TYPE-IV PILI ARE INVOLVED IN PLANT-MICROBE AND FUNGUS-MICROBE INTERACTIONS

Adherence of bacteria to eukaryotic cells is essential for the initiat ion of infection in many animal and human pathogens, e.g. Neisseria go norrhoeae and Pseudomonas aeruginosa, Adhesion-mediating type IV pill, filamentous surface appendages formed by pilin subunits, are crucial virulence factors. Here, we report that type IV pilus-dependent adhesi on is also involved in plant-bacteria and fungus-bacteria interactions , Nitrogen-fixing, endophytic bacteria, Azoarcus sp,, can infect the r oots of rice and spread systemically into the shoot without causing sy mptoms of plant disease. Formation of pill on solid media was dependen t on the pilAB locus. PilA encodes an unusually short (6.4 kDa) putati ve pilin precursor showing 100% homology to the conserved N-terminus o f the Pseudomonas aeruginosa type IV pilin. PilB encodes for a 14.2 kD a polypeptide showing similarity to FimF, a component of type I fimbri ae of Escherichia coli, It was found to be extruded beyond the cell su rface by immunofluorescence studies, and it may, therefore, be part of a pilus assembly complex or the pilus itself, Both genes are involved in the establishment of bacteria on the root surface of rice seedling s, as detected by fluorescence microscopy. Moreover, both genes are ne cessary for bacterial adhesion to the mycelium of an ascomycete, which was isolated from the same rhizosphere as the bacteria. In co-culture with the fungus, Azoarcus sp. forms complex intracytoplasmic membrane s, diazosomes, which are related to efficient nitrogen fixation, Adhes ion to the mycelium appears to be crucial for this process, as diazoso mes were absent and nitrogen fixation rates were decreased in pilAB mu tants in co-culture.