Adaptation Mechanisms in the Evolution of Moss Defenses to Microbes

Abstract

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.

Keywords: adaptation mechanisms; evolution; horizontal gene transfer; moss-microbe interactions; pathogens; plant defenses.

FIGURE 1

Pathogen invasion and P. patens defense responses. P. patens has one layer of cells in most tissues facilitating microscopic analysis of pathogen colonization processes and moss defense mechanisms (Ponce de León, 2011; Overdijk et al., 2016). Leaves and rhizoids are colonized by different pathogens, which are easily visualized by different staining techniques (leaf; trypan blue and solophenyl flavine, rhizoid: solophenyl flavine) (Ponce de León and Montesano, 2013). Representative pictures of the oomycete P. irregulare and the fungus B. cinerea pathogen-infected tissues are shown. Plant defenses are activated after pathogen infection, including a HR-like response, cell wall fortification and chloroplast reorientation (red arrow), ROS accumulation, defense gene expression and increase in hormone levels (Ponce de León and Montesano, 2013). Representative pictures of defense responses are shown; HR-response, cell wall fortification (phenolic compounds accumulation, evidenced by stained cell walls) and ROS production in B. cinerea-infected leaves, callose deposition in P.c. carotovorum elictor treated protonemal cells (white arrow), and expression pattern of the defense gene Ppalpha-DOX and auxin levels in response to elicitors of P.c. carotovorum at different hours after treatment. The HR-like response was visualized by autofluorescent compounds, cell wall fortification by safranin-O staining, callose deposition by methyl blue staining, and intracellular ROS with 2′,7′-dichlorodihydrofluorescein diacetate. “Graph showing auxin levels as originally published in Alvarez et al. (2016).”

FIGURE 2

Activation of defense responses in Arabidopsis and P. patens. (A) Angiosperms like Arabidopsis thaliana sense the presence of pathogen-associated molecular patterns (PAMPs) by plasma membrane (PM) pattern recognition receptors (PRRs), such as FLS2, EFR1, CERK1, and LYK5 (Couto and Zipfel, 2016). Pathogen recognition triggers calcium (Ca²⁺) production and activates a MAPK cascade (Seybold et al., 2014). MAP kinase kinase kinases (MEKKs), MAP kinase kinase (MKKs), and MAP kinase (MPKs) are subsequently activated, leading to production of reactive oxygen species (ROS), and expression of defense genes (Meng and Zhang, 2013). The hormones salicylic acid (SA), jasmonic acid (JA) and its precursor cis-oxophytodienoic acid (OPDA), abscisic acid (ABA), auxins, and ethylene (ET) activate the expression of defense genes leading to the activation of defense mechanisms that involves pathogenesis-related (PR) proteins, incorporation of phenolics into the cell wall (CW), deposition of callose and lignin, activation of an hypersensitive response (HR) and systemic acquired resistance (SAR) (Glazebrook, 2005). In response to pathogen infection, oxylipins derived mainly from linoleic acid (18:2) and linolenic acid (18:3; C18 fatty acids) are synthesized (Ponce de León et al., 2015). (B) P. patens lacks close homologs of the receptors FLS2 and EFR, while a functional CERK1 receptor perceives fungal chitin and bacterial peptidyl glycan (Bressendorff et al., 2016). At least one MEKK, one MKK, and two MPKs (MPK4a and MPK4b) participate in P. patens defense responses to fungal chitin (Bressendorff et al., 2016). ROS, SA, and auxin activate expression of defense genes, while JA is not produced (Ponce de León and Montesano, 2013; Reboledo et al., 2015). Further studies are needed to reveal the role of OPDA, ABA and ethylene (names in gray) in moss resistance against pathogens. After pathogen assault, P. patens activates several defense mechanisms, including the expression of genes encoding PR proteins, incorporation of phenolics into the CW, callose deposition and accumulation of pre-lignin compounds (Ponce de León and Montesano, 2013; Overdijk et al., 2016). An HR-like response and SAR are also activated in infected mosses, and oxylipins derived from C18 and polyunsaturated C20 fatty acids are synthesized producing a broader range of oxylipins with possible roles in plant defense (Ponce de León et al., 2012, 2015; Winter et al., 2014). Orphan genes, some of them acquired by gene horizontal transfer from fungi and other microorganisms, could also play a role in moss defense against pathogens. Names in red or blue indicate their presence only in traqueophytes or P. patens, respectively.