By Irda Safni Induced Plant Resistance to Bacteria

Materi Patogenesis Tumbuhan 1.Induced resistance to bacteria (2 Mei 2015) 2.Phytoalexins (9 Mei 2015) 3.Effect of pathogens of plant physiology (16 Mei 2015)  Libur Kuliah Pengganti: ?

 Plants are central players in a complex food web in which many members take advantages of the plant’s resources. Introduction Food for: Human Animals Insects Microorganisms (fungi, bacteria, virus, nematode, etc)

Plant faces challenges from various herbivores & microbes at all stages and all organs

However, most plant are resistant to most potential pathogens in most times !! Because plants have evolved defense systems to protect themselves!!

 Plants has a good relationships with commensal and mutualistics microbes that provides the plants with essential services, such as enhanced mineral uptake, nitrogen fixation, growth promotion, and protection from pathogens.

 These plant microbiota are predominantly hosted by the root system  deposits up to 40% of the plant’s photosynthetically fixed carbon into the rhizosphere.  Several genera of the rhizosphere microbiota, such as plant growth–promoting rhizobacteria (PGPR) and fungi (PGPF), can enhance plant growth and improve health

Properties of PGPR -Stimulate growth N fixation Increase solubility of limiting nutrients (siderophores) Stimulate nutrient delivery and uptake Production of phytohormones Modulation of plant development (e.g. reduce ethylene enhances root growth) -Plant-mediated disease suppression Non-pathogens antagonize pathogens (competition, antibiotics, lytic enzymes) Activating plant to better defend itself (ISR) Induced resistance observed on spatially separated parts of same plant

 Some experimental studies proved evidence that PGPR can promote plant health through stimulation of the plant immune system. Examples:  The inoculation of PGPR strain Pseudomonas fluorescens in the root system of carnation, aboveground parts of the plant acquired an enhanced level of resistance against infection by the fungal pathogen Fusarium oxysporum (Van Peer et al., 1991).  Colonization of roots by different beneficial Pseudomonas and Serratia PGPR strains resulted in a significant reduction in disease symptoms after challenge inoculation of leaves with the anthracnose pathogen Colletotrichum orbiculare (Wei et al., 1991).

Mutualism relationships between nitrogen fixing bacteria & plants N 2 + 8H + 8e ATP  2NH 3 +H ADP + 16Pi

Induced resistance: The induced state of resistance in plants triggered by biological and chemical inducers, which protects non-exposed plant parts against future attacks by pathogenic microbes and herbivorous insects. Induced resistance can be triggered by certain chemicals, nonpathogens, avirulent forms of pathogens, incompatible races of pathogens or virulent pathogen under circumstances. Induced Resistance

 Plants can develop induced resistance as a results of infection: - by a pathogen or - by a herbivorous insects, or - after colonization of beneficial microbes, or - after treatments with specific chemicals.

 Generally, induced resistance is systemic.  Because of this systemic character, induced resistance is commonly referred to as systemic acquired resistance (SAR).  However, induced resistance is not always expressed systemically: Localized acquired resistance (LAR) occurs when only those tissues exposed to the primary invader become more resistant.

Figure 1 Schematic representation of biologically induced resistance triggered by pathogen infection (red arrow), insect herbivory (blue arrow), and colonization of the roots by beneficial microbes ( purple arrows).

Characteristics of induced resistance  The activation of latent defence mechanisms.  Induced resistance is expressed locally and systemically (ISR).  Induced resistance confers an enhanced level of protection against a broad spectrum of attackers.  Induced resistance is regulated by a network of interconnected signaling pathways in which plant hormones play a major regulatory role.

The Plant Immune System and Induced Resistance What is Plant immunity? It is a state of defense against infectious pathogens (fungi, bacteria, virus, nematodes, etc). Mode of entry of pathogen depend on the type of pathogen.  Fungi: haustoria  Bacteria : stomata, hydatodes, and wounds  Nematode : stylet

Gene-for-gene- Hypothesis (by H.H. Flor) Disease resistance in plants requires two complementary genes: an avirulence (Avr) gene in the pathogen and a matching, resistance (R) gene in the host.

Principle of plant immunity

Pathogen-Induced Systemic Acquired Resistance Signalling Systemic Acquired Resistance (SAR): uninfected systemic plant parts become more resistant in response to a localized infection elsewhere in the plant.

In the current concept of the plant immune system, the onset of pathogen-induced SAR is triggered upon local activation of a PTI or ETI response. In systemic tissues, SAR is characterized by increased levels of the hormone salicylic acid (SA) and pathogenesis-related proteins (PRs)..

