Desiccation Tolerance Research Articles

Reinduction of Desiccation Tolerance in Germinated Millet (Pennisetum glaucum L.) Seeds

Reinduction of Desiccation Tolerance in Germinated Millet (Pennisetum glaucum L.) Seeds

Highly efficient CRISPR-mediated gene editing in a rotifer.

Rotifers have been studied in the laboratory and field for over 100 years in investigations of microevolution, ecological dynamics, and ecotoxicology. In recent years, rotifers have emerged as a model system for modern studies of the molecular mechanisms of genome evolution, development, DNA repair, aging, life history strategy, and desiccation tolerance. However, a lack of gene editing tools and transgenic strains has limited the ability to link genotype to phenotype and dissect molecular mechanisms. To facilitate genetic manipulation and the creation of reporter lines in rotifers, we developed a protocol for highly efficient, transgenerational, CRISPR-mediated gene editing in the monogonont rotifer Brachionus manjavacas by microinjection of Cas9 protein and synthetic single-guide RNA into the vitellaria of young amictic (asexual) females. To demonstrate the efficacy of the method, we created knockout mutants of the developmental gene vasa and the DNA mismatch repair gene mlh3. More than half of mothers survived injection and produced offspring. Genotyping these offspring and successive generations revealed that most carried at least 1 CRISPR-induced mutation, with many apparently mutated at both alleles. In addition, we achieved precise CRISPR-mediated knock-in of a stop codon cassette in the mlh3 locus, with half of injected mothers producing F2 offspring with an insertion of the cassette. Thus, this protocol produces knockout and knock-in CRISPR/Cas9 editing with high efficiency, to further advance rotifers as a model system for biological discovery.

Commonalities between the Atacama Desert and Antarctica rhizosphere microbial communities.

Plant-microbiota interactions have significant effects on plant growth, health, and productivity. Rhizosphere microorganisms are involved in processes that promote physiological responses to biotic and abiotic stresses in plants. In recent years, the interest in microorganisms to improve plant productivity has increased, mainly aiming to find promising strains to overcome the impact of climate change on crops. In this work, we hypothesize that given the desertic environment of the Antarctic and the Atacama Desert, different plant species inhabiting these areas might share microbial taxa with functions associated with desiccation and drought stress tolerance. Therefore, in this study, we described and compared the composition of the rhizobacterial community associated with Deschampsia antarctica (Da), Colobanthus quitensis (Cq) from Antarctic territories, and Croton chilensis (Cc), Eulychnia iquiquensis (Ei) and Nicotiana solanifolia (Ns) from coastal Atacama Desert environments by using 16S rRNA amplicon sequencing. In addition, we evaluated the putative functions of that rhizobacterial community that are likely involved in nutrient acquisition and stress tolerance of these plants. Even though each plant microbial rhizosphere presents a unique taxonomic pattern of 3,019 different sequences, the distribution at the genus level showed a core microbiome with a higher abundance of Haliangium, Bryobacter, Bacillus, MND1 from the Nitrosomonadaceae family, and unclassified taxa from Gemmatiamonadaceae and Chitinophagaceae families in the rhizosphere of all samples analyzed (781 unique sequences). In addition, species Gemmatirosa kalamazoonesis and Solibacter usitatus were shared by the core microbiome of both Antarctic and Desert plants. All the taxa mentioned above had been previously associated with beneficial effects in plants. Also, this microbial core composition converged with the functional prediction related to survival under harsh conditions, including chemoheterotrophy, ureolysis, phototrophy, nitrogen fixation, and chitinolysis. Therefore, this study provides relevant information for the exploration of rhizospheric microorganisms from plants in extreme conditions of the Atacama Desert and Antarctic as promising plant growth-promoting rhizobacteria.

Population influences desiccation tolerance in an invasive aquatic snail, Potamopyrgus antipodarum (Tateidae, Mollusca).

