The Physcomitrella patens unique alpha-dioxygenase participates in both developmental processes and defense responses

Evolutionary Analysis of Six Gene Families Part of the Reactive Oxygen Species (ROS) Gene Network in Three Brassicaceae Species

Climate change is expected to intensify the occurrence of abiotic stress in plants, such as hypoxia and salt stresses, leading to the production of reactive oxygen species (ROS), which need to be effectively managed by various oxido-reductases encoded by the so-called ROS gene network. Here, we studied six oxido-reductases families in three Brassicaceae species, Arabidopsis thaliana as well as Nasturtium officinale and Eutrema salsugineum, which are adapted to hypoxia and salt stress, respectively. Using available and new genomic data, we performed a phylogenomic analysis and compared RNA-seq data to study genomic and transcriptomic adaptations. This comprehensive approach allowed for the gaining of insights into the impact of the adaptation to saline or hypoxia conditions on genome organization (gene gains and losses) and transcriptional regulation. Notably, the comparison of the N. officinale and E. salsugineum genomes to that of A. thaliana highlighted changes in the distribution of ohnologs and homologs, particularly affecting class III peroxidase genes (CIII Prxs). These changes were specific to each gene, to gene families subjected to duplication events and to each species, suggesting distinct evolutionary responses. The analysis of transcriptomic data has allowed for the identification of genes related to stress responses in A. thaliana, and, conversely, to adaptation in N. officinale and E. salsugineum.

Transcriptional profiling reveals conserved and species-specific plant defense responses during the interaction of Physcomitrium patens with Botrytis cinerea

Evolutionary conserved defense mechanisms present in extant bryophytes and angiosperms, as well as moss-specific defenses are part of the immune response of Physcomitrium patens. Bryophytes and tracheophytes are descendants of early land plants that evolved adaptation mechanisms to cope with different kinds of terrestrial stresses, including drought, variations in temperature and UV radiation, as well as defense mechanisms against microorganisms present in the air and soil. Although great advances have been made on pathogen perception and subsequent defense activation in angiosperms, limited information is available in bryophytes. In this study, a transcriptomic approach uncovered the molecular mechanisms underlying the defense response of the bryophyte Physcomitrium patens (previously Physcomitrella patens) against the important plant pathogen Botrytis cinerea. A total of 3.072 differentially expressed genes were significantly affected during B. cinerea infection, including genes encoding proteins with known functions in angiosperm immunity and involved in pathogen perception, signaling, transcription, hormonal signaling, metabolic pathways such as shikimate and phenylpropanoid, and proteins with diverse role in defense against biotic stress. Similarly as in other plants, B. cinerea infection leads to downregulation of genes involved in photosynthesis and cell cycle progression. These results highlight the existence of evolutionary conserved defense responses to pathogens throughout the green plant lineage, suggesting that they were probably present in the common ancestors of land plants. Moreover, several genes acquired by horizontal transfer from prokaryotes and fungi, and a high number of P. patens-specific orphan genes were differentially expressed during B. cinerea infection, suggesting that they are important players in the moss immune response.

Membrane Lipids, Waxes and Oxylipins in the Moss Model Organism Physcomitrella patens

The moss Physcomitrella patens receives increased scientific interest since its genome was sequenced a decade ago. As a bryophyte, it represents the first group of plants that evolved in a terrestrial habitat still without a vascular system that developed later in tracheophytes. It is easily transformable via homologous recombination, which enables the formation of targeted loss-of-function mutants. Even though genetics, development and life cycle in Physcomitrella are well studied nowadays, research on lipids in Physcomitrella is still underdeveloped. This review aims on presenting an overview on the state of the art of lipid research with a focus on membrane lipids, surface lipids and oxylipins. We discuss in this review that Physcomitrella possesses very interesting features regarding its membrane lipids. Here, the presence of very-long-chain polyunsaturated fatty acids (VLC-PUFA) still shows a closer similarity to marine microalgae than to vascular plants. Unlike algae, Physcomitrella has a cuticle comparable to vascular plants composed of cutin and waxes. The presence of VLC-PUFA in Physcomitrella also leads to a greater variability of signaling lipids even though the phytohormone jasmonic acid is not present in this organism, which is different to vascular plants. In summary, the research on lipids in Physcomitrella is still in its infancy, especially considering membrane lipids. We hope that this review will help to promote the further advancement of lipid research in this important model organism in the future, so we can better understand how lipids are involved in the evolution of land plants.

