ROLE FOR CYCLIC ▀-GLUCANS FROM BRADYRHIZOBIUM JAPONICUM IN SUPPRESSION OF HOST DEFENCE IN SOYABEAN
A MITHÍFER1, AA BHAGWAT2 and J EBEL1
1Botanisches Institut der Universitńt, Menzinger Str. 67, D-80638 Muenchen, Germany; 2USDA, Agricultural Research Service, BARC-W, Beltsville, MD 20705, USA and Department of Natural Resource Science and Landscape Architecture, University of Maryland, College Park, MD 20742, USA
The soyabean (Glycine max) microsymbiont Bradyrhizobium japonicum synthesizes cyclic ▀-(1-3),(1-6)-glucans which seem to function as osmoprotectants. The bacteria synthesize these ▀-glucans throughout nodule development. Although bradyrhizobial ▀-glucans share some structural similarity with one of the best characterized elicitors from the soyabean fungal pathogen Phytophthora sojae (cell wall-derived non-cyclic ▀-(1-3),(1-6)-glucans), they are weak inducers of phytoalexin synthesis in contrast to the fungal ▀-glucans . Mutations within the cyclic glucan synthesis locus resulted in two different phenotypes, AB-14 and AB-1, respectively. The AB-14 strain showed a total absence of cyclic glucans while the AB-1 strain synthesized cyclic ▀-glucan with predominantly ▀-(1-3)-glycosyl linkages. Strain AB-14 formed ineffective but differentiated nodules on soyabean roots, whereas strain AB-1 displayed delay in nodulation and formed very small nodule-like structures without viable bacteria [2, 3]. These results suggest that the structure of the cyclic ▀-glucans is important for a successful symbiotic interaction, and these carbohydrates may have a specific function in addition to their role in osmotic adaptation.
Results and conclusions
The bradyrhizobial cyclic ▀-(1-3),(1-6)-glucans were only weak elicitors of plant defence responses such as the synthesis and accumulation of phytoalexins. When tested in combination with the fungal ▀-glucans, these bacterial ▀-glucans even suppressed elicited stimulation of phytoalexin synthesis in bioassays. This effect correlated well with binding activity of the cyclic ▀-glucans to the ▀-glucan binding protein in soyabean that is responsible for pathogen recognition and the onset of defence responses upon pathogen attack or elicitor treatment. For the AB-1 cyclic ▀-glucans, binding activity was much weaker compared to the wild-type cyclic ▀-glucans, and no suppression of fungal ▀-glucan-induced defence response was detectable. Very sensitive measurements of phytoalexins in infected nodules showed higher levels of these antimicrobial compounds in the nodule tissue in AB-14- and AB-1-induced nodules than in wild-type nodules or uninfected roots. This indicates again that the presence of bacterial cyclic ▀-glucans during nodulation is necessary for the development of effective and differentiated nodules, very probably due to its role as a suppressor of plants' defence responses.
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 369), by award 96 35305 3731 USDA NRI Competitive Research Grants Program, and the binational NSF-DAAD scientist exchange program.
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3. Bhagwat AA, Gross KC, Tully RE, Keister DL, 1996. Journal of Bacteriology 178, 4635-4642.