Plant cell wall in pathogenesis, parasitism and symbiosis

The cell wall is the foremost interface at which interactions take place between plants and a wide range of other organisms including insects, nematodes, pathogenic, or symbiotic microorganisms. This Research Topic includes Mini Reviews, Reviews, Original Research Articles, and Perspective Articles that provide novel insights and detailed overviews on the dynamics of the plant cell wall in plant defense, parasitism, and symbiosis and describes experimental approaches to study plant cell wall modifications occurring during interaction of plants with different organisms. The cuticle represents the first cell wall layer encountered by pathogens. Serrano and collaborators describe the influence of the cuticle components on microbial development during pathogenesis (Serrano et al., 2014). The authors highlight also the alterations of the cuticle that induce defense responses against necrotrophs. Once the microbe has penetrated, the cell wall becomes a battleground where plants and pathogens attempt to prevail. Necrotrophic fungi produce several cell wall degrading enzymes (CWDEs) to degrade plant cell wall polysaccharides that favor plant colonization. Blanco-Ulate and co-authors present a genome-wide transcriptional profiling of Botrytis cinerea CWDEs expressed during infection of important nutritional resources such as lettuce leaves, ripe tomato fruit, and grape berries (Blanco-Ulate et al., 2014). Plants develop different cell wall related systems for sensing intruders. Malinovsky and co-workers provide an overview of the cell surface pattern recognition receptors (PRRs) that perceive a diverse set of microbial molecules referred to as microbial/pathogen-associated-molecular patterns (MAMPs/PAMPs) and trigger immune responses (Malinovsky et al., 2014). Bellincampi and co-authors illustrate the mechanisms of sensing the alteration of cell wall integrity (CWI) during biotic stress and explain how cell walls can be a source of the so-called damage-associated molecular patterns (DAMPs) (Bellincampi et al., 2014). Three manuscripts of this research topic deal with specific mechanisms of perception and signaling of cell wall damage and modulation of plant immunity. Emerging evidence indicates that the Arabidopsis plasma membranelocalized protein NDR1 (NON-RACE-SPECIFIC DISEASE RESISTANCE 1) functions as key signaling component of the loss of membrane-cell wall adhesions during pathogen infection. The research article by Lu and co-workers reports on the identification of a citrus ortholog CsNDR1 that, when overexpressed in Arabidopsis, improves disease resistance to Pseudomonas syringae and Hyaloperonospora arabidopsidis (Lu et al., 2013). Komarova and co-authors discuss on the role of methanol as signal in plant immunity (Komarova et al., 2014). Methanol (MeOH) can be released by plant pectin methylesterases, induced by the mechanical damage of plant tissues during microbe penetration. The emission of MeOH by a wounded plant can have a priming effect, enhancing the resistance of the non-wounded, neighboring receiver plants to pathogens. Tauzin and Giardina review the emerging argument of the “sweet immunity” (Tauzin and Giardina, 2014). The authors describe the involvement of sucrose and cell wall invertases as priming agents important for triggering an appropriate defense responses during pathogen invasion. During pathogen infection plants can repair to the loss of CWI by activating cell wall reinforcing mechanisms (Bellincampi et al., 2014). At sites of interaction with intruding microbial pathogens the cell wall is actively reinforced through the deposition of cell wall appositions, so-called papillae. Voigt present a perspective article that discusses the possible roles of the (1,3)-βglucan callose and the other papillae components in plant defense (Voigt, 2014). Lignin is both pathogen-induced and developmentally deposited in the secondary thickened cell wall. In their review article Miedes and collaborators update us on the effect of altered lignin amount and composition on pathogen infection and spread (Miedes et al., 2014). Pathogens, parasites, and symbionts may exploit the host cell wall metabolism to support the colonization of their hosts (Bellincampi et al., 2014). An important remodeling of the plant cell wall is also exploited during plant parasitism by cyst nematodes. Bohlmann and Sobczak present a detailed overview of the cell wall degrading and modifying enzymes that nematodes produce during migration through the root, and the cell wall modifications occurring during syncytium development due to the cell wall synthetizing, modifying, and degrading proteins of the plant (Bohlmann and Sobczak, 2014). Two articles of this research topic report on the cell wall remodeling during mutualistic symbiosis of plants with different microorganisms, useful to establish an intimate interface for developmental coordination and nutrient