Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2005
Publication Date: 9/20/2005
Citation: Okubara, P.A., Paulitz, T.C. 2005. Molecular bases of root defense responses.. Plant and Soil Journal. 274:221,232.
Interpretive Summary: Some plant pathogens infect their hosts without killing the plant cells initially- i.e. require a living cell to gain nutrients. These are deemed “biotrophic” pathogens. Others kill the plant tissue and derive nutrition from dead or necrotic tissue. These are called “necrotrophic” pathogens. Hypersensitive response and programmed cell death are two mechanisms that have evolved to stop the progress of biotrophic pathogens, but conversely, may be used by necrotrophic pathogens to increase infection and colonization. To defend themselves against microbial pathogens, plants have evolved elaborate signaling pathways to turn on these defenses, and the two classes of pathogens seemed to have evolved different pathways. There is strong evidence for the involvement of salicylic acid in the biotrophic pathway, and the involvement of jasmonic acid and ethylene in the nectrotrophic pathway.
Technical Abstract: This review will focus on the molecular and genetic mechanisms underlying defense responses of roots to fungal pathogens. Soil-borne pathogens, including Phytophthora, Pythium, Fusarium, and Bipolaris, represent major sources of biotic stress in the rhizosphere and roots of plants. Molecular recognition and signaling leading to effective resistance has been demonstrated to occur between host and Phytophthora, or Pythium. The hypersensitive response and apoptotic cell death, two oxidative processes that limit biotrophic pathogens, generally act to exacerbate disease symptoms induced by necrotrophic organisms. Although pathogenesis-related proteins can be expressed in roots during pathogen challenge, salicylic acid has not been implicated in root-mediated interactions. Jasmonic acid and ethylene have been found to mediate parallel as well as synergistic pathways that confer partial tolerance to necrotrophic pathogens, as well as induced systemic resistance to root and foliar pathogens. Genomics approaches are revealing new networks of defense-signaling pathways, and have the potential of elucidating those pathways that are important in root-defense responses.