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

The innate immune system in drosophila and mammals senses the invasion of microorganisms using the family of Toll receptors, stimulation of which initiates a range of host defense mechanisms. In drosophila antimicrobial responses rely on two signaling pathways: the Toll pathway and the IMD pathway. In mammals there are at least 10 members of the Toll-like receptor (TLR) family that recognize specific components conserved among microorganisms. Activation of the TLRs leads not only to the induction of inflammatory responses but also to the development of antigen-specific adaptive immunity. The TLR-induced inflammatory response is dependent on a common signaling pathway that is mediated by the adaptor molecule MyD88. However, there is evidence for additional pathways that mediate TLR ligand-specific biological responses.

Toll-like receptors.

The transient gene expression system using Arabidopsis mesophyll protoplasts has proven an important and versatile tool for conducting cell-based experiments using molecular, cellular, biochemical, genetic, genomic and proteomic approaches to analyze the functions of diverse signaling pathways and cellular machineries. A well-established protocol that has been extensively tested and applied in numerous experiments is presented here. The method includes protoplast isolation, PEG-calcium transfection of plasmid DNA and protoplast culture. Physiological responses and high-throughput capability enable facile and cost-effective explorations as well as hypothesis-driven tests. The protoplast isolation and DNA transfection procedures take 6-8 h, and the results can be obtained in 2-24 h. The cell system offers reliable guidelines for further comprehensive analysis of complex regulatory mechanisms in whole-plant physiology, immunity, growth and development.

/pdf/arabidopsis-mesophyll-protoplasts-a-versatile-cell-system-3i7d0dbqvh.pdf

Arabidopsis mesophyll protoplasts: a versatile cell system for transient gene expression analysis.

Plants cannot move to escape environmental challenges. Biotic stresses result from a battery of potential pathogens: fungi, bacteria, nematodes and insects intercept the photosynthate produced by plants, and viruses use replication machinery at the host's expense. Plants, in turn, have evolved sophisticated mechanisms to perceive such attacks, and to translate that perception into an adaptive response. Here, we review the current knowledge of recognition-dependent disease resistance in plants. We include a few crucial concepts to compare and contrast plant innate immunity with that more commonly associated with animals. There are appreciable differences, but also surprising parallels.

Plant pathogens and integrated defence responses to infection.

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

Contrasting Mechanisms of Defense Against Biotrophic and Necrotrophic Pathogens

There is remarkable conservation in the recognition of pathogen-associated molecular patterns (PAMPs) by innate immune responses of plants, insects and mammals. We developed an Arabidopsis thaliana leaf cell system based on the induction of early-defence gene transcription by flagellin, a highly conserved component of bacterial flagella that functions as a PAMP in plants and mammals. Here we identify a complete plant MAP kinase cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) and WRKY22/WRKY29 transcription factors that function downstream of the flagellin receptor FLS2, a leucine-rich-repeat (LRR) receptor kinase. Activation of this MAPK cascade confers resistance to both bacterial and fungal pathogens, suggesting that signalling events initiated by diverse pathogens converge into a conserved MAPK cascade.

MAP kinase signalling cascade in Arabidopsis innate immunity

Plants and animals recognize microbial invaders by detecting pathogen-associated molecular patterns (PAMPs) such as flagellin. However, the importance of flagellin perception for disease resistance has, until now, not been demonstrated. Here we show that treatment of plants with flg22, a peptide representing the elicitor-active epitope of flagellin, induces the expression of numerous defence-related genes and triggers resistance to pathogenic bacteria in wild-type plants, but not in plants carrying mutations in the flagellin receptor gene FLS2. This induced resistance seems to be independent of salicylic acid, jasmonic acid and ethylene signalling. Wild-type and fls2 mutants both display enhanced resistance when treated with crude bacterial extracts, even devoid of elicitor-active flagellin, indicating the existence of functional perception systems for PAMPs other than flagellin. Although fls2 mutant plants are as susceptible as the wild type when bacteria are infiltrated into leaves, they are more susceptible to the pathogen Pseudomonas syringae pv. tomato DC3000 when it is sprayed on the leaf surface. Thus, flagellin perception restricts bacterial invasion, probably at an early step, and contributes to the plant's disease resistance.

Bacterial disease resistance in Arabidopsis through flagellin perception.