Sunagoke Moss (Racomitrium japonicum) Used for Greening Roofs Is Severely Damaged by Sclerotium delphinii and Protected by a Putative Bacillus amyloliquefaciens Isolate

Mosses are ecologically important plants also used for greening, gardening, and decorative purposes. Knowledge of the microbial flora associated with mosses is expected to be important for control and preservation of global and local environments. However, the moss-associated microbial flora is often poorly known. Moss-associated fungi and bacteria may promote plant growth and pest control, but they may be alternative hosts for pathogens of vascular plants. In this study, the fungus Sclerotinia delphinii was identified for the first time as a pathogen that causes severe damage to Sunagoke moss (Racomitrium japonicum). This moss is used for greening roofs and walls of buildings in urban environments owing to its notable tolerance of environmental stresses. Inoculation with the S. delphinii strain SR1 of the mono- and dicotyledonous seed plants Hordeum vulgare, Brassica rapa var. pekinensis, Lactuca sativa, and Spinacia oleracea, in addition to the liverwort Marchantia polymorpha and the moss Physcomitrella patens, showed that the fungus has a wide host range. Colonization with SR1 progressed more rapidly in non-vascular than in vascular plant species. Studies with P. patens under controlled conditions showed that SR1 secreted a fluid during colonization. Treatment with the secretion induced production of reactive oxygen species in the moss. Endogenous peroxidase partially inhibited SR1 colonization of P. patens. A bacterial isolate, most likely Bacillus amyloliquefaciens, that coexists with R. japonicum was antagonistic to SR1 growth. Taken together, the present results suggest that fungal colonization of mosses may be prevented by a peroxidase secreted by the moss and an antagonistic bacterium coexisting in the moss habitat. The findings suggest that there is potential to apply biological control measures for protection of mosses against fungal pathogens.

Manipulation of Bryophyte Hosts by Pathogenic and Symbiotic Microbes

The colonization of plant tissues by pathogenic and symbiotic microbes is associated with a strong and directed effort to reprogram host cells in order to permit, promote and sustain microbial growth. In response to colonization, hosts accommodate or sequester invading microbes by activating a set of complex regulatory programs that initiate symbioses or bolster defenses. Extensive research has elucidated a suite of molecular and physiological responses occurring in plant hosts and their microbial partners; however, this information is mostly limited to model systems representing evolutionarily young plant lineages such as angiosperms. The extent to which these processes are conserved across land plants is therefore poorly understood. In this review, we outline key aspects of host reprogramming that occur during plant-microbe interactions in early diverging land plants belonging to the bryophytes (liverworts, hornworts and mosses). We discuss how further knowledge of bryophyte-microbe interactions will advance our understanding of how plants and microbes co-operated and clashed during the conquest of land.

Adaptation Mechanisms in the Evolution of Moss Defenses to Microbes

Bryophytes, including mosses, liverworts and hornworts are early land plants that have evolved key adaptation mechanisms to cope with abiotic stresses and microorganisms. Microbial symbioses facilitated plant colonization of land by enhancing nutrient uptake leading to improved plant growth and fitness. In addition, early land plants acquired novel defense mechanisms to protect plant tissues from pre-existing microbial pathogens. Due to its evolutionary stage linking unicellular green algae to vascular plants, the non-vascular moss Physcomitrella patens is an interesting organism to explore the adaptation mechanisms developed in the evolution of plant defenses to microbes. Cellular and biochemical approaches, gene expression profiles, and functional analysis of genes by targeted gene disruption have revealed that several defense mechanisms against microbial pathogens are conserved between mosses and flowering plants. P. patens perceives pathogen associated molecular patterns by plasma membrane receptor(s) and transduces the signal through a MAP kinase (MAPK) cascade leading to the activation of cell wall associated defenses and expression of genes that encode proteins with different roles in plant resistance. After pathogen assault, P. patens also activates the production of ROS, induces a HR-like reaction and increases levels of some hormones. Furthermore, alternative metabolic pathways are present in P. patens leading to the production of a distinct metabolic scenario than flowering plants that could contribute to defense. P. patens has acquired genes by horizontal transfer from prokaryotes and fungi, and some of them could represent adaptive benefits for resistance to biotic stress. In this review, the current knowledge related to the evolution of plant defense responses against pathogens will be discussed, focusing on the latest advances made in the model plant P. patens.

