Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

• 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.
   
• Some of the responses by which plants fight off pathogens are constitutive — always present in the plant — whereas others are induced — produced in reaction to the presence of a pathogen. Review Figure 28.1 and ANIMATED TUTORIAL 28.1
   
• Plants use physical barriers to block pathogen entry and seal off infected regions.
   
• Gene-for-gene resistance depends on a match between a plant's resistance (R) genes and a pathogen's avirulence (Avr) genes. Review Figure 28.2
   
• In the hypersensitive response to infection, cells produce phytoalexins and pathogenesis-related (PR) proteins, and the plant isolates the area of infection by forming necrotic lesions.
   
• The hypersensitive response may be followed by another defensive reaction, systemic acquired resistance, in which salicylic acid activates further synthesis of defensive compounds throughout the plant.
   

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

• Physical structures such as spines and thick cell walls deter some herbivores.
   
• Plants produce secondary metabolites as defenses against herbivores. Review Table 28.1, Figure 28.5, and WORKING WITH DATA 28.1
   
• Hormones, including jasmonate, participate in signaling pathways leading to the production of defensive compounds. Review Figure 28.6
   
• Plants protect themselves against their own toxic defensive chemicals by compartmentalizing those chemicals, by storing their precursors separately, or through modifications of their own proteins.
   

Concept 28.3 Plants Adapt to Environmental Stresses

• Xerophytes are plants adapted to dry environments. Their structural adaptations include thickened cuticles, specialized trichomes, stomatal crypts, succulence, and long taproots.
   
• Some plants accumulate solutes in their cells, which lowers their water potential so they can more easily take up water.
   
• 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 can diffuse throughout the plant. Review Figure 28.11
   
• A signaling pathway involving abscisic acid initiates a plant's response to drought stress. Review Figure 28.12
   
• Plants respond to high temperatures by producing heat shock proteins. Low temperatures can result in cold-hardening.
   
• Plants that are adapted for survival in saline soils are called halophytes. 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 and store large amounts of those metals into their tissues.
   
• Phytoremediation is the use of hyperaccumulating plants or their genes to clean up environmental pollution in soils.
   

See WEB ACTIVITY 28.1 for a concept review of this chapter.