Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2007
Publication Date: 11/1/2007
Citation: Macarisin, D., Cohen, L., Eick, A., Rafael, G., Belausov, E., Wisniewski, M.E., Droby, S. 2007. Penicillium digitatum suppresses production of hydrogen peroxide in host tissue during infection of citrus fruit. Phytopathology. 97(11):1491-1500. Interpretive Summary: Some fruits are highly susceptible to postharvest rots and appear to have very little resistance to specific fungal pathogens. The application of chemical fungicides are required to prevent the economic losses that would occur as a result of these postharvest infections, however, consumers prefer a reduction in the use of chemical pesticides. In order to develop new approaches to disease control, a better understanding of the interactions between the host fruit and the fungal pathogen are required. The present study examined the biochemical response of citrus fruit to disease causing and non-causing pathogens. An important response of plants to a pathogen is the production of hydrogen peroxide which directly impacts the invading fungus and initiates other mechanisms of host resistance. In our study, we found that the fungus, Penicillium digitatum, that causes green mold in citrus was able to suppress the production of hydrogen peroxide which is referred to as an oxidative burst and block other responses. In contrast, the non-pathogen, Penicillium expansum, induced a high level of hydrogen peroxide in citrus fruit and was not able to penetrate the host any further than the point of inoculation. The production of the anti-oxidant enzyme, catalase, by the fungus appears to play an important role in suppressing the oxidative burst in citrus. The role of hydrogen peroxide in resistance to postharvest pathogens will be explored in other fruit/pathogen systems. If results are consistent, we will try to develop non-chemical approaches to boosting the production of hydrogen peroxide in harvested commodities in order to increase resistance to postharvest rots.
Technical Abstract: During the development of green mold disease (Penicillium digitatum) on citrus fruit, there is little evidence of a host resistance response against the invading fungus. This suggests that P. digitatum has the ability to suppress host defenses. Current knowledge of plant-fungal interactions indicates that a crucial role in host resistance can be attributed to an oxidative burst (rapid and massive generation of reactive oxygen species) within host cells responsible for further activation of signaling cascades leading to a hypersensitive response. The current study demonstrates that during infection, P. digitatum suppresses the defense-related oxidative burst in host cells. In contrast, the non-compatible pathogen, P. expansum, triggers production of a significant amount of H2O2 in citrus fruit exocarp. Using laser scanning confocal microscopy, we demonstrated in vivo that P. digitatum did not induce a substantial elevation in H2O2 within 40-42 h after inoculation, while the level of H2O2 around the wounds inoculated with P. expansum exceeded by 63-fold the H2O2 level present in control. At 60-66 h after inoculation, P. digitatum continued to suppress H2O2 production in citrus fruit exocarp, and in fact, H2O2 levels were approximately three fold below that of non-inoculated controls. In contrast, the H2O2 level was still about 11-fold above the control value in wound sites inoculated with P. expansum. Studies on the effect of organic acids (as pH modulators) on the response of citrus fruit to compatible and non-compatible pathogens indicated that pathogenicity was enhanced only when host-tissue acidification was accompanied by the suppression of H2O2. Additionally, pathogenicity of both P. digitatum and P. expansum on citrus fruit was significantly enhanced by the H2O2 -scavenging enzyme catalase (CAT). Based on our study and previous reports regarding the potential involvement of citric acid and catalase in green mold pathogenesis, we suggest that these compounds are virulence factors for P. digitatum responsible for suppression of the defense-related oxidative burst in citrus fruit.