Induced Biochemical Defenses (HR)

DEVANSHU DEVPALB 3256UAS,GKVK

Hypersensitive Response Hypersensitive Response

Plant defense Plant defense response response

Cell deathCell death

•Plants have a general response to infectionanti-microbial molecules (secondary metabolites, phytoalexins)

• Plants respond to specific infections through the Hypersensitive Response (PCD) rapid accumulation of reactive oxygen species (directly kill pathogen)

•Induction of defense genes (pathogenesis-related proteins)

How do plants defend against bacteria that enter the cell?

Plant Defense Response

Hypersensitive response

Production of reactive oxygen species

Cell wall fortification

Production of antimicrobial metabolites (phytoalexins)

Defense signal transduction

Synthesis of enzymes harmful to pathogen (eg. chitinases, glucanases)

Plant Defense Response

3 aspects of response:

1. Hypersensitive

2. Local

3. Systemic

Compatible interaction diseaseIncompatible interaction resistance

Elicitors of defense responses

Any substance that has the capability of activating defense responses in plants

Include components of the cell surface as well as excreted metabolites

Elicitors

General Race specific

a) Oligosaccharide elicitors a)avr gene products

b) Protein/peptide elicitors

Plant disease resistance genesEncode proteins that recognize Avr-gene-

dependent ligandsActivate signaling cascade(s) that coordinate the

initial plant defense responses to impair pathogen ingress

Capacity for rapid evolution of specificityCommon feature of resistance proteins is a leucine-

rich repeat

Classes of resistance proteins

Gene-for-gene resistance For resistance to occur, complementary pairs of dominant genes,

one in the host and the other in the pathogen, are required (incompatibility)

A loss or alteration to either the plant resistance (R) gene or the pathogen avirulence (Avr) gene leads to disease (compatibility)

Deviations from gene-for-gene conceptOne R gene may confer specificity to more than

one ligand- RPM1 in Arabidopsis confers resistance against P.syringae

expressing either avrRpm1 and avrB

More than one R gene may exist for a given Avr gene- Pto and Prf genes encode biochemically distinct components of the same

pathway- Two genes at the Cf-2 locus furnish identical functions

(Bent, 1996)

Guard hypothesis

Key points

a) An effector acting as a virulence factor has a target(s) in the host

b) By manipulating or altering this target(s) the effector contributes to pathogen success in susceptible host genotypes

c) Effector perturbation of a host target generates a “pathogen induced modified self” molecular pattern, which activates the corresponding NB-LRR protein, leading to ETI

(Jones et al.,2006)

a) 9

Guard hypothesis

Programmed cell death Programmed cell death is a genetically regulated process of

cell suicide that is central to the development, homeostasis and integrity of multicellular organisms

General mechanism of three PCDs inplants

Model of plant programmed cell death

Necrosis vs apoptosis

Purpose of cell death

Cells that are produced in excessCell that have no functionCells that are produced in excessCell that develop improperlyCell that have finished their functionCells that are harmful

Hypersensitive responseRapid, localized plant cell death upon contact

with avirulent pathogens. HR is considered to be a key component of multifaceted plant defense responses to restrict attempted infection by avirulent pathogens

Rapid - within 24 hNot always needed for resistanceHR also contributes to the establishment

of the long-lasting systemic acquired resistance against subsequent attack by a broad range of normally virulent pathogens

HR Includes: oxidative bust (production of reactive oxygen species)Disruption of cell membranesopening of ion channelsCross linking of phenolics with cell wall componentProduction of anti-microbial phytoalexins and PR proteinapoptosis (programmed cell death)

•Bacteria like Pseudomonas syringae inject effector proteins (bacterial avirulence and virulence proteins) into plant cells using the Type-III secretion system.

•Plants that are resistant to the bacteria have resistance proteins that recognize the effector proteins and cause the infected cell to commit suicide (apoptosis/PCD/Hypersensitive Response)

•prevents the bacteria from infecting the rest of the plant by directly killing them and depleting nutrients

The Hypersensitive ResponseThe Hypersensitive Response

• Pectic enzymes- Soft rot Erwinia spp.- Multiple isozymes, some plant regulated

• Toxins- e.g., coronatine acts as JA mimic to downregulate plant defense

• Extracellular polysaccharides- Important in many diseases, esp. vascular diseases- Postulated roles in protection from plant-derived antimicrobials, osmotic stress,

dessication; evading recognition; causing wilt, etc.

