recap

38.1 recap

Plants have both constitutive and induced defense mechanisms to protect themselves against pathogens. General and specific immunity involve several common signaling pathways, but specific immunity is a stronger response that usually leads to hypersensitive cell death. Specific immunity involves reactions between genetically determined molecules of pathogen and host plant. Systemic acquired resistance provides a longer-lasting, more general immunity throughout the plant.

learning outcomes

You should be able to:

  • Compare and contrast general immunity and specific immunity.

  • Apply the concept of the genetic "arms race" between plants and their pathogens to the evolution of resistance genes and avirulence genes.

  • Describe and discuss the influence of salicylic acid in systemic acquired resistance.

Question 1

A tomato plant can be infected with the fungus Cladosporium. The host plant and pathogen can have various genes involved in the hypersensitive response. Complete the table below to show the effects of each fungal strain on tomato leaves:

Tomato genotype
Cladosporium genotype R1R2 R3R4 R1R4
Avr1Avr2
image
image
image
Avr2Avr3
Avr1Avr4


Avr2Avr3 Healthy Healthy Diseased
Avr1Avr4 Healthy Healthy Healthy

Question 2

Describe how a researcher could determine whether an induced response in a plant cell represents general immunity or specific immunity.

Both general and specific immunity involve similar signaling pathways. For general immunity, the researcher could look for PAMPs on the pathogen, and a weaker response. For specific immunity, the researcher could look for gene-for-gene resistance (Avr genes) and programmed cell death and the formation of a necrotic lesion at the site of pathogen invasion.

Question 3

A researcher makes a small incision in a leaf of a healthy plant and applies salicylic acid to the cut. How might the researcher expect the plant to respond?

Salicylate initiates systemic resistance. Application of this molecule at the site of a wound would trigger the production of protective PR proteins against further infection. This would be a long-term effect.

Not all biological threats to plants come from pathogens. Another threat comes from the many animals that eat plants.