Fighting for their lives: Plants and
pathogens
Photos courtesy of CIMMYT, Scott Bauer, USDA Agricultural Research Service, IRRI, IRRI, IITA
Nematodes are
large,
multicellular
animals
Bacteria are
prokaryotes
Viruses are
non-cellular,
and merely
packaged
nucleic acids
Fungi and
oomyctes are
eukaryotes
Bacteria
Oomycete
Fungus Fungus
Many kinds of organisms
cause plant disease
Large, E.C. 1940. Advance of the Fungi. Jonathan Cape, London.
What makes an interaction into a
disease? (Disease triangle)
Plants are exposed to
countless microbes, but
very, very few of these
interactions lead to
disease. Why?
Environment
The disease
triangle
(it takes three)
The pathogen must
be able to overcome
plant defenses
The environment must
tip the balance in favor
of the pathogen
The host plant
must be
susceptible to
the pathogen
Humans influence diseases
Photos courtesy Scott Bauer and Bob Nichols, USDA, and Geovantage, Inc.
Monoculture
Introduced pathogens
and vectors
Growing
practices
Migrations
of people
and plants Humans add another
dimension (making a
disease pyramid….)
Strategies of pathogenicity
A successful pathogen must:
• Find the host and attach to it
• Gain entry through the plant’s
impermeable defenses
• Avoid the plant’s defense
responses
• Grow and reproduce
• Spread to other plants
Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org
Gray mold (Botrytis cinerea)
Wind, water, insects and chemotaxis
help pathogens reach their hosts
Jim Plaskowitz, USDA, and Stan Diffie, University of Georgia
Pathogens must be able to penetrate
or circumvent physical barriers
Reprinted by permission from Macmillan Publishers Ltd: Giraldo, M.C., Valent, B. (2013). Filamentous plant pathogen effectors in action. Nat. Rev. Microbiol. 11: 800-814; Kleemann, J., Rincon-Rivera, L.J., Takahara,
H., Neumann, U., van Themaat, E.V.L., van der Does, H.C., Hacquard, S., Stüber, K., Will, I., Schmalenbach, W., Schmelzer, E. and O'Connell, R.J. (2012). Sequential delivery of host-induced virulence effectors by
appressoria and intracellular hyphae of the phytopathogen Colletotrichum higginsianum. PLoS Pathog. 8: e1002643;
Appressorium
Some pathogens
produce non-
melanized but
effective
appressoria
Melanized
appressoria
build up high
pressure to
puncture the
cell wall
Some pathogens
enter through
stomata and grow
extracellularly
Pathogens are biotrophs,
necrotrophs or hemibiotrophs
Reprinted by permission from Macmillan Publishers Ltd: Pieterse, C.M.J., Leon-Reyes, A., Van der Ent, S. and Van Wees, S.C.M.
(2009). Networking by small-molecule hormones in plant immunity. Nat Chem Biol. 5: 308-316, copyright 2009.
Necrotrophs kill
cells and then
consume the
contents Biotrophs live
within host tissue
without causing
death
Hemibiotrophs can
switch from biotroph
to necrotroph
Summary - strategies of
pathogenicity
Pathogens must overcome
formidable plant defenses
Once inside the plant, they
can either co-habitate or kill
Plant immune responses
Plants resist pathogens
through active processes that
include recognition of the
pathogen and defense
responses to fight it
The zig-zag model of plant –
pathogen interactions
Adapted from Jones and Dangl (2006) The plant immune system. Nature. 444: 323-329.
Am
plit
ude o
f defe
nse
Low
High
Pathogen is
recognized:
Pattern
Triggered
Immunity
Effector is
“recognized”:
Effector
Triggered
Immunity
Pathogen effectors
suppress defense
response:
Effector Triggered
Susceptibility
Defense responses
Defense responses
How are pathogens recognized?
Pattern recognition receptors (PRRs)
Many PRRs have an
extracellular leucine-
rich repeat domain
that recognizes
conserved microbial
elements…..
