Biotic Stress II

1. Agents of Biotic Stress

2. Physical and chemical barriers and pathogen recognition

3. Basal Immune System

4. SAR

5. Hipersensitivity and programmed cell death

6. Resistance Genes

Defenses involve:

Biotrophic pathogens: SA-dependent signalling pathways, and culminates in a localized cell death in the vicinity of the pathogen entry site;

Necrotrophic: pathways utilizing genes-responces JA and ET;

Cross-communicating between signaling pathways allow quick plant adaptation ;

Model of interaction between Plant-Patogen

Bent & Mackey, 2007

The plants have a other layer for defence against differents pathogens:

• The SAR is tiggered for three major hormones : AS, JA and ET mainly against biotrophic and hemi-biotrophic pathogens.

• Mutants that are affected in the accumulation of SA or areinsensitive to SA show enhanced susceptibility to biotrophicand hemi-biotrophic pathogens.

• The PCD also have major function in defence acquired in plants also trough of production de ROS both in site (HR) and distant of infection (SAR).

Levels and forms of regulations

≠ PROCESS and NAMES SAR: Systemic Acquired Resistance:

Requires both local and systemic salicylic acid (SA) accumulation and the induction of a subset of the pathogenesis-related (PR) genes, but SA itself is not the mobile signals.

ISR: Induced Systemic Resistance:

Is associated with the ability of rhizobacterial strains that promote plant growth to protect against a variety of fungal and bacterial pathogens. ISR is less ‘broad spectrum’ in nature and generally requires of JA and ET.

HR and death cell (PCD)It is characterized by rapid and localized collapse of plant tissue around the infection site, including the formation of lesions around of infection caused by programmed cell death (PCD).

These lesions avoiding the pathogen to other plant tissues, stopping the manifestation of the disease !!

The induction of a large number of soluble proteins pathogenesis-related proteins (PR-proteins), especially peroxidases, chitinases and glucanases.

Other responses parallel to infection are increased expression of phenylalanine ammonia lyase (PAL) and deposition of lignin and increase the levels of SA.

HR again the necrotroffics ...

If the HR active the PCD causing necroses, this pathway not would interesting for pathogens necrotroffics and hemi-biotroffics ...

In the interactions related this types pathogens in plants, the HR mechanisms not yet fully understood since these pathogens could benefit to obtain nutrients from dead cells…

Systemic Acquired Resistance

• SAR occurs at sites distant from primary and secondary immune responses and protects plants from subsequent pathogen attacks.

• Induced by pathogens that usually infect leaves or stems of plants and is induced simultaneously with local primary and secondary immune responses effective against a broad range of pathogens and is dependent on different plant hormones including salicylic acid (SA),jasmonic acid (JA),ethylene (ET),abscisic acid (ABA) or combinations thereof.

R Proteins ...

R proteins are most commonly intracellular NB-LRR proteins or extracellular LRR carrying receptors, but other proteins types can also be classified as R proteins.

Some NB-LRR proteins are present in both the cytoplasm and the nucleus.

• Can act in the different levels of immunity ;

• Are activating the signaling cascades for SAR ;

Partners in SAR ...

The identification of signals mobile and the mechanisms by which they are perceived in distal sites triggered a effective systemic immunty remains understood.

Signalling molecules ??

The efforts are focused on the conserved central regulator positive of SAR, NON-EXPRESSED OF PATHOGENESIS-RELATED1 (NPR1 or NIM1).

This is a type of protein R in plants that act core play in signalling in distants sites.

The action of NPR1... Function is downstream of Salicylic Acid ( SA ) ;

Accumulation of SA induces changes in redox potential of cell

Triggered changes its shape cytosolic to activate monomers

nucleus

In the nucleus occurs the interaction these monomers of NPR1 com TFs type TGA

TGA binding in the promoter of genes PR triggered transcriptional remodeling contributes to the establishment of SAR response .

NPR1-interacting...

