Fungitoxic compounds

Biotic Agents. It is well-established that treatment of plants with virulent or avirulent forms of a pathogen, or with a non-pathogen, may induce the formation of fungitoxic compounds (phytoalexins) which prevent or retard subsequent infection by a pathogen. The phytoalexins are formed at the site of inoculation and are not transported to other plant parts. 

Since in organic compounds the presence of such groups is of outstanding significance for their physiological properties, it was decided to look for ways to synthesize functionally-substituted organo-tin compounds and to study their fungitoxicity. 

Carboxln. Carboxin is used primarily to control Basidiomycetes such as rusts, bunts, and smuts. It is used primarily for seed treatment, but also has soil and foliar applications. The mode of action of this and related compounds is probably the best understood of any fungitoxic mechanism. Carboxin blocks the transfer of electrons from... 

The compound for which the best biochemical evidence has been reported for sensitization of host plant response to pathogens, rather than direct or indirect fungitoxicity or nonspecific phytoalexin induction, is 2,2-d ich 1 oro-3,3-d ime thy lcyc 1 opropane carboxylic acid (108-110). Neither the compound nor any of its metabolites generated after treatment of rice plants are directly inhibitory to the rice blast fungus, Piricularia oryzae. Constitutive phytoalexin production was not induced in rice by the cyclopropane derivative. However, infection of plants treated with the compound results in rapid localized cell death, me lanization, and production of the phytoalexins, momilactones A and B. 

Aliette (aluminum tris-0 ethylphosphonate) has been reported to enhance defense reactions and phytoalexin accumulation in grapes and tomatoes in response to infection by Plasmopara viticola and Phytophthora spp., respectively, and to trigger phenolic accumulation and hypersensitive cell death in tomatoes, peppers, and beans in response to infection while possessing little direct fungitoxicity (116,117). However, recent data cast doubt on the earlier reports of the low activity of Aliette as an inhibitor of Phytophthora sporulation mi vitro (118), and have attributed the protective properties of the compound to phosphorous acid which is formed in plant tissues or in certain buffer solutions of Aliette (119,120). Toxicity of phosphorous acid to Oomycetes is reversible by phosphate ion, and this may explain Aliette s lack of fungitoxicity in certain growth media. 

Metalaxyl is clearly fungicidal i n vitro to a variety of phycomycetous fungi. Its activity in vivo may also be enhanced by stimulation of host plant defenses including hypersensitive cell death (104), accumulation of phytoalexins such as glyceollin (121), and cal lose encasement of hyphae (122) after fungal infection of metalaxy1-treated soybeans. However, the mi vivo concentration of the compound in cell sap may be sufficient for fungitoxicity alone to account for its protective activity (123). 

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