Many plant-associated microbes are pathogens that impair plant growth and reproduction. Plants respond to infection using a two-branched innate immune system. The first branch recognizes and responds to molecules common to many classes of microbes, including non-pathogens. The second responds to pathogen virulence factors, either directly or through their effects on host targets. These plant immune systems, and the pathogen molecules to which they respond, provide extraordinary insights into molecular recognition, cell biology and evolution across biological kingdoms. A detailed understanding of plant immune function will underpin crop improvement for food, fibre and biofuels production.

http://www.segenetica.es/cng2/cng2008/biblio/Ref-Marc-Valls---Jones-and-Dangl-Nat06.pdf

The plant immune system

Microbe-associated molecular patterns (MAMPs) are molecular signatures typical of whole classes of microbes, and their recognition plays a key role in innate immunity. Endogenous elicitors are similarly recognized as damage-associated molecular patterns (DAMPs). This review focuses on the diversity of MAMPs/DAMPs and on progress to identify the corresponding pattern recognition receptors (PRRs) in plants. The two best-characterized MAMP/PRR pairs, flagellin/FLS2 and EF-Tu/EFR, are discussed in detail and put into a phylogenetic perspective. Both FLS2 and EFR are leucine-rich repeat receptor kinases (LRR-RKs). Upon treatment with flagellin, FLS2 forms a heteromeric complex with BAK1, an LRR-RK that also acts as coreceptor for the brassinolide receptor BRI1. The importance of MAMP/PRR signaling for plant immunity is highlighted by the finding that plant pathogens use effectors to inhibit PRR complexes or downstream signaling events. Current evidence indicates that MAMPs, DAMPs, and effectors are all perceived as danger signals and induce a stereotypic defense response.

A Renaissance of Elicitors: Perception of Microbe-Associated Molecular Patterns and Danger Signals by Pattern-Recognition Receptors

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses. Significant progress has been made in identifying the key components and understanding the role of salicylic acid (SA), jasmonates (JA) and ethylene (ET) in plant responses to biotic stresses. Recent studies indicate that other hormones such as abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids (BR) and peptide hormones are also implicated in plant defence signaling pathways but their role in plant defence is less well studied. Here, we review recent advances made in understanding the role of these hormones in modulating plant defence responses against various diseases and pests.

Role of plant hormones in plant defence responses.

The plant hormone abscisic acid (ABA) acts as a developmental signal and as an integrator of environmental cues such as drought and cold. Key players in ABA signal transduction include the type 2C protein phosphatases (PP2Cs) ABI1 and ABI2, which act by negatively regulating ABA responses. In this study, we identify interactors of ABI1 and ABI2 which we have named regulatory components of ABA receptor (RCARs). In Arabidopsis, RCARs belong to a family with 14 members that share structural similarity with class 10 pathogen-related proteins. RCAR1 was shown to bind ABA, to mediate ABA-dependent inactivation of ABI1 or ABI2 in vitro, and to antagonize PP2C action in planta. Other RCARs also mediated ABA-dependent regulation of ABI1 and ABI2, consistent with a combinatorial assembly of receptor complexes.

http://science.sciencemag.org/content/sci/324/5930/1064.full.pdf

Regulators of PP2C Phosphatase Activity Function as Abscisic Acid Sensors

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

Hormone Crosstalk in Plant Disease and Defense: More Than Just JASMONATE-SALICYLATE Antagonism

Plant Stomata Function in Innate Immunity against Bacterial Invasion

Erwinia amylovora is a highly virulent, necrogenic, vascular pathogen of rosaceous species that produces the exopolysaccharide amylovoran, a known pathogenicity factor, and levan, a virulence factor. An in vitro crystal violet staining and a bright-field microscopy method were used to demonstrate that E. amylovora is capable of forming a biofilm on solid surfaces. Amylovoran and levan production deletion mutants were used to determine that amylovoran was required for biofilm formation and that levan contributed to biofilm formation. In vitro flow cell and confocal microscopy were used to further reveal the architectural detail of a mature biofilm and differences in biofilm formation between E. amylovora wild-type (WT), Δams, and Δlsc mutant cells labeled with green fluorescent protein or yellow fluorescent protein. Scanning electron microscopy analysis of E. amylovora WT cells following experimental inoculation in apple indicated that extensive biofilm formation occurs in xylem vessels. However, Δams mutant cells were nonpathogenic and died rapidly following inoculation, and Δlsc mutant cells were not detected in xylem vessels and were reduced in movement into apple shoots. These results demonstrate that biofilm formation plays a critical role in the pathogenesis of E. amylovora.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-99-11-1237

Contribution of Erwinia amylovora exopolysaccharides amylovoran and levan to biofilm formation: implications in pathogenicity.

Biofilm formation plays a critical role in the pathogenesis of Erwinia amylovora and the systemic invasion of plant hosts. The functional role of the exopolysaccharides amylovoran and levan in pathogenesis and biofilm formation has been evaluated. However, the role of biofilm formation, independent of exopolysaccharide production, in pathogenesis and movement within plants has not been studied previously. Evaluation of the role of attachment in E. amylovora biofilm formation and virulence was examined through the analysis of deletion mutants lacking genes encoding structures postulated to function in attachment to surfaces or in cellular aggregation. The genes and gene clusters studied were selected based on in silico analyses. Microscopic analyses and quantitative assays demonstrated that attachment structures such as fimbriae and pili are involved in the attachment of E. amylovora to surfaces and are necessary for the production of mature biofilms. A time course assay indicated that type I fimbriae function earlier in attachment, while type IV pilus structures appear to function later in attachment. Our results indicate that multiple attachment structures are needed for mature biofilm formation and full virulence and that biofilm formation facilitates entry and is necessary for the buildup of large populations of E. amylovora cells in xylem tissue.

Cell Surface Attachment Structures Contribute to Biofilm Formation and Xylem Colonization by Erwinia amylovora

Mechanisms of fire blight control by the shoot-growth regulator prohexadione-calcium (ProCa) were investigated by comparing disease development in ProCa-treated potted apple trees (cv. Gala) to paclobutrazol (another shoot-growth regulator)-treated and nontreated trees and in ProCa-treated cv. McIntosh trees in the field. Twenty-eight days after inoculation with Erwinia amylovora Ea110, disease incidence on ProCa- and paclobutrazol-treated shoots was significantly reduced compared with that on nontreated shoots. Disease severity (percent shoot length infected) was also significantly lower on both ProCa- and paclobutrazol-treated shoots than on nontreated shoots. However, bacterial populations within inoculated shoots were high and bacterial growth occurred in all treatments. In addition, the mean cell wall width of the cortical parenchyma midvein tissue of the first and second youngest unfolded leaves of ProCa- and paclobutrazol-treated shoots was significantly wider both 0.5 and 2 cm from the le...

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-99-5-0591

Jessica M. Koczan

Papers

Plant Stomata Function in Innate Immunity against Bacterial Invasion

Contribution of Erwinia amylovora exopolysaccharides amylovoran and levan to biofilm formation: implications in pathogenicity.

Cell Surface Attachment Structures Contribute to Biofilm Formation and Xylem Colonization by Erwinia amylovora

Evidence that Prohexadione-Calcium Induces Structural Resistance to Fire Blight Infection

Biofilm formation in Erwinia amylovora: Implications in pathogenicity