Plants use defense systems to deter herbivory, prevent infection, and combat pathogens.
Herbivory, animals eating plants, is a stress that plants face in any ecosystem.
Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds.
Some plants even “recruit” predatory animals that help defend against specific herbivores.
Slide 70
A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore.
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
Synthesis and
release of
volatile attractants
Chemical
in saliva
Wounding
Signal transduction
pathway
1
1
2
3
4
Slide 71
Plants damaged by insects can release volatile chemicals to warn other plants of the same species.
Methyljasmonic acid can activate the expression of genes involved in plant defenses.
A plant’s first line of defense against infection is the epidermis and periderm. If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread.
Slide 72
A virulent pathogen is one that a plant has little specific defense against. An avirulent pathogen is one that may harm but does not kill the host plant.
Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes.
An R protein recognizes a corresponding molecule made by the pathogen’s Avr gene.
R proteins activate plant defenses by triggering signal transduction pathways.
Slide 73
The Hypersensitive Response
The hypersensitive response
Causes cell and tissue death near the infection site
Induces production of phytoalexins and PR proteins, which attack the pathogen
Stimulates changes in the cell wall that confine the pathogen.
Slide 74
Signal
Hypersensitive
response
Signal transduction
pathway
Avirulent
pathogen
Signal
transduction
pathway
Acquired
resistance
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Slide 75
Review: Signal Transduction Pathway
Plasma membrane
Reception
Response
CELL
WALL
CYTOPLASM
Transduction
Receptor
Hormone or
environmental