recap

38.2 recap

Many plants use secondary metabolites as constitutive defenses against herbivory. Other defenses are induced by herbivory through signal transduction pathways. The plant hormone jasmonate stimulates local and systemic responses to herbivores.

learning outcomes

You should be able to:

  • Describe physical methods plants use to defend themselves against herbivores.

  • Discuss the range of effects that secondary metabolites might have on organisms attacking plants.

  • Compare constitutive and induced chemical defenses against herbivores.

  • Give examples of types of elicitors known to trigger signal transduction pathways.

  • Describe the key components of signal transduction pathways that are activated by herbivory or wounding.

  • Describe and analyze the process by which jasmonate defends plants against herbivory.

Question 1

Latex is harvested from certain trees to make rubber. After a region of the tree has been drained of latex, slugs are observed consuming the leaves in that region. What does this indicate about the role of latex in the tree?

It indicates that latex is important in defense against herbivores such as slugs.

815

Question 2

Tobacco plants produce nicotine as a secondary metabolite. The plants also have induced defenses against herbivores. How would these defenses work together to protect the plant from herbivores?

Because nicotine is a secondary metabolite that is always present in tobacco leaves, it is a constitutive defense that can deter many herbivores. Induced responses would be triggered by herbivores that are not affected by the nicotine.

Question 3

Armyworm larvae feed on corn leaves. They deposit their frass, or waste, in crevices where corn leaves meet stalks. The frass releases chemicals that signal the plant that it is being attacked by armyworms, and the corn plant mounts a defense against armyworms.

  1. Does this represent a constitutive or an induced defense? Explain your answer.

  2. How would chemicals in armyworm frass affect the corn plant’s response?

  1. This is an induced defense, as the frass contains an elicitor.
  2. Chemicals in the frass act as elicitors that trigger signal transduction pathways. One pathway involves changes in electric potential of the cell membrane, which is transmitted via the symplast to the whole plant. Another pathway triggers production of reactive oxygen species, which act as signaling molecules in pathways that lead to change in gene expression. Yet another pathway leads to production of jasmonate and other hormones, which trigger systemic defenses, and may even travel to neighboring plants to stimulate their defense responses.

Question 4

Explain how plants attacked by herbivores can use jasmonate to "call for help."

Jasmonates are made in response to an elicitor from herbivory. Jasmonates can induce the synthesis of volatile molecules, which diffuse through the air to other plants or organs in the same plant.

A plant’s survival depends not only on successful defenses against pathogens and herbivores but also on coping with a sometimes hostile physical environment. Next we will consider how plants deal with environmental stresses.