Induced Resistance

Induced-systemic resistance has been observed on cucumber plants treated with PGPF from zoysiagrass (Hyakumachi 1997; Koike et al. 2001; Meera et al. 1994). Almost all of these PGPF could induce resistance against anthracnose in cucumber. In contrast, Ishiba et al. (1981) reported that only 1.9-2.4% of the soil fungi isolated from cucumber rhizosphere, were able to induce systemic resistance against anthracnose in cucumber plants. Different types of PGPF have been isolated from all over the world and it would be interesting to know if any of these have as high performance of induced systemic resistance as PGPF isolated from zoysiagrass. Recently, induced systemic resistance caused by binucleate Rhizoctonia and Trichoderma has been reported in plants. Some isolates within these fungi have growth-promoting ability and others have been used as biological control agents. The induced resistance on cucumber caused by PGPF isolated from zoysiagrass can last as long as 9 weeks under glass house conditions and for up to 6 weeks under field conditions (Meera et al. 1995). Lignin deposition is known as one of the mechanisms of induced systemic resistance (Hammaerschimidt and Kuc 1982). Koike et al. (2001) reported that lignification of cucumber seedling hypocotyls was induced by culture filtrates of PGPF, following challenge inoculation with C. orbiculare. The result showed enhanced lignin deposition in cucumber after infection by C. orbiculare as compared to the control. The elicitor activity of culture filtrates of PGPF has been evaluated by chemiluminescence to determine the emission of active oxygen species from tobacco callus and cucumber fruit disks (Koike et al. 2001). The oxidative burst is characterized by a rapid and transient generation of active oxygen species immediately following fungal elicitor treatment. From these results, the > 12,000 MW fraction and both > 12,000 MW fraction and lipid fraction from the culture filtrate elicited the highest superoxide generation, respectively. A high correlation between superoxide generation ability and lignification ability was reported.

Localized and induced-systemic resistance against Phyto-phthora parasitica caused by the AM fungi, Glomus mosseae, has been observed in tomato roots (Cordier et al. 1998). The phenomena were demonstrated by use of a split-root experimental system. Decreased pathogen development in mycorrhizal and nonmycorrhizal parts of the root system was associated with an accumulation of phenolics and plant cell defense responses. G. mosseae-containing cortical cells in the mycorrhizal tissues were immune to the pathogen infection and exhibited a localized resistance response with the formation of cell wall appositions reinforced by callose adjacent to intercellular hyphae. The systemically induced resistance in nonmycorrhizal root parts was characterized by elicitation of host wall thickenings containing nonesterified pectins and PR-1a protein in reaction to the intercellular hyphae of the pathogen. Systemic resistance was also characterized by the formation of callose-rich encasement material around P. parasitica hyphae that were penetrating root cells and PR-1a protein was detected in the pathogen wall only in these tissues. None of these cell reactions were observed in nonmycorrhizal pathogen-infected root systems, where disease development resulted in host cell death. Increased chitinase activities have also been reported in AM symbiosis as part of the induced defense reaction by these mycorrhizal fungi. Pozo et al. (1999) studied b-1,3-glucanase in tomato roots which were either colonized by AM fungi and/or infected by the pathogen Phytophthora parasitica. b-1,3-glucanase activity was higher in mycorrhizal roots compared to the non-mycorrhizal roots. Nonmycorrhizal roots infected by P. parasitica showed high levels of activity but the pathogen did not induce b-1,3-glucanases in AM colonized roots. There was strong evidence to suggest that these hydrolases are antifungal proteins. Increased chitinase activities have also been reported in ectomycorrhizal symbiosis (Albrecht et al. 1994; Sauter and Hager 1989).

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