•The proteins NPR1 interacts with three structurally related nuclear localised NIM1-INTERACTING (NIMIN);

• Overexpression of NIMIN1 suppresses resistance to Pseudomonas syringae (Pst) strain and attenuates SAR and PR-1 expression. NIMIN1 knock down enhanced PR1 expression but did not modify pathogen responses;

• As overexpression in rice of NPR1 (or the rice orthologue NH1) confers enhanced resistance to the pathogen Xanthomonas oryzae pv. oryzae, NPR1 function appears to be conserved between monocots and dicots;

NPR1-interacting...• In rice, ectopic expression of an unrelated rice nuclear localized protein, NEGATIVE REGULATOR OF RESISTANCE (NRR), which interacts with both NH1 and NPR1, generates pathogen responses that are similar to those resulting from the overexpression of NIMIN1;

• NIMIN1 and NRR share only a conserved NPR1-binding domain;

Collectively, these data suggest that NRR and NIMIN1 have conserved functions as transcriptional repressors that negatively regulate a subset of the NPR1 signalling network ;

NPR1 also modulates cross-talk between SA and JA signalling;

NPR1-dependent ISR signalling does not require redox-activated of NPR1 nuclear localization, consistent with a role for JA:IAA as an inducing signal;

Other possible targets are WRKY transcription factors that bind W boxes in PR genes, such as the positive regulator of SA signalling WRKY70;

Alternatively, the targets might be interactions with negative regulators of SA signalling, such as mitogen-activated protein (MAP) kinase 4 (MAPK4), which negatively regulates SA and positively regulates JA defense responses. In an interaction walk, MKS1 interacted with WRKY25 and WRKY33, thereby coupling MAPK4 to activation of SAR.

Involviment of NPR1 in others pathways ...

•The identification of MKS1, a MAPK4 substrate, established a biochemical link between WRKY transcription factors and SA. MKS1 functions downstream of SA and independently of JA signalling pathways to confer SA dependent resistance.

In an interaction walk, MKS1 interacted with WRKY25 and WRKY33, thereby coupling MAPK4 to activation of SAR .

SA

MKS1

Regulatory Interactions

MKS1

WRK25/WRKY33

SAR

WRKY

SA - dependent

Lipids as signals in systemic immunity

SAR is generally studied as a response to classical gene for gene recognition. Nitric oxide (NO), SA, JA, ethylene and lipid-derived molecules are all implicated in systemic signalling.

Intracellular signals that are derived from lipid metabolism are emerging as mediators of basal, gene-for-gene and more recently systemic resistance;

SFD1 encodes dihydroxyacetone phosphate reductase, which forms the glycerol-3-phosphate backbone of glycerolipids.

SFD1 is required for both local and distant SA accumulation , basal resistance involved is unaffected in sfd1 mutants, suggesting that SFD1 functions in the generation, translocation and/or perception of a mobile SAR signal.

It’s could be JA ? Several studies have demonstrated that concentrations of JA increase locally in response to pathogen infection or tissue damage and exogenous application of JA induced the expression of defence related genes;

Rapid accumulation of JA in phloem exudates of leaves challenged with an avirulent strain of P. syringae and increased accumulation of JA biosynthetic gene transcripts as well as JA levels in systemic leaves suggests that JA could act as a mobile signal in Arabidopsis pathogen immunity ;

However, suggestions and evidences demonstrate that the mobile signal in SAR is likely to be molecules

related as jasmonates and not JA itself…

Signalling Molecules ??

Defective In Induced Resistance1 (DIR1) is an extracellular Lipid Transfer Proteins (LTP) that is involved in the local generation, or subsequent translocation, of a mobile systemic signal; ONLY ??

The importance of JA as an essential component of the long-distance signal for wounding also is challenge ; ONLY ??

Considering the localisation of JA-biosynthetic enzymes to the sieve elements and companion cells in tomato , and the extensive apoplastic exchange, release and retrieval of phloem constituents along the phloem pathway , a mobile JA signal could iterate itself along the phloem network to provide a robust, global inductive signal…

Future Issues

What are the MAMPs and PAMP receptors that most strongly affect defense activation, especially for the most destructive diseases of valuable crop species?

As pathway of lipids-dependence (LTP1-JA), but not of LTP1 or JA alone, resulted in enhanced resistance in tobacco against Phytophthora parasitica, these data support a role for JA as a systemic signal.

This is conserved in others species ??