Traits in species that influence invasion success may vary in populations across its invaded range. The aquatic New Zealand mud snail, Potamopyrgus antipodarum, reproduces parthenogenetically in its invaded range, and a few distinct clonal genotypes have been identified in North America. Much of the spread of the snail in North America has been the result of unintentional overland transport by recreational water users. Thus, desiccation tolerance may play an important role in the invasion success of this species. The primary goal of these experiments is to determine if variation in desiccation tolerance exists between populations of this species. Here we compared multiple multi-locus genotypes (MLGs) and populations within those genotypes with regard to their desiccation tolerance. We conducted three experiments. The first compared the survival rate over time of snails from three populations and two MLGs with regard to their ability to survive being completely removed from water. The second experiment examined different size classes of snails from the same population, and the third experiment compared four different populations and two MLGs genotypes with respect to their survival rate over time when removed from water but being kept in moist conditions. We found larger snails tolerate desiccation longer than smaller snails with snails between 4 and 4.6 mm surviving at a rate of more than 50% after 12 h while smaller snails survived at a less than 5% rate after 12 h. We also found significant variation both between and within MLGs in their survival rate when completely removed from water and dried with the MLG from the western US having a more than 50% greater survival probability than the eastern MLGs at both 18 and 24 h out of water. When removed from water and kept moist all MLGs had a near 100% survival rate at 60 days at 7 °C, and most survived at a greater than 90% rate at 60 days at 17 °C, while no MLG's survived past 30 days at 27 °C. The results demonstrate that variation for desiccation tolerance exists between populations of this invader which could influence the invasiveness of different populations.

Desiccation tolerance of Termitidae termites in relation to their nest type.

The family Termitidae is renowned for its diverse nesting behaviors, with the evolution of epigeal and arboreal nests hypothesized to increase desiccation stress due to greater exposure to air. However, these nests may also alleviate desiccation stress through humidity regulation. To explore the implications of acquiring epigeal and arboreal nests, we investigated desiccation tolerance traits in 16 Termitidae termite species with varying nest types and analyzed trait correlations. Principal component analysis revealed that termites constructing epigeal and arboreal nests exhibited reduced water loss rates and enhanced survival under desiccated conditions. Furthermore, termites building arboreal nests displayed a notably higher water content. Redundancy analysis demonstrated that nest types accounted for a substantial portion (57.2%) of the observed variation in desiccation tolerance. These findings support the hypothesis that epigeal and arboreal nests in termites are associated with increased desiccation stress and increased desiccation tolerance. These findings highlight the role of nest type in influencing desiccation tolerance mechanisms and water regulation strategies in termites.

Comparative analysis of desiccation tolerance in Oeosporangium elegans and Mickelopteris cordata

Comparative analysis of desiccation tolerance in Oeosporangium elegans and Mickelopteris cordata

Production of dried Beauveria bassiana conidia in packed-column bioreactor using agro-industrial palm oil residues

Agro-industrial waste byproducts are commonly inexpensive organic nutritional sources for production of filamentous fungi, and selection of an optimal bioreactor is a key factor for attaining satisfactory biomass yield and quality. The insect-pathogenic fungus Beauveria bassiana (Hypocreales: Cordycipitaceae) has been extensively mass produced by solid-state fermentation using precooked cereal grains. Here, we propose low-cost palm oil residues as the main substrate using a cylindrical packed-column aerobic bioreactor prototype designed for conidial production of B. bassiana. Fermentation treatments using B. bassiana strain CG1229 assessed the impact of temperature, air flow, substrate moisture content, and ratio between palm fiber (PF) and palm kernel cake (PKC) on conidial yield and desiccation tolerance. Results showed significant enhanced productivity (2 ×1010 conidia g−1 dry matter) and reduced fermentation time (from 168 h to 120 h) by this method compared with the conventional tray bioreactor. Optimal substrate conditions were 60% initial moisture and 30% PF + 70% PKC content, fermentation at 26 °C with aeration rate ≥ 0.2 L min−1. Air drying conidia inside the column yielded > 95% germination. Maximum spore production was achieved in 120 h. Our findings demonstrate the feasibility of using a packed-column bioreactor for mass production of B. bassiana conidia using low-cost agricultural residues, which contributes to a sustainable production method of this biopesticide in Brazil.