Component & mechanism involved in ISR, HIR,SAR triggered by beneficial microbes

 Systemic acquired resistance (SAR) is typically activated in healthy systemic tissues of locally infected plants.  Induced systemic resistance (ISR) is typically activated upon colonization of plant roots by beneficial microorganisms

Hormonal Regulation of Induced Systemic Resistance by Beneficial Microbes Induced Systemic Resistance  The plant hormone Jasmine Acid (JA) and Ethylene (ET) are the central player of in the regulation of rhizobacteria-mediated ISR. Systemic Acquired Resistance  Several PGPR have been reported to trigger an SA- dependent type of ISR that resembles pathogen induced SAR

The Roots of Induced Systemic Resistance Root colonization Initiation of ISR requires beneficial microbes to efficiently colonize the root system of host plants For a successful mutualistic association, host plants and microbes need to respond to reciprocal signals and accordingly prioritize their responses so as to develop a lifestyle that provides mutual benefits

Root colonization by beneficial soil bacteria Pieterse et al. (2014) Annu. Rev. Phytopathol. 52:347-75

Plant growth-promoting effects of P. fluorescens WCS417r Pieterse et al. (2014) Annu. Rev. Phytopathol. 52:347-75

Systemic protection against Cucumber mosaic virus Kloepper Phytopathology. 94: Bacillus pumulis strain SE34Nonbacterized

Modulation of Root Immunity Like pathogens, beneficial microbes need to overcome or evade plant immune responses in order to establish a prolonged and intimate mutualistic interaction with the host. Molecules and strategies commonly used by pathogens to suppress host immunity are also employed by soilborne ISR-inducing microbes.

 Many bacterial pathogens deliver immune suppressive effectors in the plant cell via a type III secretion system.  Along with suppressing local host defenses to facilitate colonization, PGPR effectors may also function as host-range specificity determinants under control of host resistance (R) proteins, as in the case of the Rhizobium-legume symbiosis.  This would allow host plants to utilize components of their immune system to select for their mutualistic partners.

The Rhizosphere Microbiome and Induced Systemic Resistance Coevolution of plant-beneficial microbe interactions for the benefit of plant health occurs in nature is evidenced by the existence of disease-suppressive soils. The disease suppressiveness of these soils is generally based on specific microbial populations that antagonize pathogens.

Disease-suppressive soils occur worldwide, and some develop following prolonged monoculture of a specific crop. E.g. Pseudomonas (the most important player), Trichoderma, Fusarium, Streptomyces, Bacillus, Actinomyces spp.

Table 1. Bacterial determinants and types of systemic induced resistance Bacterial StrainPlant species: bacterial determinant Type Pseudomonas aeruginosa : strain 7NSK2Tobacco: salicylic acidSAR Bean: salicylic acidSAR Pseudomonas fluorescens: CHAOTobacco: siderophoreSAR Radish: lipopolysaccharide, siderophore, iron-regulated factor ISR WCS417Carnation: lipopolysaccharideISR Radish: lipopolysaccharide, iron- regulated factor ISR Arabidopsis: lipopolysaccharideISR Pseudomonas putida: WCS358Arabidopsis: lipopolysaccharide, siderophore ISR

Possible mechanisms of disease suppression:  Competition for space and (micro)nutrients  Hyperparasitism  Antagosism via microbial production of secondary metabolites, such as iron-chelating siderophores, antibiotics, and lytic enzymes  Elicitation of ISR

Many Pseudomonas spp. strains that have been isolated worldwide for their excellent plant protective properties appear to be genetically very closely related. Some of these closely related strains were isolated from different plant species and thus might embody a group of universal PGPR, whereas others were isolated from the same plant species and could represent plant species- specific beneficials.

SUMMARY POINTS 1.Beneficial microbes produce different MAMPs and elicitors that can trigger ISR. 2. Local suppression of root immune responses is a common feature of ISR-eliciting beneficial microbes that possibly aids in root colonization. 3. ISR triggered by beneficial soilborne microbes is often regulated by a JA/ET-dependent signaling pathway, but beneficial microbes that elicit the SA-dependent SAR pathway exist as well.

4. Plants have mechanisms by which they enrich their microbiome with beneficial microbes that provide protection against diseases. 5. ISR is a plant immune function mediated by the root microbiome. 6. Disease-suppressive soils are enriched with beneficial microbes that promote plant health.