• Quorum sensing (cell-cell signaling) and global regulation of virulence

- Soft rot Erwinia spp. regulation of virulence associated genes, including pectic enzymes

- Cell wall degradation products elicit plant defense, so benefit to repressing pectolytic activity until high numbers of bacteria accumulate.

• The Type III secretion (Hrp) pathway- Essential for necrogenic Gram negative pathogens

Major findings

• A type III secretion pathway, broadly conserved among gram-negative pathogens of plants and animals

• Macromolecular structure,Hrp pilus, acts as conduit for traffic(called needle complex in animal pathogens)

• Encoded by clustered hrp genes

• Required for hypersensitive reaction and pathogenicity

• Expression induced in plant and in defined minimal media

• Capable of delivering proteins into host cells

• Secretes and delivers “effector proteins”a) virulence factorsb) avirulence factors

The Hrp pathway

Type III effector proteins in plant-bacterial interactions

Exopolysaccharides

(gum)

Major pathogenicity determinants in Major pathogenicity determinants in XanthomonasXanthomonas

Regulation networks

rpf locus

DSFDSF

Plant cell wall degrading enzymes(cellulases,

polygalacturonases…)

hrpG

Environmental stimuli

Metabolic signals ??

hrpX

Type III secretion system

(hrp/hrc)

Effectors

Type II secretion system

(xps,xcs)

HR HR vs.vs. Disease Disease

Disease:

Chlorosis: A common disease symptom in pathogen infection in which the leaf tissue appears yellow due to the loss of chlorophyll.

Necrosis: A common, slow-developing disease symptom caused by necrotrophic pathogens. Tissue necrosis appears at very late stage of disease development.

Tobacco

Tomato

The Hypersensitive ResponseThe Hypersensitive Response

Host Cell

Bacterium

Effector protein

Type III secretion

Resistance protein

The Hypersensitive ResponseThe Hypersensitive ResponseHost cell recognizes the bacterium and initiates programmed cell death to restrict the growth of the pathogen, which thus does not cause disease.

Avirulent pathogen

HRlesions

Resistant plant

Incompatible interaction, no disease

Plant diseasePlant disease

Plant diseasePlant disease

Disease symptoms

Susceptible plant

Virulent pathogen

Compatible interaction, disease

Plant disease and plant resistance Plant disease and plant resistance Plant HostPathogenInteraction

Virulent

Avirulent

Compatible

Incompatible

Susceptible(Tomato)

Resistant(Tobacco)

Disease

HR

Systemic Acquired Resistance Systemic Acquired Resistance (SAR)(SAR) SAR is a mechanism of induced defense that

confers long-lasting protection against a broad spectrum of microorganisms.

Enhance resistance against subsequent attack by a wide array of pathogen.

The vasculature provide the excellent channel for transport of systemic signals.

SAR takes 24-48 h to start, can last for monthsInvolves gene activation and a transmitted signal. Genes induced:

chitinasesβ 1,3- glucanasesother PR proteins

Complex signalling networks orchestrate different types of plant-inducible defences to prevent microbial growth.

Pathogen recognition triggers a number of rapid cellular responses, including ionic changes, and phosphorylation cascades, which precede the accumulation of reactive oxygen species, nitric oxide, and salicylic acid

(SA) and the transcriptional activation of defence-related genes.

Interplay between reactive oxygen species, nitric oxide, and SA contributes to the establishment of HR. SA also has a key role in establishing local and systemic resistance to many virulent biotrophic pathogens, whereas jasmonic acid (JA) and ethylene (ET) are more often associated with resistance to necrotrophic pathogens. Considerable interactions occur within and between these hormone signalling networks, resulting in an overall mutual antagonism between SA and JA/ET signalling

SAR can also be transmitted to the next generation progeny.

Defense Proteins

Salicylic Acid

The plant defense proteins provide the plant resistance to a

variety of plant pathogens.

Disease organisms and nonpathogenic microbes stimulates the plant above or

belowground to produce the hormone

salicylic acid.

An increase in the hormone salicylic acid

causes the plant to produce many types of pant defense proteins.

Plant hormones Jasmonate and

Ethylene increase throughout the plant and induce resistance

to a wide variety of plant pathogens.

Jasmonate

Ethylene

Plant growth promoting

rhizobacteria (PGPR) stimulate plants roots, causing production of

plant defense hormones

ISRInduced Resistance

SARInduced Resistance

a) Systemic Acquired Resistance b) Induced Systemic Resistance

Types of induced resistance to plant diseases (modified from Vallad and Goodman (2006) by Heather Darby).

conclusion

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