Impact of osmotic stress on production, morphology, and fitness of Beauveria bassiana blastospores.

Beauveria bassiana is a cosmopolitan entomopathogenic fungus that can infect over 1000 insect species. During growth inside the host, B. bassiana transitions from hyphal to yeast-like unicellular growth as blastospores. Blastospores are well suited as an active ingredient in biopesticides due to their ease of production by liquid fermentation. Herein, we investigated the impact of hyperosmotic growth environments mediated by ionic and non-ionic osmolytes on two strains of B. bassiana (ESALQ1432 and GHA) relevant to growth morphology, blastospore production, desiccation tolerance, and insecticidal activity. Polyethylene glycol (PEG200) increased osmotic pressure in submerged cultures leading to decreased blastospore size but higher blastospore yields for one strain. Morphologically, decreased blastospore size was linked to increased osmotic pressure. However, smaller blastospores from PEG200 supplemented cultures after air-drying exhibited delayed germination. Ionic osmolytes (NaCl and KCl) generated the same osmotic pressure (2.5-2.7MPa) as 20% glucose and boosted blastospore yields (> 2.0 × 109 blastospores mL-1). Fermentation performed in a bench-scale bioreactor consistently promoted high blastospore yields when using NaCl (2.5MPa) amended media within 3days. Mealworm larvae (Tenebrio molitor) were similarly susceptible to NaCl-grown blastospores and aerial conidia in a dose-time-dependent manner. Collectively, these results demonstrate the use of hyperosmotic liquid culture media in triggering enhanced yeast-like growth by B. bassiana. Understanding the role of osmotic pressure on blastospore formation and fitness will hasten the development of viable commercial fungal biopesticides. KEY POINTS: • Osmotic pressure plays a critical role in submerged fermentation of B. bassiana. • Ionic/non-ionic osmolytes greatly impact blastospore morphology, fitness, and yield. • Desiccation tolerance and bioefficacy of blastospores are affected by the osmolyte.

Oligo-dT anchored cDNA-SRAP and cDNA-SCoT aided identification of transcripts differentially expressed during the early stages of recovery of resurrection plant Haberlea rhodopensis Friv. from freezing-induced desiccation

Resurrection plants are a unique group of flora showing an extraordinary ability to survive extreme water shortages. Among them, the Bulgarian endemic plant Haberlea rhodopensis Friv. has been used as a model to study the mechanisms underlying desiccation tolerance induced by water and low temperature stress. Although extensive transcriptome data, including two RNAseq experiments, are available for drought induced desiccation and recovery, the gene expression during recovery after freezing-induced desiccation (RAF) remains largely unexplored. In this study we performed a differential screening using a new approach, based on oligo-dT anchored cDNA-SRAP (copy DNA sequence-related amplified polymorphism) and a modified oligo-dT anchored cDNA-SCoT (copy DNA start codon targeted) assays to provide new information on genes involved in the tolerance to low temperature stress and the recovery of H. rhodopensis. The putative functions of 19 identified transcript derived fragments were established by searching for homology with known sequences in public databases and the expression of 10 of them during the early stages of recovery was confirmed by quantitative reverse transcription polymerase chain reaction. Some of the identified genes have known functions in the response to abiotic stress in other plant species including resurrection plants, which confirms the effectiveness of the approach used in the present study. The results of this study will contribute to obtaining new information on the defence mechanisms in the recovery process of H. rhodopensis, especially at the early stages of recovery, which is the most vulnerable period for plants.

