Living plant cells released from the root cap: A regulator of microbial populations in the rhizosphere?

A group of cells at the periphery of the root cap separate from plant roots as they move through the soil. Experimentally, these cells can be separated from the root by gentle agitation in water. Detached root cap cells provide a convenient system for comparing cellular and whole-plant responses to fungal and bacterial pathogens. Results with several hos-parasite combinations indicate that these cells express host-specific traits with respect to chemotaxis, binding, and infection. Mutants of Agrobacterium tumefaciens were used to test the hypothesis that recognition of the cells plays a role in establishment of microbial-plant associations. Tn 5 mutants deficient in chemotaxis to the cells exhibit reduced ability to colonize the rhizosphere and to induce crown gall tumorigenesis on pea plants grown in soil. The discovery that cells shed from the cap express hos-specific genes suggests that some microorganisms may have greater access to constituents of the cells than others. This cellular selectivity could have a significant impact on rhizosphere populations.