Plant Responses to Internal and External Signals

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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