Conserving useful plants for a sustainable future: species coverage, spatial distribution, and conservation status within the Millennium Seed Bank collection

A substantially rich diversity of the world’s recorded useful plants (UPs) is captured within the Millennium Seed Bank collection hosted by the Royal Botanic Gardens, Kew, at Wakehurst, UK with 13,598 species (34%) belonging to 3696 genera and 325 families. This constitutes just over half of the total accessions and one third of the species and covers all 9 continents, 8 realms, 14 biomes, 34 biodiversity hotspots and 175 countries. The most common beneficial use category within the captured diversity is medicinal, then species with environmental, material, and human food value. About 86% of conserved UP species have a ‘Least Concern’ conservation status but 8% are identified as globally threatened. The advantages of mutual, continued, and long-term partnership (e.g., Mexico) are showcased when conserving plants important for local communities and addressing conservation challenges beyond seed banking. However, the geographic coverage suffers from a lack of partnerships with some parts of the world. Also, a low number of accessions contributed from many countries means that insufficient native range is yet to be captured for many species. This is particularly the case for restricted or narrow distribution species from families or genera with a high incidence of recalcitrant or short-lived seeds. Future planning must tailor better to cover the spatial distribution patterns for individual species, thereby improving the number of accessions and geographical coverage for those captured. Moreover, seed biology research should characterise desiccation tolerance and develop complementary, novel conservation methods, such as cryopreservation, to strengthen conservation options for UP species.

Transcriptomic analysis reveals novel desiccation tolerance mechanism of Cronobacter based on type VI secretion system inhibition

Cronobacter malonaticus (C. malonaticus) is a food-borne pathogen inducing severe infections both in infants and adults, and it could survive in dry powdered infant formula (PIF) for a long time, implying its strong tolerance to desiccation. However, the thorough molecular mechanism of resistance to desiccation remains elusive. When C. malonaticus was exposed to desiccation conditions (7, 15, and 30 d), transcriptomic analysis provided a universal adaptation strategy to withstand desiccation with the increased compatible solutes accumulation, activated stress resistance-related regulators, suppressed protein export and bacterial secretion system, and reduced other unessential survival functions including adhesion, invasion, virulence, and flagellar motility. Importantly, type VI secretion system (T6SS) genes exhibited significantly downregulated expressions, as well as markedly increased survival and viability of their mutants after desiccation treatment, revealing the negative regulation of T6SS in desiccation tolerance. Meanwhile, the decreased expressions of T6SS structure genes in other six species further confirmed the vital role of T6SS in desiccation tolerance of Cronobacter spp. Thus, our studies present a novel hypothesis of desiccation resistance in Cronobacter based on type VI secretion system inhibition, causing the reduction of macromolecule secretion such as effectors and hyperosmolality development within the cytomembrane, which allow Cronobacter to survive in desiccation.

Transcriptomic Profiling of Tomato Leaves Identifies Novel Transcription Factors Responding to Dehydration Stress.

Drought is among the most challenging environmental restrictions to tomatoes (Solanum lycopersi-cum), which causes dehydration of the tissues and results in massive loss of yield. Breeding for dehydration-tolerant tomatoes is a pressing issue as a result of global climate change that leads to increased duration and frequency of droughts. However, the key genes involved in dehydration response and tolerance in tomato are not widely known, and genes that can be targeted for dehydration-tolerant tomato breeding remains to be discovered. Here, we compared phenotypes and transcriptomic profiles of tomato leaves between control and dehydration conditions. We show that dehydration decreased the relative water content of tomato leaves after 2 h of dehydration treatment; however, it promoted the malondialdehyde (MDA) content and ion leakage ratio after 4 h and 12 h of dehydration, respectively. Moreover, dehydration stress triggered oxidative stress as we detected significant increases in H2O2 and O2- levels. Simultaneously, dehydration enhanced the activities of antioxidant enzymes including peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), and phenylalanine ammonia-lyase (PAL). Genome-wide RNA sequencing of tomato leaves treated with or without dehydration (control) identified 8116 and 5670 differentially expressed genes (DEGs) after 2 h and 4 h of dehydration, respectively. These DEGs included genes involved in translation, photosynthesis, stress response, and cytoplasmic translation. We then focused specifically on DEGs annotated as transcription factors (TFs). RNA-seq analysis identified 742 TFs as DEGs by comparing samples dehydrated for 2 h with 0 h control, while among all the DEGs detected after 4 h of dehydration, only 499 of them were TFs. Furthermore, we performed real-time quantitative PCR analyses and validated expression patterns of 31 differentially expressed TFs of NAC, AP2/ERF, MYB, bHLH, bZIP, WRKY, and HB families. In addition, the transcriptomic data revealed that expression levels of six drought-responsive marker genes were upregulated by de-hydration treatment. Collectively, our findings not only provide a solid foundation for further functional characterization of dehydration-responsive TFs in tomatoes but may also benefit the improvement of dehydration/drought tolerance in tomatoes in the future.

