Plant Defense against Pathogen Attack

1
Plant Defense against Pathogen Attack
2
Learning Objectives

• Plants have a variety of responses to pathogen
  attack
• Some defenses are constitutively present
• Some defenses are induced by the presence of the
  pathogen
• These processes are dictated by host and pathogen
  genes

3
Most problems in biology are solved by a
combination of physics and chemistry
4
Constitutive defenses - Structural

• Cuticle
• Host Cell Wall
• Bark
• Trichomes

5
Type II cell wall found in grasses, gingers,
bromeliads, palms, cycads, cypresses
glucuronarabinoxylans and mixed-link glucans are
major hemicelluloses Type I cell wall found in
all other higher plant species xyloglucan is
major hemicellulose From Yokoyama R and
Nishitani K (2004) Genomic basis for cell-wall
diversity in plants. A comparative approach to
gene families in rice and Arabidopsis. Plant Cell
Physiol 45(9)1111-21
6
Constitutive defenses - Chemical

• Toxins
• Plant lectins
• Plant glucanses and chitinases
• Phytocystatins

7
Induced Defenseshost must recognize pathogen and
respond
Pathogen signal molecules (elicitors)
Host response
8
Elictors

• Fungal Cell Wall
• Fatty Acids
• Pathogen proteins

9
Induced Defenses - Structures

• Papillae
• Cork
• Abscission layers
• Tyloses
• Gum deposition
• Callus formation
• Wound healing

10

• Steps to Infection
• Spore germination
• Growth over the leaf
• Appressorium formation
• Penetration peg

11

• Plant Response
• Papillae form on inner surface of CW
• (hypothesis) Anchors invading hyphae by forming
  tight seal between neck of haustorium and plasma
  membrane

12
Induced Defenses - Structures

• Papillae
• Cork
• Abscission layers
• Tyloses

13
Wound healing zone around wound/infection
sacrificed for defense

• Infused with phytoalexins and other phenoic
  compounds (tannins), gum, resin
• Tissue dies
• Cork barrier forms outside zone
• Tissue differentiation
• Parenchyma return to meristematic state
• Cork cambium
• Wound isolated from living part of plant body

14
Induced defenses - biochemical

• Hypersensitive response (HR)
• Pathogenesis related (PR) proteins
• Phytoalexins
• Systemic Acquired Resistance (SAR)

15
Hypersensitive response programmed cell death

• Recognition
• Host membrane changes
• Chemical attack
• Cessation of cytoplasmic streaming
• Membrane collapse

16
(No Transcript)
17
(No Transcript)
18
What do the Avirulence genes (avr genes) code for?

• They are VERY diverse!
• Bacteria
• Cytoplasmic enzymes involved in the synthesis of
  secreted elicitor
• Fungi
• Secreted proteins
• Fungal toxins

19
What do the Resistance genes (R genes) code for?

• They are ALSO diverse!
• Enzymes
• Inactivate toxins
• Signal initiation of the defense responses
• Membrane receptor proteins
• Proteins that interact with other proteins

20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
Systemic Acquired Resistance (SAR)

• Generalized resistance AFTER exposure to pathogen
• Enhanced capacity to mobilize infection-induced,
  cellular defense responses
• Reduces severity of disease from all pathogen
  types
• First localized
• Next spreads systemically

28
(No Transcript)
29
PLANT-PATHOGEN INTERACTIONS
INCOMPATIBLE
COMPATIBLE
Host R-gene Pathogen Avr gene(s)
Host does not recognize pathogen
HR
?
Salicylic acid
SAR
infection
Local lesions, necrosis, no spread
Systemic spread, symptoms
30
(No Transcript)