Pratylenchus attracted to root hairs

1
Attraction to plant
Pratylenchus attracted to root hairs
2
Zoospores swim to nutrients
Drop sugar
Microscope slide
3
Germtube Septoria tritici growing through stomata
4
Adhesion to Plant

• Adhesion to Plant
• Nonspecific adhesion to surface e.g.
  Magnaporthe grisea conidia gt tip mucilage
  (glue)

Micrographs by Rick Howard Lab, DuPont
5
Spore Tip Mucilage
Glues the spore to the hydrophobic rice surface
through rain or dew, which are critical for
penetration.
6
Adhesion to Plant cont.

• Non specific
• mistletoe (viscin)
• Specific adhesion carbohydrate (lectin) or
  glycoprotein binding of pathogen to host surface,
  especially in wounds.
• E.g. Agrobacterium
• Nematodes suction with lips

7
Urediospore germination and appressorium
8
Plant Cell Walllipid and sugar polymers with
imbedded proteins
CW layer
Biochemical composition
Pathogen tool
9
Other enzymes

• Proteinases
• Amylases - starch
• Lipases, phospholipses - oils, fats, membranes

10
Cellulose
b 1-4 D glucan chains held together by hydrogen
bonds

11
Mixed linked b glucans

12
Pectin structure and where enzymes attack
galacturonic acid
rhamnose
arabinose
galacturonic acid
13
Erwinia soft rots

• 2 cellulases
• 8 endopectate lyases
• 1 Exopectate lyases
• 1 Pectin acetylesterase
• 2 Pectin methylesterases
• 3 Pectin lyases
• 1 Polygalacturonase
• 1 Rhamnogalacturonate lyase
• 3 Endopolygalacturonases

14
Wood Rotting Fungi
Brown rot only cellulase
White rot ligninases followed by cellulase
15
Phytotoxins, Host Specific

• Victorin
• Cochliobolus victoria, Victoria Blight of Oats
• inhibition of glycine decarboxylase, involved in
  photorespiration
• T-toxin
• Cochliobolus heterostrophus, race T southern corn
  leaf blight
• uncouples electrolyte gradient in mitochondria
• HC toxin
• Cochliobolus carbonum, Carbonum leaf spot of corn
• inhibition of histone deacetylase
• Ptr toxin
• Pyrenophora tritici-repentis, tan spot of wheat

16
Tan Spot of Wheat Pyrenophora tritici-repentis
Courtesy of Dr.William Bockus, Kansas State Univ.
17
Toxin alone causes chlorosis on susceptible wheat
Lines
Katepwa S Chl
6B365 R
fungus
Glenlea R
Salamouni R
S Chl
R
Prt toxin
R
R
S Control
18
Pyrenophora tritici-repentis isolates lacking the
Prt gene are less virulent on susceptible wheat
Susceptible wheat
R wheat
19
Phytotoxins, Non Host Specific

• Tabtoxin
• Pseudomonas syringae pv. tabaci, wildfire disease
  of tobacco
• Glutamine synthetase gt toxic conc ammonia
• Phaseolotoxin
• Pseudomonas syringae pv. phaseolicola, halo
  blight of bean
• Ornithine carbamoyltransferase gt alters amino
  acid pools and inhibits pyrimidine biosynthesis
• Pyricularin
• Pyricularia (Magnaporthe) grisea, rice blast
  disease
• Tentoxin
• Alternaria alternata, seedling disease
• Disrupts chloroplast development and inhibits
  polyphenol oxidase
• Cerato-ulmin
• Ophiostoma ulmi, Dutch elm disease

20
Detoxification

• Plants produce low molecular weight antimicrobial
  molecules
• Pathogens can degrade these molecules
• E.g. Saponins are plant molecules with antifungal
  properties.
• Take all fungus Gaeumannomyces graminis f. sp.
  avenae produces avenacinase which detoxifys the
  oat saponin called avenacin

21
Growth Regulators

• Can be formed or induced by pathogen
• Auxins (cell elongation)
• Pathogen derived inhibitors of auxin
• Cytokinins (cell division)
• Mistletoes on conifers
• Gibberellins (cell elongation)
• Gibberella fujikuroi, foolish seedling disease of
  rice
• Elsinoë brasiliensis, superelongation disease of
  cassava
• Ethylene (many functions)
• Implicated in premature defoliation of some
  plants diseases

22
Giant cell formation by Melodigyne
Female nematode
23
Galls
24
Foolish seedling disease in rice
25
avr and Type III Secretion

• Avr genes encode (or direct production) of small
  molecules recognized by host plants.
• Resistant host plants respond in rapid cell
  defense
• Avr proteins promote pathogen growth and disease
  development in susceptible hosts
• Proven for bacterial Avr proteins
• Avr proteins may be injected into host cells by
  type III secretion systems