Trehalose delivered by cold-responsive nanoparticles improves tolerance of cumulus-oocyte complexes to microwave drying.

Trehalose is a non-permeable protectant that is the key to preserve live cells in a dry state for potential storage at ambient temperatures. After intracellular trehalose delivery via cold-responsive nanoparticles (CRNPs), the objective was to characterize the tolerance of cat cumulus-oocyte complexes (COCs) to different levels of microwave-assisted dehydration. Trehalose was first encapsulated in CRNPs. After exposure to trehalose-laden CRNPs, different water amounts were removed from cat COCs by microwave drying. After each dehydration level, meiotic and developmental competences were evaluated via in vitro maturation, fertilization, and embryo culture. In addition, expressions of critical genes were assessed by quantitative RT-PCR. CRNPs effectively transported trehalose into COCs within 4h of co-incubation at 38.5°C followed by a cold-triggered release at 4°C for 15min. Intracellular presence of trehalose enabled the maintenance of developmental competence (formation of blastocysts) as well as normal gene expression levels of HSP70 and DNMT1at dehydration levels reaching up to 63% of water loss. Intracellular trehalose delivery through CRNPs improves dehydration tolerance of COCs, which opens new options for oocyte storage and fertility preservation at ambient temperatures.

Jinggangmycin stimulates reproduction and increases CHCs-dependent desiccation tolerance in Drosophila melanogaster

Jinggangmycin (JGM), an agricultural antibiotic compound, is mainly used against the rice sheath blight (RSB) Rhizoctonia solani. However, its application may lead to unexpected consequences in insects. In this study, the effects of JGM on the physiological parameters of Drosophila melanogaster were investigated. The results showed that 0.005 g/ml JGM exposure increased female daily egg production and extended the oviposition period, while there was no significant effect on reproduction at 0.016 g/ml. At the same time, desiccation tolerance increased in flies fed 0.005 g/ml JGM. The RT-qPCR results revealed that FAS1 and FAS3 expression were upregulated in 0.005 g/ml JGM treated flies. Consistently, the amount of CHCs accumulated on the cuticle surface increased upon JGM treatment at 0.005 g/ml. Moreover, RNAi for FAS3 decreased desiccation tolerance of JGM-treated flies. These results suggest that JGM affects fatty acid biosynthesis, which in turn enhances reproduction and desiccation tolerance in Drosophila.

Acquisition of Freezing Tolerance of Resurrection Species from Gesneriaceae, a Comparative Study.

Resurrection plants have the unique ability to restore normal physiological activity after desiccation to an air-dry state. In addition to their desiccation tolerance, some of them, such as Haberlea rhodopensis and Ramonda myconi, are also freezing-tolerant species, as they survive subzero temperatures during winter. Here, we compared the response of the photosynthetic apparatus of two other Gesneriaceae species, Ramonda serbica and Ramonda nathaliae, together with H. rhodopensis, to cold and freezing temperatures. The role of some protective proteins in freezing tolerance was also investigated. The water content of leaves was not affected during cold acclimation but exposure of plants to -10 °C induced dehydration of plants. Freezing stress strongly reduced the quantum yield of PSII photochemistry (Y(II)) and stomatal conductance (gs) on the abaxial leaf side. In addition, the decreased ratio of Fv/Fm suggested photoinhibition or sustained quenching. Freezing-induced desiccation resulted in the inhibition of PSII activity, which was accompanied by increased thermal energy dissipation. In addition, an increase of dehydrins and ELIPs was detected, but the protein pattern differed between species. During recovery, the protein abundance decreased and plants completely recovered their photosynthetic activity. Thus, our results showed that R. serbica, R. nathaliae, and H. rhodopensis survive freezing stress due to some resurrection-linked traits and confirmed their freezing tolerance.

