyllous spores of two ferns. They were subjected to four different storage regimes resulting from light/dark and normoxia/hypoxia combinations. Lipophilic and hydrophilic antioxidants, reactive oxygen species (ROS), and photosynthetic pigments were analysed in parallel to germination and the recovery of Fv/Fm over a storage period of up to 22-months. We show that light and oxygen accelerate the ageing process, but their mechanisms (ROS, increase, antioxidant capacity decrease, loss of efficiency of the photosystem II, pigment degradation) appear the same under all conditions tested. The end of the asymptomatic phase of longevity, when a sudden drop of germination occurs, seems to be determined by a threshold in the depletion of antioxidants. Our results support the hypothesis that ageing kinetics in dry plant propagules is determined by the antioxidant system, but also suggests an active role of the photosynthetic machinery during ageing, even in darkness and hypoxia.

Non-chlorophyllous and crypto-chlorophyllous fern spores differ in their mobilisation of fatty acids during priming

During fern spore germination, lipid hydrolysis primarily provides the energy to activate their metabolism. In this research, fatty acids (linoleic, oleic, palmitic and stearic) were quantified in the spores exposed or not to priming (hydration-dehydration treatments). Five fern species were investigated, two from xerophilous shrubland and three from a cloud forest. We hypothesised that during the priming hydration phase, the fatty acids profile would change in concentration, depending on the spore type (non-chlorophyllous and crypto-chlorophyllous). The fatty acid concentration was determined by gas chromatograph-mass spectrometer. Chlorophyll in spores was vizualised by epifluorescence microscopy and quantified by high-resolution liquid chromatography with a DAD-UV/Vis detector. Considering all five species and all the treatments, the oleic acid was the most catabolised. After priming, we identified two patterns in the fatty acid metabolism: (1) in non-chlorophyllous species, oleic, palmitic, and linoleic acids were catabolised during imbibition and (2) in crypto-chlorophyllous species, these fatty acids increased in concentration. These patterns suggest that crypto-chlorophyllous spores with homoiochlorophylly (chlorophyll retained after drying) might not require the assembly of new photosynthetic apparatus during dark imbibition. Thus, these spores might require less energy from pre-existing lipids and less fatty acids as 'building blocks' for cell membranes than non-chlorophyllous spores, which require de novo synthesis and structuring of the photosynthetic apparatus.

The relationship between chlorophyllous spores and mycorrhizal associations in ferns: evidence from an evolutionary approach

PREMISE

Approximately 14% of all fern species have physiologically active chlorophyllous spores that are much more short-lived than the more common and dormant achlorophyllous spores. Most chlorophyllous-spored species (70%) are epiphytes and account for almost 37% of all epiphytic ferns. Chlorophyllous-spored ferns are also overrepresented among fern species in habitats with waterlogged soils, of which nearly 60% have chlorophyllous spores. Ferns in these disparate habitat types also have a low incidence of mycorrhizal associations. We therefore hypothesized that autotrophic chlorophyllous spores represent an adaptation of ferns to habitats with scarce mycorrhizal associations.

METHODS

We evaluated the coevolution of chlorophyllous spores and mycorrhizal associations in ferns and their relation to habitat type using phylogenetic comparative methods.

RESULTS

Although we did not find support for the coevolution of spore type and mycorrhizal associations, we did find that chlorophyllous spores and the absence of mycorrhizal associations have coevolved with epiphytic and waterlogged habitats. Transition rates to epiphytic and waterlogged habitats were significantly higher in species with chlorophyllous spores compared to achlorophyllous lineages.

CONCLUSIONS

Spore type and mycorrhizal associations appear to play important roles in the radiation of ferns into different habitat types. Future work should focus on clarifying the functional significance of these associations.

Exploring the High Variability of Vegetative Desiccation Tolerance in Pteridophytes

In the context of plant evolution, pteridophytes, which is comprised of lycophytes and ferns, occupy an intermediate position between bryophytes and seed plants, sharing characteristics with both groups. Pteridophytes is a highly diverse group of plant species that occupy a wide range of habitats including ecosystems with extreme climatic conditions. There is a significant number of pteridophytes that can tolerate desiccation by temporarily arresting their metabolism in the dry state and reactivating it upon rehydration. Desiccation-tolerant pteridophytes exhibit a strategy that appears to be intermediate between the constitutive and inducible desiccation tolerance (DT) mechanisms observed in bryophytes and angiosperms, respectively. In this review, we first describe the incidence and anatomical diversity of desiccation-tolerant pteridophytes and discuss recent advances on the origin of DT in vascular plants. Then, we summarize the highly diverse adaptations and mechanisms exhibited by this group and describe how some of these plants could exhibit tolerance to multiple types of abiotic stress. Research on the evolution and regulation of DT in different lineages is crucial to understand how plants have adapted to extreme environments. Thus, in the current scenario of climate change, the knowledge of the whole landscape of DT strategies is of vital importance as a potential basis to improve plant abiotic stress tolerance.

