Chapter Summary
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38.1 Plants Respond to Pathogens with Constitutive and Induced Responses
Plants and pathogens have evolved together in a continuing “arms race”: pathogens have evolved mechanisms for attacking plants, and plants have evolved mechanisms for defending themselves against those attacks.
Constitutive defenses include plants’ ability to strengthen their cell walls and block plasmodesmata when attacked, limiting the ability of viral pathogens to move from cell to cell.
Induced defenses are triggered by a wide range of molecular elicitors and fall into two main categories: general immunity and specific immunity. Review Focus: Key Figure 38.2, Animation 38.1
There is an ongoing search for genes involved in plant defense. Review Investigating Life: A Gene for Resistance to Wheat Rust
The gene-
for- gene concept depends on a match between a plant’s resistance (R) genes and a pathogen’s Avirulence (Avr) genes. Review Figure 38.3 In the hypersensitive response to infection by bacteria or fungi, cells produce two kinds of defensive molecules: phytoalexins and pathogenesis-
related (PR) proteins. Some cells around the infected area die, sealing off the pathogens and the damage they have caused. The hypersensitive response is often followed by systemic acquired resistance, in which salicylic acid activates further synthesis of defensive compounds.
Plants use RNA interference to develop specific immunity to invading RNA viruses.
38.2 Plants Have Mechanical and Chemical Defenses against Herbivores
Some plants produce secondary metabolites as defenses against herbivores. Review Table 38.1
Hormones, including jasmonates, participate in signal transduction pathways leading to the production of defensive compounds. Review Figure 38.5, Activity 38.1
Plants protect themselves against their own toxic defensive chemicals by sequestering them in vacuoles or storing them as harmless precursors until used.
38.3 Plants Can Adapt to Environmental Stresses
Plants cope with environmental stresses by adaptation (genetically encoded resistance) or acclimation (increased tolerance). Review Table 38.2
Xerophytes are plants that are adapted to dry environments.
Some xerophytic adaptations are structural, including thickened cuticles, specialized trichomes, stomatal crypts, succulence, and long taproots.
Some plants accumulate solutes, making their water potential lower so they can tolerate drought.
Adaptations to water-
saturated habitats include pneumatophores, extensions of roots that allow oxygen uptake from the air, and aerenchyma, tissue in which oxygen can be stored and used to supply the rest of the plant. A signaling pathway involving abscisic acid initiates a plant’s response to drought stress. Review Figure 38.12
Membranes and proteins can be damaged by extremely high or low temperatures. Plants respond to extreme temperatures by producing heat shock proteins.
Some plants undergo cold-
hardening, an acclimation process that includes changes in membrane lipids and production of heat shock proteins. Some plants resist freezing by producing antifreeze proteins.
Most halophytes accumulate salt. Some have salt glands that excrete salt to the leaf surface.
Some plants living in soils that are rich in heavy metals are hyperaccumulators that take up large amounts of those metals into their tissues.
Phytoremediation is the use of hyperaccumulating plants or their genes to clean up environmental pollution.
See Activity 38.2 for a concept review of this chapter.
Go to LearningCurve (in LaunchPad) for dynamic quizzing that helps you solidify your understanding of this chapter. LearningCurve adapts to your responses, giving you the practice you need to master each key concept.