When Phased without Water: Biophysics of Cellular Desiccation, from Biomolecules to Condensates.

The molecular machinery that enables life has evolved in water, yet many of the organisms around us are able to survive even extreme desiccation. Especially remarkable are single-cell and sedentary organisms that rely on specialized biomolecular machinery to survive in environments that are routinely subjected to a near-complete lack of water. In this review, we zoom in on the molecular level of what is happening in the cellular environment under water stress. We cover the various mechanisms by which biochemical components of the cell can dysfunction in dehydrated cells and detail the different strategies that organisms have evolved to eliminate or cope with these desiccation-induced perturbations. We specifically focus on two survival strategies: (1) the use of disordered proteins to protect the cellular environment before, during, and in the recovery from desiccation, and (2) the use of biomolecular condensates as a self-assembly mechanism that can sequester or protect specific cellular machinery in times of water stress. We provide a summary of experimental work describing the critical contributions of disordered proteins and biomolecular condensates to the cellular response to water loss and highlight their role in desiccation tolerance. Desiccation biology is an exciting area of cell biology, still far from being completely explored. Understanding it on the molecular level is bound to give us critical new insights in how life adapted/can adapt to the loss of water, spanning from the early colonization of land to how we can deal with climate change in our future.

ITRAQ-Based Proteomic and Physiological Analyses Reveal the Mechanisms of Dehydration and Cryopreservation Tolerance of Sophora tonkinensis Gagnep. Seeds.

Sophora tonkinensi is a shrub of the genus Sophora in the family Fabaceae with anti-inflammatory and pain-relieving effects. While the cultivation, chemical makeup, and medicinal properties of S. tonkinensis have been reported, the physiological mechanisms governing its dehydration and cryopreservation tolerance of seeds remain unclear. In this study, we investigated the morphological, physiological, biochemical, and protein expression characteristics of S. tonkinensis seeds subjected to dehydration and cryopreservation techniques via the observation of cell microstructure, determination of antioxidant enzyme activity, and iTRAQ-based proteomic analysis, respectively. The results of the study demonstrated that the seeds possessed a certain level of tolerance to dehydration. The highest germination percentage of 83.0% was observed after 2 h of dehydration (10.1% water content), which was identified as the optimal time point for cryopreservation. However, the germination percentage was reduced to only 30.5% when the water content reached 5.4%, indicating that S. tonkinensis seeds exhibit intermediate storage behavior. Further investigation revealed that during seed dehydration and cryopreservation treatment, liposomes were gradually and highly fused, whereas the activities of ROS scavenging and stress defense were significantly enhanced. During dehydration, the seed tissues formed a protective mechanism of stress resistance based on protein processing in the endoplasmic reticulum and antioxidant system, which was related to the dehydration tolerance. Moreover, only three differentially expressed LEA proteins were identified, and it is speculated that the strengthening of intracellular metabolism and the absence of specific LEA and dehydrins could be crucial factors for the reduced germination percentage after excessive dehydration and cryopreservation.

Intra-Specific Variation in Desiccation Tolerance of Citrus sinensis 'bingtangcheng' (L.) Seeds under Different Environmental Conditions in China.

