7c: Plant immunity part 3 Flashcards
Defence mechanisms of plants to ward of pathogens
Plant defences against pathogens:
Stomatal closure
Ion fluxes
Oxidative burst
Phyto-Hormone action
Induced systemic resistance
Systemic Acquired Resistance
Stomatal closure
Stomata are natural opening through pathogen can easily enter into the apoplast
Stomatal closure is part of the innate immune response to restrict bacterial invasion.
Plant Stomata Function in Innate
Immunity against Bacterial Invasion:
(See Melotto et al)
I) Stomata actively closes as
an initial response to both
pathogenic bacteria
(2)Pst DC3000 has a
mechanism to reopen stomata 3
hr after incubation
(3)InocuIum concentration 1 x
107 cfu/ml Ihr-closure & 3hr-
Reopen
Bacteria and PAMPs Trigger Stomatal Closure in Arabidopsis
Ion fluxes
Ion fluxes
Membrane permeability changes rapidly leading to a loss of cellular electrolytes
such as K+ and and uptake of H+.
At the same time, there is often an influx of Ca2+, a key intracellular signal in plants that is involved in the activation of enzymes and gene expression.
The experimental blocking of Ca2+ transport across membranes in inoculated bean cells also inhibits gene activation and subsequent defence responses.
Oxidative burst
It is a rapid, transient, production of huge amounts of reactive oxygen species (ROS)
Produced from membrane localized NADPH oxidase
See Nuhse et al 2007
&
Rarl is required for Mla12 R gene mediated resistance against mildew pathogen in barley Shirasu etal..1999
^ infected cell is destroyed by an oxidative burst
Phytohormones
Salicylic acid is the most fundamentally important
see notes for full list
Phytohormones:
JA- and ET- dependent signaling
Effective against necrotrophic pathogens
Increase in JA levels and induction of effector genes (PDF1.2, VSP1)
Induction of transcription factors ERF1, RAP2.6 and JIN1
which activates many defense related genes
Some JA regulated genes also regulated by ET (PDF1.2, ERF1)
JA levels regulated by cellulose synthases
JAR1 involved in conversion of JA to active form by conjugation with amino acids like isoleucine
In Arabidopsis, all activities of JA requires the function of CO11.
Some responses to JA require the function of an MAP kinase encoded by MPK4
(Glazebrook, 2005)
phytohormones:
Cross-talk between SA and JA/ET signaling
(SA = salicylic acid)
Helps the plant to minimize energy costs and create a flexible signaling network that allows the plant to finely tune its defense response to the invaders encountered
Most reports indicate a mutually antagonistic interaction between SA- and JA dependent signaling.
(Koornneef et al., 2008)
Phytohormones:
Hypersensitive response generates SA
Rapid death of cells in the local region surrounding an infection.
Restrict the growth and spread of pathogens to other parts of the plant.
Favor growth of pathogens with a necrotrophic lifestyle
HR generates salicylic acid
Process of HR:
*Burst of oxygen reactive species around infection site
*Synthesis of antimicrobial phytoalexins
*Accumulation of Salicylic Acid (SA)
*Directly kill and damage pathogens
*Strengthen cell walls, and triggers apoptosis
*Restrict pathogen from spreading
*Rapid and local
The different life pathways of the pathogen determine the plant signalling response:
Biotrophic: pathogens propagate in living plant tissue and generally do not cause necrosis as a result of infection.
Necrotrophic: pathogens actively induce necrosis in infected tissues, often through the production of toxins.
Hemibiotroph: An organism that is parasitic in living tissue for some time and then continues to live in dead tissue
Systemic Acquired Resistance (SAR)
Systemic Acquired Resistance (SAR)
It is secondary resistance response
Because, once plant defense responses are activated at the site of infection, a systemic defense response is triggered in distal plant parts to protect these undamaged tissues against subsequent invasion by the pathogen.
Long-lasting and broad-spectrum induced disease resistance
Act non-specifically through out the plant and reduce disease severity
Characterized by the accumulation of Salicylic Acid (SA)
SAR timeline over 24 hours
SAR signal is generated within 4hrs of pathogen inoculation
SA could be detected in phloem of leaves 8hr after inoculation
Increased level of SA in phloem of leaf above the incubated leaf
Expression of SAR occurs within 24hr after inoculation
Phytohormone Salicylic Acid (SA) information
Salicylic Acid (SA)
*Made in chloroplasts
*Accumulates in both local and systemic tissues (not the systemic signal)
*Removal of SA (as in nahG plants) prevents induction of SAR
*Analogs: INA or BTH
THE MECHANISM INVOLVED IN SYSTEMIC ACQUIRED RESISTANCE
1.Lignifications and other structural barriers
2.Accumulation of Salicylic acid
3.Pathogenesis Related proteins
- Lignification and other structural Barriers
Deposition of lignin in cell wall is called as lignification. Is observed in many plants
It is an important mechanism for disease resistance Lignin incorporation strengthen plant mechanically. Lignified cell starve the pathogen Lignin pre cursors might exert toxic effect.