26
4.c. Altered Genetic Control

• Pathogen nucleic acids
• Viruses
• Agrobacterium tumefaciens

27
4.d. Interference with Transport

• Plugging vascular tissues
• Wilting
• Pathogen
• Mycelium and spores
• Extracellular polysaccharide (EPS)
• May mask elicitors of resistant response
• Host
• Tyloses, gums, gels
• Girdling by cankers
• Root damage
• Nematodes
• fungi

28
Fusarium in vascular tissue
29
Tyloses
Overgrowth of the protoplast of a parenchyma cell
into an adjacent xylelm vessel or tracheid
30
Cankersfungal and bacterial
31
Defending oneself against the enemy
32
Most problems in biology are solved by a
combination of physics and chemistry
33
I. Constitutive defenses

• 1. Structural
• Cuticle
• hard to wet impedes spore germination
  bacterial multiplication. Net negative charge
• Host Cell Wall
• Structure and composition can determine whether
  pathogen recognizes host and whether it can
  invade tissue
• Bark
• impervious to water organisms
• Trichomes
• trap aphids, suppress virus transmission, e.g.
  Solanum bertholdii
• Xylem vessels
• smaller diameter correlated with increased
  resistance to Ophiostoma novo-ulmi (dutch elm
  disease)

34
Cuticle
Image from U of Hawaii Dept of Botany
35
Cell walls enable remarkable structural diversity
36
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
37
Trichomes on Solanum species traps aphid
38
Chemical Responses
39
I. Constitutive defenses
2. Pre-formed chemicals a. Toxins (often
phenolics) i) chlorogenic acid
potatoes/Verticillium ii) protocatechuic acid
purple onions/smudge disease caused by
Colletotrichum circinans iii) saponins
(non-phenolic) e.g. Avenacin in oats and
tomatine in tomatoes b. Plant lectins
sugar-binding proteins - can lyse and/or
inhibit growth of some fungi c. Plant glucanases
and chitinases can break down pathogen cell
wall d. Phytocystatins inhibit enzyme activity
of nematodes, smaller females and less eggs
40
II. Induced Defenseshost must recognize pathogen
and respond
Pathogen signal molecules (elicitors)
Host response
41
Elicitorsextracellular products or wall-bound
molecules

• plant cell wall (oligogalacturonides) of specific
  size range lt 20 sugars in length
• E.g., Polygalacturonase from Botrytis cinerea
  (indirect gt oligogalacturonides from plant cell
  wall).
• fungal cell wall chitin monomers, ß-1-3 glucans
  
• fatty acids Phytophthora infestans, arachidonic
  acid
• Pathogen proteins
• E.g., Erwinia amylovora, harpin encoded by hrp
  genes
• E.g. avr gene products
• E.g. Tobacco Mosaic Virus (TMV) and other
  viruses- coat protein

42
Receptors

• Location of host receptors recognizing elicitors
  not known for the most part
• Once elicitor recognized, plant responds with
  biochemical and structural defenses
• Speed of response dictates degree of resistance
  (fast is better)

43
Induced Defenses Structures(pp. 214-217 Agrios)

• 1. Papillae (cell wall appositions) response
  to fungi, particularly powdery mildew
• 2. Cork -- lignified and suberized protective
  layer.
• 3. Abscission layers leave cuts off infected
  area
• 4. Tyloses (xylem vessels)
• 5. Gum deposition, impenetrable barrier
• 6. Callus formation around cankers on woody
  plants
• 7. Wound healing

44
Papillae and cell wall appositions

• A cell wall modification formed opposite to
  penetration peg
• Papillae contain callose, phenolics,
  hydroxyproline rich proteins
• Callose is a ?-1,3-glucan polymer, different than
  cellulose in the connections of the sugars
• Enhance cell wall mediated defense
• Part of the basal defense response
• In resistance reactions, papillae may block /
  delay penetration and haustorium development

45
Papillae
Papillae
Papillae formed below powdery mildew appressorium
46



Resistance (HR)
Susceptible
Resistance
Huckelhoven, R., et al. Plant Physiol.
19991191251-1260
47
Formation of Cork Layers- inhibits further
invasion- prevents spread of toxins- deprives
tissue and pathogen of nutrition.
48
Abcission Layers
- similar function as cork layers but associated
with leaves - diseased leaf can be sloughed off
tree
49
Tylosescan both help restrict movement of
pathogen and cause injury to plant
50
Callus tissue formed around pathogen canker
51
Wound healing

• A zone of tissue around the wound or infection is
  sacrificed for defense.
• Becomes infused with phytoalexins and other
  phenolic compounds (tannins), gum, resin.
• This tissue then dies is a temporary barrier
  impervious to water and refractory or impervious
  to pathogens.
• Just beyond this zone, a cork barrier forms
• Parenchyma dedifferentiate, return to
  meristematic state
• Differentiate a cork cambium
• Lesion or wound is thus isolated from living part
  of plant body

52
II. Induced Defenses, Biochemicals

• 1. HR (hypersensitive response) (217, 221-232)
• 2. PR (pathogenesis related) proteins (232 -234)
• 3. Phytoalexins (235-236)
• 4. Systemic Acquired Resistance (237-242)