8/8/2019 BiotStress

1/25

Biotic Stress and Plant Defense

Responses (Whos Minding the

Fort?)

Land plants are attacked by:

Herbivores (insects, mammals, etc.)

Pathogens

8/8/2019 BiotStress

2/25

Pathogen Strategies

1. Necrotrophic plant tissue killed and then colonized;

broad host range

e.g., rotting bacteria (Erwinia)

2. Biotrophic plant cells remain alive, narrow host range

(1 plant species)

e.g., viruses, nematodes, fungal mildews

3. Hemibiotrophic biotrophic phase, then necrotrophic,

intermediate host range

e.g., Phytophthora (potato blight disease)

8/8/2019 BiotStress

3/25

Major Pathogens

1) Viruses - most are RNA virusesHave a small compact genome which encodes:

coat protein

RNA-dependent RNA polymerase (for replication)

movement protein(s)

Some plant RNA viruses also encode:

protease for processing polyprotein (Potyvirus)

capping enzyme

RNA helicase

Some also exist as multiple particles, and a segmentedRNA genome (e.g., Bromoviruses, 3 particles needed forinfection).

8/8/2019 BiotStress

4/25

An encapsulated SSRNA virus: Tobacco

mosaic virus

DS DNA virus,CauliflowerMosaic Virus

Fig. 21.10, Buchanan et al.

8/8/2019 BiotStress

5/25

8/8/2019 BiotStress

6/25

Fig. 21.11 , Buchanan et al.

Genome of CauliflowerMosaic

Virus (CaMV)

8/8/2019 BiotStress

7/25

1a) Viroids- naked RNA infectious

agents

Single strands of RNA

Lack the protein coat of viruses Do not code for any specific protein

Replicate in the nuclei of infected cells

e.g., Potato spindle tuber viroid was thefirst to be identified.

8/8/2019 BiotStress

8/25

Viroid structure

8/8/2019 BiotStress

9/25

2) Bacteria3) Fungi - plant pathogens found among 4

major groups

4) Nematodes - major root parasites, alsoincrease infection by microorganisms

Insects and other herbivores can alsotransmit plant diseases.

8/8/2019 BiotStress

10/25

8/8/2019 BiotStress

11/25

8/8/2019 BiotStress

12/25

Distribution of Oak Wilt in the US

8/8/2019 BiotStress

13/25

Hill Country Oak

Leaves from Infected tree

Fungus - Ceratocystis

fagacearum

8/8/2019 BiotStress

14/25

Modes of Transmission of Oak Wilt fungus

Natural root grafts Sap beetle

8/8/2019 BiotStress

15/25

Plant Defenses

1) Physical barriers: cuticle, thorns, cell walls

2) Constitutively produced chemicals (e.g., phytoalexins)and proteins (e.g., Ricin)

1 and 2 together provide non-host resistance againstmost potential pathogens. However, some pathogensget around these protections (but only for certain plants),

and generate induced responses, which work to limit thedisease.

3) Induced responses

8/8/2019 BiotStress

16/25

Plant Defense Response

3 aspects of response:

1. Hypersensitive

2. Local

3. Systemic

Compatible interaction

diseaseIncompatible interaction resistance

8/8/2019 BiotStress

17/25

Hypersensitive response

Rapid - within 24 h

Not always needed for resistance

Includes: oxidative reactions (production of hydrogen

peroxide)

deposition of callose (related to cellulose)

opening of ion channels

apoptosis (programmed cell death)

8/8/2019 BiotStress

18/25

Disease symptoms

(necrotic lesions

surrounded bychlorosis) caused by

the phytobacterium

Pseudomonas

syringae pv. tomato

(Pst) strain DC3000

on inoculated leavesof a susceptible

Arabidopsis thaliana

Col-0 plant. Green

leaves were not

inoculated. [Photo byR. Thilmony]

8/8/2019 BiotStress

19/25

Local responses

Cessation of cell cycle

Induction of genes that promote resistance

Phenylpropanoid pathway induced: products include

salicylic acid (secondary inducer: induces otherpathogenesis-related proteins), lignins (cell wall), and

flavonoids

Pathogenesis-related (PR) proteins

Phytoalexins increased

Fortification of cell walls with lignin,

hydroxyproline-rich glycoproteins (HRGPs), etc.

8/8/2019 BiotStress

20/25

Systemic Response or Systemic

Acquired Resistance (SAR)

SAR takes 24-48 h to start, can last for months

Involves gene activation and a transmitted signal.

Genes induced:

chitinases

1,3- glucanases

other PR proteins

8/8/2019 BiotStress

21/25

Signaling Cascade for Defense

Responses

Molecular nature ofelicitors:

1. Cell wall proteins (e.g., Harpin)

2. Intracellular proteins (defined geneticallyin a bacterium by cloning avirulent

loci)

3. Peptide derived from a larger protein(from a fungus)

4. Heptaglucan (small oligosaccharide)

8/8/2019 BiotStress

22/25

Secondary Signals (3)

1. Ca+2 , required for subsequent steps

May mediate phosphorylation-dephos. events

involved in transcriptional or post-

transcriptional gene regulation (there are a

number of genes whose transcription

increases, and some decrease)

Some defense genes also induced by

blue-UV light or other stresses

8/8/2019 BiotStress

23/25

2. H2O2 (hydrogen peroxide)

Plays multiple roles:

induces defense-related genes

induces apoptosis

causes cross-linking of cell wall proteins

(more resistant to wall-degrading enzymes)

may directly kill pathogens

8/8/2019 BiotStress

24/25

3. Salicylic acid

required for SAR

levels increase locally and at distance

from infection Systemic Signal? Probably not. Still

unknown.

8/8/2019 BiotStress

25/25

Model derived

mostly fromstudies in cell

culture using

specific elicitors.

However, there is

evidence for

induction in inact

plants by R

genes. Some

aspects are also

constitutive and

help block mostmicrobes (non-

host resistance).