An Innate Immunity Pathway in the Moss Physcomitrella patens

MAP kinase (MPK) cascades in Arabidopsis thaliana and other vascular plants are activated by developmental cues, abiotic stress, and pathogen infection. Much less is known of MPK functions in nonvascular land plants such as the moss Physcomitrella patens Here, we provide evidence for a signaling pathway in P. patens required for immunity triggered by pathogen associated molecular patterns (PAMPs). This pathway induces rapid growth inhibition, a novel fluorescence burst, cell wall depositions, and accumulation of defense-related transcripts. Two P. patens MPKs (MPK4a and MPK4b) are phosphorylated and activated in response to PAMPs. This activation in response to the fungal PAMP chitin requires a chitin receptor and one or more MAP kinase kinase kinases and MAP kinase kinases. Knockout lines of MPK4a appear wild type but have increased susceptibility to the pathogenic fungi Botrytis cinerea and Alternaria brassisicola Both PAMPs and osmotic stress activate some of the same MPKs in Arabidopsis. In contrast, abscisic acid treatment or osmotic stress of P. patens does not activate MPK4a or any other MPK, but activates at least one SnRK2 kinase. Signaling via MPK4a may therefore be specific to immunity, and the moss relies on other pathways to respond to osmotic stress.

Activation of Shikimate, Phenylpropanoid, Oxylipins, and Auxin Pathways in Pectobacterium carotovorum Elicitors-Treated Moss

Plants have developed complex defense mechanisms to cope with microbial pathogens. Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are perceived by pattern recognition receptors (PRRs), leading to the activation of defense. While substantial progress has been made in understanding the activation of plant defense by PAMPs and DAMPs recognition in tracheophytes, far less information exists on related processes in early divergent plants like mosses. The aim of this study was to identify genes that were induced in P. patens in response to elicitors of Pectobacterium carotovorum subsp. carotovorum, using a cDNA suppression subtractive hybridization (SSH) method. A total of 239 unigenes were identified, including genes involved in defense responses related to the shikimate, phenylpropanoid, and oxylipin pathways. The expression levels of selected genes related to these pathways were analyzed using quantitative RT-PCR, confirming their rapid induction by P.c. carotovorum derived elicitors. In addition, P. patens induced cell wall reinforcement after elicitor treatment by incorporation of phenolic compounds, callose deposition, and elevated expression of Dirigent-like encoding genes. Small molecule defense markers and phytohormones such as cinnamic acid, 12-oxo-phytodienoic acid, and auxin levels all increased in elicitor-treated moss tissues. In contrast, salicylic acid levels decreased while abscisic acid levels remained unchanged. P. patens reporter lines harboring an auxin-inducible promoter fused to β-glucuronidase revealed GUS activity in protonemal and gametophores tissues treated with elicitors of P.c. carotovorum, consistent with a localized activation of auxin signaling. These results indicate that P. patens activates the shikimate, phenylpropanoid, oxylipins, and auxin pathways upon treatment with P.c. carotovorum derived elicitors.

Moss Pathogenesis-Related-10 Protein Enhances Resistance to Pythium irregulare in Physcomitrella patens and Arabidopsis thaliana

Plants respond to pathogen infection by activating signaling pathways leading to the accumulation of proteins with diverse roles in defense. Here, we addressed the functional role of PpPR-10, a pathogenesis-related (PR)-10 gene, of the moss Physcomitrella patens, in response to biotic stress. PpPR-10 belongs to a multigene family and encodes a protein twice the usual size of PR-10 proteins due to the presence of two Bet v1 domains. Moss PR-10 genes are differentially regulated during development and inoculation with the fungal pathogen Botrytis cinerea. Specifically, PpPR-10 transcript levels increase significantly by treatments with elicitors of Pectobacterium carotovorum subsp. carotovorum, spores of B. cinerea, and the defense hormone salicylic acid. To characterize the role of PpPR-10 in plant defense against pathogens, we conducted overexpression analysis in P. patens and in Arabidopsis thaliana. We demonstrate that constitutive expression of PpPR-10 in moss tissues increased resistance against the oomycete Pythium irregulare. PpPR-10 overexpressing moss plants developed less symptoms and decreased mycelium growth than wild type plants. In addition, PpPR-10 overexpressing plants constitutively produced cell wall depositions in protonemal tissue. Ectopic expression of PpPR-10 in Arabidopsis resulted in increased resistance against P. irregulare as well, evidenced by smaller lesions and less cellular damage compared to wild type plants. These results indicate that PpPR-10 is functionally active in the defense against the pathogen P. irregulare, in both P. patens and Arabidopsis, two evolutionary distant plants. Thus, P. patens can serve as an interesting source of genes to improve resistance against pathogen infection in flowering plants.

