Interactions between Rhizobium, antagonistic bacteria and fungal pathogens in faba bean

Abstract: Plant associated micro-organisms such as nitrogen-fixing Rhizobium, and plant growth- promoting rhizobacteria (PGPR) Pseudomonas spp. and Serratia spp. are well recognised for their vital role in soil fertility and plant health. Most cultivated soils contain large populations of such micro-organisms. A diversity of interactions occur between rhizobia and PGPR with leguminous plants in natural conditions, however, their associations and functions in field crops such as faba bean is less studied. In Sweden, faba bean cultivation is increasing because of increasing demand for self-sufficiency of native protein feed. Two of the most serious diseases to limit the yield of faba beans are chocolate spot disease and Ascochyta blight. This thesis contains several studies. One study was done on the microbial communities naturally associated with faba bean seeds of two different cultivars. In another study, the role of selected bacteria for health of faba bean was examined. Furthermore, the antagonistic ability of Rhizobium and PGPR towards faba bean pathogens, and the compatibility and root colonization competence of the two beneficial bacteria was investigated. Cultivation-dependent and cultivation-independent approaches used to study the seed microbial community structure revealed high diversity, which seemed to be affected by the health status of the seeds. Enterobacteriaceae dominated the bacterial flora and Serratia, Pseudomonas and Burkholderia were among the numerous genera that inhabited the seeds. Higher bacterial growth in cv. Aurora seed exudates compared to cv. Fuego suggested differences in the chemical composition between the two varieties. Both rhizobial and PGPR isolates were shown to be antagonistic but PGPR proved to be stronger antagonists than rhizobia. Serratia proteamaculans S4 was strongest antagonist and produced both diffusible and volatilic antifungal metabolites. In greenhouse, rhizobia and PGPR colonised roots of both cultivars after single or co- inoculation but the level of colonisation differed depending on the cultivar and the isolate combination. Colonisation by Rhizobium leguminosarum was stimulated by fluorescent Pseudomonas sp. Positive effects of seed bacterisation with Rhizobium in greenhouse were evident in terms of emergence and plant growth in both Aurora and Fuego. The results presented here highlight the importance of multifarious interactions between bacteria, plant and pathogens for sustainability in crop production of faba bean.