The oomycetes form a phylogenetically distinct group of eukaryotic microorganisms that includes some of the most notorious pathogens of plants. Oomycetes accomplish parasitic colonization of plants by modulating host cell defenses through an array of disease effector proteins. The biology of effectors is poorly understood but tremendous progress has been made in recent years. This review classifies and catalogues the effector secretome of oomycetes. Two classes of effectors target distinct sites in the host plant: Apoplastic effectors are secreted into the plant extracellular space, and cytoplasmic effectors are translocated inside the plant cell, where they target different subcellular compartments. Considering that five species are undergoing genome sequencing and annotation, we are rapidly moving toward genome-wide catalogues of oomycete effectors. Already, it is evident that the effector secretome of pathogenic oomycetes is more complex than expected, with perhaps several hundred proteins ded...

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A Catalogue of the Effector Secretome of Plant Pathogenic Oomycetes

Plants are sessile organisms that are under constant attack from microbes. They rely on both preformed defenses, and their innate immune system to ward of the microbial pathogens. Preformed defences include for example the cell wall and cuticle, which act as physical barriers to microbial colonization. The plant immune system is composed of surveillance systems that perceive several general microbe elicitors, which allow plants to switch from growth and development into a defense mode, rejecting most potentially harmful microbes. The elicitors are essential structures for pathogen survival and are conserved among pathogens. The conserved microbe-specific molecules, referred to as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), are recognized by the plant innate immune systems pattern recognition receptors (PRRs). General elicitors like flagellin (Flg), elongation factor Tu (EF-Tu), peptidoglycan (PGN), lipopolysaccharides (LPS), Ax21 (Activator of XA21-mediated immunity in rice), fungal chitin, and β-glucans from oomycetes are recognized by plant surface localized PRRs. Several of the MAMPs and their corresponding PRRs have, in recent years, been identified. This review focuses on the current knowledge regarding important MAMPs from bacteria, fungi, and oomycetes, their structure, the plant PRRs that recognizes them, and how they induce MAMP-triggered immunity (MTI) in plants.

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MAMP (Microbe-Associated Molecular Pattern) triggered immunity in Plants

▪ Abstract Recognition is the earliest step in any direct plant-microbe interaction. Recognition between Phytophthora pathogens, which are oomycetes, phylogenetically distinct from fungi, has been studied at two levels. Recognition of the host by the pathogen has focused on recognition of chemical, electrical, and physical features of plant roots by zoospores. Both host-specific factors such as isoflavones, and host-nonspecific factors such as amino acids, calcium, and electrical fields, influence zoospore taxis, encystment, cyst germination, and hyphal chemotropism in guiding the pathogen to potential infection sites. Recognition of the pathogen by the host defense machinery has been analyzed using biochemical and genetic approaches. Biochemical approaches have identified chemical elicitors of host defense responses, and in some cases, their cognate receptors from the host. Some elicitors, such as glucans and fatty acids, have broad host ranges, whereas others such as elicitins have narrow host ranges. M...

Molecular basis of recognition between phytophthora pathogens and their hosts

The cellulose binding elicitor lectin (CBEL) from Phytophthora parasitica nicotianae contains two cellulose binding domains (CBDs) belonging to the Carbohydrate Binding Module1 family, which is found almost exclusively in fungi. The mechanism by which CBEL is perceived by the host plant remains unknown. The role of CBDs in eliciting activity was investigated using modified versions of the protein produced in Escherichia coli or synthesized in planta through the potato virus X expression system. Recombinant CBEL produced by E. coli elicited necrotic lesions and defense gene expression when injected into tobacco (Nicotiana tabacum) leaves. CBEL production in planta induced necrosis. Site-directed mutagenesis on aromatic amino acid residues located within the CBDs as well as leaf infiltration assays using mutated and truncated recombinant proteins confirmed the importance of intact CBDs to induce defense responses. Tobacco and Arabidopsis thaliana leaf infiltration assays using synthetic peptides showed that the CBDs of CBEL are essential and sufficient to stimulate defense responses. Moreover, CBEL elicits a transient variation of cytosolic calcium levels in tobacco cells but not in protoplasts. These results define CBDs as a novel class of molecular patterns in oomycetes that are targeted by the innate immune system of plants and might act through interaction with the cell wall.

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Cellulose Binding Domains of a Phytophthora Cell Wall Protein Are Novel Pathogen-Associated Molecular Patterns

The cell wall of the oomycete plant pathogen Phytophthora
parasitica var. nicotianae contains a protein called CBEL that
shows cellulose-binding (CB), elicitor (E) of defense in plants and
lectin-like (L) activities. The biological role of this molecule in
 Phytophthora was investigated by generating transgenic strains
suppressed in CBEL expression. Phenotypic characterization of these
strains showed that they were severely impaired in adhesion to a cellophane
membrane, differentiation of lobed structures in contact with cellophane, and
formation of branched aggregating hyphae on cellophane and on flax cellulose
fibres. Infection assays revealed that the strains suppressed in CBEL 
expression were not greatly affected in pathogenicity and formed branched
aggregating hyphae in contact with the roots of the host plant, thereby
indicating that CBEL is involved in the perception of cellulose rather than in
the morphogenesis of hyphal aggregates. Interestingly, the absence of CBEL was
correlated with abnormal formation of papillae-like cell wall thickenings in
vitro, suggesting that CBEL is involved in cell wall deposition in
 Phytophthora . Reverse genetics in oomycetes has long been hampered by
their diploid nature and difficulties in transformation and regeneration. The
gene inactivation approach reported in this work provides the first direct
evidence for intrinsic functions of an elicitor and cell wall protein in
oomycetes.

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The CBEL glycoprotein of Phytophthora parasitica var-nicotianae is involved in cell wall deposition and adhesion to cellulosic substrates.

A glycoprotein of 34 kDa (GP 34) was solubilized at acidic pH from the mycelium of Phytophthora parasitica var. nicotianae and was purified by ion exchange and gel permeation chromatography. Whole tobacco plants treated with GP 34 through their roots showed an enhanced lipoxygenase activity as well as hydroxyproline-rich glycoprotein accumulation, indicating that this molecule had elicitor properties. An antiserum raised against the pure glycoprotein allowed localization of GP 34 by immunogold-labeling on the cell surface of the mycelium when the fungus was grown in vitro. In the wall-less zoospores, GP 34 was limited to the flagellum surface. It was then abundantly synthesized at the onset of encystment. During infection of tobacco plants, labeling was very faint at early stages of colonization, particularly in the susceptible host cultivar. It appeared earlier in the resistant host cultivar and was restricted to the living fungus, declining with mycelium cell death.

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Purification, Elicitor Activity, and Cell Wall Localization of a Glycoprotein from Phytophthora parasitica var. nicotianae, a Fungal Pathogen of Tobacco.

F. Villalba Mateos

Papers

Purification, Elicitor Activity, and Cell Wall Localization of a Glycoprotein from Phytophthora parasitica var. nicotianae, a Fungal Pathogen of Tobacco.