….and an intracellular
kinase domain
They are leucine-rich
repeat receptor
kinases (LRR-RKs)
Bacterial pathogen
Fungal or
oomycete
pathogen
PRRs recognize
pathogens
outside the cell
and initiate
defense
responses
Pathogens produce effectors that
enhance their virulence
Kinase cascade
leading to
transcriptional
responses
Calcium
ion influx
Reactive
oxygen
production
Microbial
effectors
suppress the
plant’s immune
response and /
or contribute to
the pathogen’s
viability
enzymes
Some effectors alter plant behaviour
and development
Reprinted from Hogenhout, S.A., Van der Hoorn, R.A.L., Terauchi, R. and Kamoun, S. (2009). Emerging concepts in
effector biology of plant-associated organisms. Mol. Plant-Microbe Interact. 22: 115-122.
Resistance proteins – intracellular
immune receptors
effector
R protein
R protein
R proteins recognize
effectors intracellularly
Defense
responses
R protein activation leads to
enhanced defense: ETI
Adapted from Jones and Dangl (2006) The plant immune system. Nature. 444: 323-329.
Am
plit
ude o
f defe
nse
Low
High
PTI
ETI
Activated R proteins signal
danger, and trigger a heightened
defense response that includes:
• Production of the stress
hormone salicylic acid (SA)
• Production of reactive oxygen
species (ROS)
• The hypersensitive cell death
response (HR)
• Expression of pathogenesis-
related (PR) proteins
• Systemic signals and systemic
acquired resistance (SAR)
Summary - PTI is suppressed by
effectors, which sometimes trigger ETI
Effectors
suppress PTI
R proteins
recognize
effectors and
induce ETI
PAMP-triggered
immunity
Defense
Responses Defense
Responses
Effector-triggered
immunity
Defense
Responses
Effector triggered
suppression
Adapted from Macmillan Publishers Ltd: Pieterse, C.M.J., Leon-Reyes, A., Van der Ent, S. and Van Wees, S.C.M. (2009). Networking by small-molecule hormones in plant immunity. Nat Chem Biol. 5: 308-316,
Strategies to prevent and manage
disease
Environment
The disease
triangle
(it takes three)
Avoid or eliminate
the pathogen
Manipulate the
environment to favor
the plant
Make the plant
resistant through
genetic or other
methods
Photos by Photo by James Tourtellotte, US Customs and Border Protection : Keith Parker, Marin County Fire Department.
The best way to prevent disease is
to keep pathogens away
Agricultural inspectors check imported
plants for pest and pathogens, but
many pathogens are spread by wind
and water….
Phytophthora ramorum causes
sudden oak death
Pathogens’ effects can be minimized
by hygiene and rotation
Courtesy J.H. Graham (left) and G. Allen (right). Reproduced by permission from Maloy, O. C. (2005). Plant Disease Management. The Plant Health Instructor. DOI: 10.1094/PHI-I-2005-0202-01.
Rotating crops helps reduce
pathogen load in soils
Continuously
cultivated Rotated with corn
Removing and burning an infected citrus
grove to eradicate bacterial canker
Chemical controls are critical for
eradicating pathogens
Photos by Ken Hammond; J.K. Lindsey
Azostrobulin, a widely used
fungicide derived from a
defensive compound
produced by Strobilurus fungi
Compounds must
be safe and
effective, and
application
protocols must be
followed to slow
the development of
resistance
Because pathogens
develop resistance,
finding novel compounds
to eradicate pathogens is
an ongoing process
Biocontrol agents:
• attack the pathogen
• compete with the pathogen
• enhance the plant’s
defenses through induced
systemic resistance (ISR)
• often have multiple effects
Biocontrol refers to the
use of other organisms
to ward off pathogenic
microorganisms and
disease
Reprinted by permission from Macmillan Publishers Ltd: Haas, D. and Defago, G. (2005). Biological control of soil-borne pathogens by fluorescent pseudomonads. Nat. Rev. Micro. 3: 307-319.
Biological control of
pathogens
Summary – strategies to prevent and
manage disease
Quarantines
Crop rotation
Good hygiene
Control of viral vectors
Biocontrols
Chemical
controls
• Adequate nutrition
• Well drained soils
• Genetic diversity to
avoid epidemics
• R-gene mediated
resistance
• Quantitative
disease resistance
• Enhancement of
immune and
defense responses
Weaken
pathogen
Separate plant
& pathogen
Strengthen
plant
Plants and Pathogens: Summary
• Plant diseases are major
threats to food production
• Pathogens have diverse
modes of pathogenicity and
rapidly evolving effectors
• Plants are not passive victims
- they have sophisticated
surveillance and defense
mechanisms
• Human practices, particularly
migrations and monocultures,
have contributed to the
magnitude of plant diseases