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Chapter 19
Secondary Metabolites with Diverse Activities toward Phytopathogenic Zoospores of Aphanomyces cochlioides in Host and Nonhost Plants 1
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Satoshi Tahara and Md. Tofazzal Islam 1
Laboratory of Ecological Chemistry, Graduate School of Agriculture, Hokkaido University, Kita-Ku, Sapporo 060-8589, Japan Permanent address: School of Agriculture and Rural Development, Bangladesh Open University, Gazipur-1705, Bangladesh 2
Our knowledge on molecular basis of life cycle development of oomycete phytopathogens has been compiled in last two decades, which is indicating that the inter-relationships between host plant and phytopathogen are elaborately regulated by chemical signaling substances. In contrast, chemical weapons in nonhost plants which can affect/interfere the life cycle development of oomycete phytopathogens, are supposed to be responsible for their incompatibility to Oomycetes. These facts suggest us some cues for biorational regulation of soilborne oosporogenic and zoosporogenic pathogens which are often difficult to eradicate. The research trend of oomycete pathogens based on ecological chemistry has been briefly reviewed with special reference to Aphanomyces cochlioides.
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© 2005 American Chemical Society
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Introduction Oomycetes (Peronosporomycetes in the new classification) are phylogenetic relatives of brown algae that cause many destructive diseases of plants, as well as several animal and human diseases (I). They are mostly water or soil inhabiting organisms (2). The motile zoospores which are propelled by flagella are an important means of pathogen distribution and often the key infectious stage of the phytopathogenic Oomycetes. Disease caused by these Oomycetes can be multi-cyclic, resulting in severe epidemics that can destroy whole crops within a single season (3). Our knowledge of their biology is limited, since their physiology differs from that of fungi, many fungicides are ineffective against Oomycetes (4, 5). New approaches are needed to identify novel targets and to develop biorational control measures to minimize the economic impact of these phytopathogens. Zoospores have high-affinity receptor based recognition systems for locating hosts by chemotaxis (6, 7, 8). They can accumulate at the potential infection sites of host roots by chemotaxis, after which they undergo a series of morphological changes before penetrating the root tissues (9). Recent reports suggest that all these key pre-infection events of zoosporic pathogens are triggered by hostspecific chemical signals (20,11). A few host-derived zoospore chemical signals have been identified (Table 1). All of these mediate chemotaxis at micromolar to nanomolar levels. In contrast to susceptible plants, non-susceptible plants may possess some chemical means to ward-off zoosporic phyopathogens (7). Surveys of nonsusceptible plants using the Aphanomyces cochlioides zoospore bioassay revealed that some of the non-susceptible plants possess diverse chemical weapons those can directly affect the viability or regulate some steps of the life cycle development of this Oomycetes. However, very restricted chemical factors disturbing the life cycle of such organisms have been known at present (Table 1), even though understanding well about such chemical factors seems to be very significant to establish new techniques for control of those soilborne oomycete phytopathogens still problematic in agriculture. Here we briefly review our research results concerning the activities of chemical signals in host plants regulating the life cycle development of oomycete phytopathogens. In addition, the diverse activities of some nonhost zoospore regulating metabolites (repellents, cytotoxins and inhibitors of zoospore motility) are discussed in relation to the biorational regulation of soilborne oomycete phytopathogens.
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Table I. Diverse Stimuli Triggering Characteristic Behaviors and/or Morphological Changes of Aphanomyces cochlioides or Other Peronosporomycetes (Oomycetes) Zoospores (1) Host-specific signal substances triggering life cycle development a) Aphanomyces raphani 3-indolecarbaldehyde (1)* b) A. euteiches prunetin (5,4'-
