Evidence that associated soil bacteria may influence root hair infection of actinorhizal plants by Frankia.

Nodulation of actinorhizal plants (Alnus rubra Bong, and others) by isolated Frankia strains occurred either at a low frequency or not at all under axenic conditions. But nodulation was achieved under nonsterile conditions and four strains of bacteria were isolated which promoted nodulation when plants were inoculated with the bacteria plus Frankia. Four strains of Pseudomonas cepacia also promoted nodulation. Root hair deformation occurred when roots of A. rubra were inoculated with these bacterial isolates, or with the bacteria plus Frankia, but rarely or not at all when roots were inoculated with the actinomycete alone. The nonendophytic bacterial strains were not detected within the nodule tissue. It is proposed that the bacterial isolates aid in the infection process at the host root hair surface, by causing root hair deformation; this may allow intimate contact between the Frankia filament and the hair wall.