Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2003
Publication Date: 11/17/2003
Citation: OKUBARA, P.A., PAULITZ, T.C. ROOT DEFENSE RESPONSES TO PATHOGENS AND PARASITES: A MOLECULAR PERSPECTIVE. PLANT AND SOIL JOURNAL. 2003. Interpretive Summary: Losses sustained from root diseases of vegetable, fruit, and field crops are estimated at $4 billion each year in the US. This review article provides an overview of root diseases, the life cycle of a typical soilborne pathogen, and the defenses that plants mount against pathogens, parasitic plants, and nematodes. Certain plants have effective resistance to nematodes, parasitic plants, and wilt-causing pathogens. Resistance can be triggered by an exchange of chemical or metabolic signals between such pests and their hosts. On the other hand, plants generally lack effective resistance to pathogens that cause root rot. Many such pathogens produce toxins and enzymes that cause tissue death, leading to root rot. Two plant hormones, jasmonic acid and ethylene, have been shown to provide marginal protection against such pathogens. Naturally-occurring metabolites and morphological traits have also been implicated in defenses against root pathogens. Current knowledge of and application of genomics to root defense mechanisms are discussed.
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. Metabolic and morphological characteristics of roots are also known to govern responses to 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.