Responses 39
Plant to Damaging Organisms
39 Plant Responses to Environmental Challenges
How Do Plants Deal with Pathogens and with Herbivores?
Pathogens evolve mechanisms to attack plants, plants evolve defenses against pathogens. Plants use both mechanical and chemical defenses.
39.1 How Do Plants Deal with Pathogens?
One of plant cell's first lines of defense is to deposit more polysaccharides on the inside of the cell wall to reinforce this barrier. The polysaccharides block the plasmodesmata, and serve as a base for laying down lignin. Lignin is a mechanical barrier, and the precursors are toxic.
Figure 39.1 Signaling between Plants and Pathogens
39.1 How Do Plants Deal with Pathogens?
Plants produce many kinds of defensive compounds.
Phytoalexins are toxic to many fungi and bacteria. Most are phenolics or terpenes, and protect against herbivores as well. Also toxic plant cells
Table 39.1 Secondary Plant Metabolites Used in Defense
39.1 How Do Plants Deal with Pathogens?
Enzymes from a pathogenic fungus cause plant cell walls to release signaling molecules, oligosaccharins, which trigger phytoalexin production. The action is nonspecific, so they can kill many species in addition to the one that triggered their production.
39.1 How Do Plants Deal with Pathogens?
Pathogenesis-related proteins (PR proteins):
Some are enzymes that break down the cell walls of pathogens. Chemicals released from the walls can trigger other defense mechanisms.
Other PR proteins serve as signals attack of to other cells.
39.1 How Do Plants Deal with Pathogens?
Hypersensitive response: cells around site of infection die, preventing nutrients from reaching site. Some produce phytoalexins before they die. Dead tissue is called a necrotic lesion, contains and isolates what is left of the microbial invasion: seals of infection
Figure 39.2 The Aftermath of a Hypersensitive Response
39.1 How Do Plants Deal with Pathogens?
One chemical produced during the hypersensitive response is salicylic acid, from which aspirin is derived.
It often evokes a second complex defensive response. Systemic acquired resistance: general increase in resistance of whole plant to a wide range of pathogens. Can be longlasting.
39.1 How Do Plants Deal with Pathogens?
Plant response to RNA viruses: Plant uses its own enzymes to convert virus RNA to double-stranded RNAs (dsRNA), and to chop it into small pieces--small interfering RNAs (siRNA).
Some of the viral RNA is transcribed, but the siRNAs degrade the mRNA, blocking viral replication.
39.1 How Do Plants Deal with Pathogens?
This is an example of RNA interference (RNAi), or posttranscriptional gene silencing.
Immunity conferred by RNAi spreads quickly through the plant. Plant viruses fight back by evolving mechanisms to confound RNAi.
Figure 39.7 A Signaling Pathway for Synthesis of a Defensive Secondary Metabolite
AN INVITATION
INTERESTED IN PLANT ADAPTATIONS? Take 020.333 Adaptations of Plants to Their Environments next spring!