Physcomitrella patens Activates Defense Responses against the Pathogen Colletotrichum gloeosporioides

The moss Physcomitrella patens is a suitable model plant to analyze the activation of defense mechanisms after pathogen assault. In this study, we show that Colletotrichum gloeosporioides isolated from symptomatic citrus fruit infects P. patens and cause disease symptoms evidenced by browning and maceration of tissues. After C. gloeosporioides infection, P. patens reinforces the cell wall by the incorporation of phenolic compounds and induces the expression of a Dirigent-protein-like encoding gene that could lead to the formation of lignin-like polymers. C. gloeosporioides-inoculated protonemal cells show cytoplasmic collapse, browning of chloroplasts and modifications of the cell wall. Chloroplasts relocate in cells of infected tissues toward the initially infected C. gloeosporioides cells. P. patens also induces the expression of the defense genes PAL and CHS after fungal colonization. P. patens reporter lines harboring the auxin-inducible promoter from soybean (GmGH3) fused to β-glucuronidase revealed an auxin response in protonemal tissues, cauloids and leaves of C. gloeosporioides-infected moss tissues, indicating the activation of auxin signaling. Thus, P. patens is an interesting plant to gain insight into defense mechanisms that have evolved in primitive land plants to cope with microbial pathogens.

Leaf oil bodies are subcellular factories producing antifungal oxylipins

Oil bodies act as lipid storage compartments in plant cells. In seeds they supply energy for germination and early seedling growth. Oil bodies are also present in the leaves of many vascular plants, but their function in leaves has been poorly understood. Recent studies with oil bodies from senescent Arabidopsis thaliana leaves identified two enzymes, peroxygenase (CLO3) and α-dioxygenase (α-DOX), which together catalyze a coupling reaction to produce an antifungal compound (2-hydroxy-octadecanoic acid) from α-linolenic acid. Leaf oil bodies also have other enzymes including lipoxygenases, phospholipases, and triacylglycerol lipases. Hence, leaf oil bodies might function as intracellular factories to efficiently produce stable compounds via unstable intermediates by concentrating the enzymes and hydrophobic substrates.

Oxylipins in moss development and defense

Oxylipins are oxygenated fatty acids that participate in plant development and defense against pathogen infection, insects, and wounding. Initial oxygenation of substrate fatty acids is mainly catalyzed by lipoxygenases (LOXs) and α-dioxygenases but can also take place non-enzymatically by autoxidation or singlet oxygen-dependent reactions. The resulting hydroperoxides are further metabolized by secondary enzymes to produce a large variety of compounds, including the hormone jasmonic acid (JA) and short-chain green leaf volatiles. In flowering plants, which lack arachidonic acid, oxylipins are produced mainly from oxidation of polyunsaturated C18 fatty acids, notably linolenic and linoleic acids. Algae and mosses in addition possess polyunsaturated C20 fatty acids including arachidonic and eicosapentaenoic acids, which can also be oxidized by LOXs and transformed into bioactive compounds. Mosses are phylogenetically placed between unicellular green algae and flowering plants, allowing evolutionary studies of the different oxylipin pathways. During the last years the moss Physcomitrella patens has become an attractive model plant for understanding oxylipin biosynthesis and diversity. In addition to the advantageous evolutionary position, functional studies of the different oxylipin-forming enzymes can be performed in this moss by targeted gene disruption or single point mutations by means of homologous recombination. Biochemical characterization of several oxylipin-producing enzymes and oxylipin profiling in P. patens reveal the presence of a wider range of oxylipins compared to flowering plants, including C18 as well as C20-derived oxylipins. Surprisingly, one of the most active oxylipins in plants, JA, is not synthesized in this moss. In this review, we present an overview of oxylipins produced in mosses and discuss the current knowledge related to the involvement of oxylipin-producing enzymes and their products in moss development and defense.