The antibiosis of nodule-endophytic agrobacteria and its potential effect on nodule functioning of Phaseolus vulgaris

The effect of the nodule-endophytic Agrobacterium strain 10C2 on nodulation, plant growth and nodule functioning of Phaseolus vulgaris was investigated using two rhizobial strains differing in their sensitivity to the in vitro antibiosis of the Agrobacterium strain. In the case of the sensitive strain, Agrobacterium sp. 10C2 induced a significant decrease in the proportion of pink nodules, probably by an antibiosis effect leading to the reduction in the number of bacteroids and thereby a decrease in total soluble proteins, leghaemoglobin content, photosynthesis and nitrogen fixation. In this case, the Agrobacterium strain behaved like a plant pathogen and the nodule reacted by increasing guaiacol peroxidase (POX) activity, which assures some physiological processes linked to pathogen control. By contrast, in the case of the resistant strain, the proportion of pink nodules increased, and thereby total soluble proteins, leghaemoglobin content, biomass production and nitrogen fixation were enhanced. The Agrobacterium strain is regarded in this case as a plant growth-promoting rhizobacterium and the POX-pathogen reaction was not observed. There was even a decrease in superoxide dismutase activity. The results suggested also that the Agrobacterium strain may be also involved in retarding nodule senescence in the case of the resistant strain.