Mycelia of colletotrichum lagenarium become lignified in vitro
Glycine rich proteins accumulate systemically in cell wall of tobacco plants infected with TMV and virus infected rice plants
2.Accumulation of Salicylic acid:
See cards 10 and 11
3: PR proteins (PRPs)
Proteins produced in plants when it is attacked by pathogen, they are antimicrobial/viral/ insecticidal
Its extremely acidic/ basic in nature, therefore it is highly soluble an highly reactive.
Crosslink the molecules of cell wall and acts as barricade by accumulation of lignin which helps the cell wall to protrude as papillae.
Gives alarming signals to neighbouring cells
It present in both resistant and susceptible plant, but concentration is differs. When there is infection its concentration increases and vice versa.
see notes for table of plants in which PRP detected
Mutants that affect salicylic acid synthesis
Mutants that affect salicylic acid synthesis
Elevated SA accumulation :
*dnd1 (defense, no death 1): increased SA, but reduced HR, DND1 gene encodes cyclic-nucleotide-gated ion channel
*mpk4: constitutive SA accumulation
*edr1 (enhanced disease resistance 1): defective MAPKKK
Reduced SA accumulation:
*eds1 (enhanced disease susceptibility 1): lipase homolog
*pad4 (phytoalexin deficient 4): another lipase homolog
*sid1 and sid2 (salicylic acid induction-deficient): defects in chorismate pathway
Mutant screen
*Aimed at identifying regulatory genes of SAR
*Strategy: Transform Arabidopsis with GUS reporter driven by SA- and INA-responsive promotor from BGL2 gene
*npr1 (non-expresser of PR genes) mutant: reduced induction of reporter gene with or without SA, INA
*cpr (constitutive expresser of PR genes) mutants: constitutively express reporter genes
NPR1: non-expresser of PR genes
*Also known as NIM1 or SAI1
*Positive regulator of SAR
*Downstream of SA, upstream of PR genes
*npr1 mutants are susceptible to various pathogens
*Overexpression of NPR1 generates broad-spectrum resistance
*Unique, but similar to Iκ-B (negative regulator of immunity in animals)
Signaling steps between SA and PR protein expression and disease resistance.
Example:
The Arabidopsis NPR1 Gene That Controls Systemic Acquired Resistance Encodes
a Novel Protein Containing Ankyrin Repeats
Hui Cao, Jane Glazebrook, Joseph D. Clarke, Sigrid Volko, and Xinnian Dong
(1997) Cell 88, 57-63,
Previously: Linked a PR protein promoter called BGL2 to GUS.
Screened thousands of mutant transgenic BGL2-GUS plants for
ABSENCE of GUS activity induced by SA treatment.
Using standard Arabidopsis genetic mapping methods, identified a
single mutant gene, npr1. Phenotype:
Complete absence of GUS activity in response to SA
Absence of PR-1, PR-5, BGL2 expression in response to SA
Is now susceptible to Peronospora parasitica and to
Pseudomonas syringae pv maculicola (Psm).
Cao et al. (1997) Cell 88, 57–63,
Structural features of NPR1
*593 amino acids, 67 kD
*Two protein-protein interaction domains: BTB/POZ and Ankyrin repeats
*Contains NLS
*Multiple phosphorylation sites
*No DNA binding domain
NPRI interacts with TGA transcription factors
- Found to interact with NPRI through
yeast-two hybrid - bZIP transcription factors
- Six members in Arabidopsis (TGA1-6)
Might be redundant - Bind to as-I DNA element
TGA transcription factors bind to TGACG Sequences found in the promoter of genes such as PR-I
NPRI enhances TGAI binding to the as-I element under reducing conditions
See notes Despres et al. (2003) Plant cell
A Model for signal transduction in SAR
Direct SA binding to NPRI
SA binding to modulates its activity. In unstressed conditions, the C-terminal transactivation domain of NPRI is repressed by the N-terminal transactivation domain, keeping NPRI in an inactive state (green).
NPRI perceives SA through Cys521/529 via the transition metal copper, which triggers a
conformation change of NPRI, resulting in de-repression of the transactivation domain and activation of NPRI (yellow).
Front. Plant sci., 09 December 2014 http://dxd0Lorg/103389/fpls.201400697
Another Model for NPR proteins dependent SAR
see: Yuli Ding, Tongjun Sun, Kevin Ao, Yujun Peng, Yaxi Zhang, Xin Li, Yuelin Zhang
Opposite Roles of Salicylic Acid Receptors NPR1 and NPR3/NPR4 in Transcriptional Regulation of Plant Immunity, Cell, Volume 173, Issue 6, 31 May 2018, Pages
1454-1467.e15