Differences in biochemical, gas exchange and hydraulic response to water stress in desiccation tolerant and sensitive fronds of the fern Anemia caffrorum

Desiccation tolerant plants can survive extreme water loss in their vegetative tissues. The fern Anemia caffrorum produces desiccation tolerant (DT) fronds in the dry season and desiccation sensitive (DS) fronds in the wet season, providing a unique opportunity to explore the physiological mechanisms associated with desiccation tolerance. Anemia caffrorum plants with either DT or DS fronds were acclimated in growth chambers. Photosynthesis, frond structure and anatomy, water relations and minimum conductance to water vapour were measured under well-watered conditions. Photosynthesis, hydraulics, frond pigments, antioxidants and abscisic acid contents were monitored under water deficit. A comparison between DT and DS fronds under well-watered conditions showed that the former presented higher leaf mass per area, minimum conductance, tissue elasticity and lower CO₂ assimilation. Water deficit resulted in a similar induction of abscisic acid in both frond types, but DT fronds maintained higher stomatal conductance and upregulated more prominently lipophilic antioxidants. The seasonal alternation in production of DT and DS fronds in A. caffrorum represents a mechanism by which carbon gain can be maximized during the rainy season, and a greater investment in protective mechanisms occurs during the hot dry season, enabling the exploitation of episodic water availability.

Born to revive: molecular and physiological mechanisms of double tolerance in a paleotropical and resurrection plant

Resurrection plants recover physiological functions after complete desiccation. Almost all of them are native to tropical warm environments. However, the Gesneriaceae include four genera, remnant of the past palaeotropical flora, which inhabit temperate mountains. One of these species is additionally freezing-tolerant: Ramonda myconi. We hypothesise that this species has been able to persist in a colder climate thanks to some resurrection-linked traits. To disentangle the physiological mechanisms underpinning multistress tolerance to desiccation and freezing, we conducted an exhaustive seasonal assessment of photosynthesis (gas exchange, limitations to partitioning, photochemistry and galactolipids) and primary metabolism (through metabolomics) in two natural populations at different elevations. R. myconi displayed low rates of photosynthesis, largely due to mesophyll limitation. However, plants were photosynthetically active throughout the year, excluding a reversible desiccation period. Common responses to desiccation and low temperature involved chloroplast protection: enhanced thermal energy dissipation, higher carotenoid to Chl ratio and de-epoxidation of the xanthophyll cycle. As specific responses, antioxidants and secondary metabolic routes rose upon desiccation, while putrescine, proline and a variety of sugars rose in winter. The data suggest conserved mechanisms to cope with photo-oxidation during desiccation and cold events, while additional metabolic mechanisms may have evolved as specific adaptations to cold during recent glaciations.

Desiccation Tolerance in Chlorophyllous Fern Spores: Are Ecophysiological Features Related to Environmental Conditions?

Fern spores of most species are desiccation tolerant (DT) and, in some cases, are photosynthetic at maturation, the so-called chlorophyllous spores (CS). The lifespan of CS in the dry state is very variable among species. The physiological, biochemical, and biophysical mechanisms underpinning this variability remain understudied and their interpretation from an ecophysiological approach virtually unexplored. In this study, we aimed at fulfilling this gap by assessing photochemical, hydric, and biophysical properties of CS from three temperate species with contrasting biological strategies and longevity in the dry state: Equisetum telmateia (spore maturation and release in spring, ultrashort lifespan), Osmunda regalis (spore maturation and release in summer, medium lifespan), Matteuccia struthiopteris (spore maturation and release in winter, medium-long lifespan). After subjection of CS to controlled drying treatments, results showed that the three species displayed different extents of DT. CS of E. telmateia rapidly lost viability after desiccation, while the other two withstood several dehydration-rehydration cycles without compromising viability. The extent of DT was in concordance with water availability in the sporulation season of each species. CS of O. regalis and M. struthiopteris carried out the characteristic quenching of chlorophyll fluorescence, widely displayed by other DT cryptogams during drying, and had higher tocopherol and proline contents. The turgor loss point of CS is also related to the extent of DT and to the sporulation season: lowest values were found in CS of M. struthiopteris and O. regalis. The hydrophobicity of spores in these two species was higher and probably related to the prevention of water absorption under unfavorable conditions. Molecular mobility, estimated by dynamic mechanical thermal analysis, confirmed an unstable glassy state in the spores of E. telmateia, directly related to the low DT, while the DT species entered in a stable glassy state when dried. Overall, our data revealed a DT syndrome related to the season of sporulation that was characterized by higher photoprotective potential, specific hydric properties, and lower molecular mobility in the dry state. Being unicellular haploid structures, CS represent not only a challenge for germplasm preservation (e.g., as these spores are prone to photooxidation) but also an excellent opportunity for studying mechanisms of DT in photosynthetic cells.