Intra-specific variation in seed storage behaviour observed in several species has been related to different maternal environments. However, the particular environmental conditions and molecular processes involved in intra-specific variation of desiccation tolerance remain unclear. We chose Citrus sinensis 'bingtangcheng' for the present study due to its known variability in desiccation tolerance amongst seed lots. Six seed lots of mature fruits were harvested across China and systematically compared for drying sensitivity. Annual sunshine hours and average temperature from December to May showed positive correlations with the level of seed survival of dehydration. Transcriptional analysis indicated significant variation in gene expression between relatively desiccation-tolerant (DT) and -sensitive (DS) seed lots after harvest. The major genes involved in late seed maturation, such as heat shock proteins, showed higher expression in the DT seed lot. Following the imposition of drying, 80% of stress-responsive genes in the DS seed lot changed to the stable levels seen in the DT seed lot prior to and post-desiccation. However, the changes in expression of stress-responsive genes in DS seeds did not improve their tolerance to desiccation. Thus, higher desiccation tolerance of Citrus sinensis 'bingtangcheng' seeds is modulated by the maternal environment (e.g., higher annual sunshine hours and seasonal temperature) during seed development and involves stable expression levels of stress-responsive genes.

Sporogenesis in Physcomitrium patens: Intergenerational collaboration and the development of the spore wall and aperture.

Although the evolution of spores was critical to the diversification of plants on land, sporogenesis is incompletely characterized for model plants such as Physcomitrium patens. In this study, the complete process of P. patens sporogenesis is detailed from capsule expansion to mature spore formation, with emphasis on the construction of the complex spore wall and proximal aperture. Both diploid (sporophytic) and haploid (spores) cells contribute to the development and maturation of spores. During capsule expansion, the diploid cells of the capsule, including spore mother cells (SMCs), inner capsule wall layer (spore sac), and columella, contribute a locular fibrillar matrix that contains the machinery and nutrients for spore ontogeny. Nascent spores are enclosed in a second matrix that is surrounded by a thin SMC wall and suspended in the locular material. As they expand and separate, a band of exine is produced external to a thin foundation layer of tripartite lamellae. Dense globules assemble evenly throughout the locule, and these are incorporated progressively onto the spore surface to form the perine external to the exine. On the distal spore surface, the intine forms internally, while the spiny perine ornamentation is assembled. The exine is at least partially extrasporal in origin, while the perine is derived exclusively from outside the spore. Across the proximal surface of the polar spores, an aperture begins formation at the onset of spore development and consists of an expanded intine, an annulus, and a central pad with radiating fibers. This complex aperture is elastic and enables the proximal spore surface to cycle between being compressed (concave) and expanded (rounded). In addition to providing a site for water intake and germination, the elastic aperture is likely involved in desiccation tolerance. Based on the current phylogenies, the ancestral plant spore contained an aperture, exine, intine, and perine. The reductive evolution of liverwort and hornwort spores entailed the loss of perine in both groups and the aperture in liverworts. This research serves as the foundation for comparisons with other plant groups and for future studies of the developmental genetics and evolution of spores across plants.

Comparative Transcriptomic and Proteomic Analyses Provide New Insights into the Tolerance to Cyclic Dehydration in a Lichen Phycobiont.

Desiccation tolerance (DT) is relatively frequent in non-vascular plants and green algae. However, it is poorly understood how successive dehydration/rehydration (D/R) cycles shape their transcriptomes and proteomes. Here, we report a comprehensive analysis of adjustments on both transcript and protein profiles in response to successive D/R cycles in Coccomyxa simplex (Csol), isolated from the lichen Solorina saccata. A total of 1833 transcripts and 2332 proteins were differentially abundant as a consequence of D/R; however, only 315 of these transcripts/proteins showed similar trends. Variations in both transcriptomes and proteomes along D/R cycles together with functional analyses revealed an extensive decrease in transcript and protein levels during dehydration, most of them involved in gene expression, metabolism, substance transport, signalling and folding catalysis, among other cellular functions. At the same time, a series of protective transcripts/proteins, such as those related to antioxidant defence, polyol metabolism and autophagy, was upregulated during dehydration. Overall, our results show a transient decrease in most cellular functions as a result of drying and a gradual reactivation of specific cell processes to accommodate the hydration status along successive D/R cycles. This study provides new insights into key mechanisms involved in the DT of Csol and probably other dehydration-tolerant microalgae. In addition, functionally characterising the high number of genes/proteins of unknown functions found in this study may lead to the discovery of new DT mechanisms.