The flagellum is an important virulence factor for bacteria pathogenic to animals and plants. Here we demonstrate that plants have a highly sensitive chemoperception system for eubacterial flagellins, specifically targeted to the most highly conserved domain within its N terminus. Synthetic peptides comprising 15-22 amino acids of this domain acted as elicitors of defence responses at sub-nanomolar concentrations in cells of tomato and several other plant species. Peptides comprising only the central 8 to 11 amino acids of the active domain had no elicitor activity but acted as specific, competitive inhibitors in tomato cells. These antagonists suppressed the plant's response to flagellin, crude bacterial extracts and living bacterial cells. Thus, plants have a highly sensitive and selective perception system for the flagellin of motile eubacteria.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.1999.00265.x

Plants have a sensitive perception system for the most conserved domain of bacterial flagellin

The antimicrobial arsenal of plants is thought to consist mainly of secondary metabolites, among which the phytoalexins are the best-studied1–3. But plants may also possess antimicrobial proteins4,5: it has been reported that wheat-germ agglutinin, a chitin-binding lectin from wheat embryos, inhibits growth of the fungus Trichoderma viride4. This has led to the notion that plant lectins, with their intriguing biochemical similarity to animal antibodies, have an antibody-like antimicrobial function4,6,7. We report here that the main proteinaceous inhibitor of fungal growth in bean leaves is chitinase, an enzyme that can be induced by the plant hormone ethylene, or by pathogen attack. Among commercial preparations of purified chitin-binding lectins (from wheat germ, tomato, potato, pokeweed and gorse), only those containing contaminating chitinase activity inhibit fungal growth. Our data indicate that plant chitinases, but not chitin-binding lectins, are important antifungal proteins in plants.

Plant chitinases are potent inhibitors of fungal growth

190 INTRODUCTION .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .... .... . . . . . .. . . . . . 190 SURVEY OF MICROBIAL SIGNALS . . . .. . . . . . . . . . .. . . . . ........ . .. Oligosaccharides . . . . . . . . . . . . . .. . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lipo-chitooligosaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glycopeptides . . . . . . . . . . . . . . . .. . . . ... . . . . . . . . . . . . . . . . .... . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . ... .... . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peptides and Proteins . .. . .. . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . ... . . . . . . . . Microbial Enzymes Releasing Elicitor-Active Compounds from Plant Cells . . . . . . . . . . . . . . . . . . . . . . Lipophilic Substances . .. . . . . . . . .... . . . . . . . . . . ...... ........ . , .. " .. " .. " .. ,,', .. ,'," , .. .... , .. PERCEPTION AND TRANSDUCTION OF MICROBIAL SIGNALS . Receptors ........ . , ... , .... , ... . .... . .. . . .... .. .... .... .. , . ... , .... .... . , .. . . .... . . . . .. .... . . ..... ... .. Second Messengers and Protein Phosphorylation ........ . . . . . . ..... . .... .... . . . . . . . . . . . . , ... . . .. . . ... . . , ...... . Desensitization and Refractory State .. , .... . . " .... .... """" ... ..... ........ ... , ... , . ... .. .. INTERACTIONS OF MICROBIAL SIGNALS .. Suppressors ...... ........ ........ .. ,' .", .. " "" ...... .. "" , ... .... , ... , .... .. ,' .. ,' .," " .... .. Synergistic Effects ..... . . . . . . . . . . . . . .. . .. .. . .. . . .. . . .. . . . . . .... ......... . .... ...... .... ........ . . .... . . SIGNIFICANCE OF MICROBIAL SIGNALS FOR THE PLANTS .. .... .... ...... . .... ..... .... . . .. . . . . Recognition of Non-Self: Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Recognition of Pathogens: Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Recog nition of M utualistic Symbionts: Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , .. . . ... . .... .. .. OUTLOOK ......... ... . . . . . . . . . . . . . . ... . . ... . . . . . . . . . . . . . .. . .. 0066-4294950601-0189 05.00 191 19 1 192 193 193 196 196 197 197 199 202

Thomas Boller

Papers

MAP kinase signalling cascade in Arabidopsis innate immunity

Bacterial disease resistance in Arabidopsis through flagellin perception.

Plants have a sensitive perception system for the most conserved domain of bacterial flagellin

Plant chitinases are potent inhibitors of fungal growth

Chemoperception of microbial signals in plant cells