Agrobacterium tumefaciens Mutants Deficient in Chemotaxis to Root Exudates. Martha C. Hawes. Departments of Plant Pathology, Molecular and Cellular Biology, University of Arizona, Tucson 85721. Laura Y. Smith(1), and Alan J. Howarth(1). Departments of (1) Plant Pathology and (2) Molecular and Cellular Biology, University of Arizona, Tucson 85721.. MPMI 1:182-186. Accepted 22 June 1988. Copyright 1988 The American Phytopathological Society.

A directional swarm assay on semisolid agar was used to measure chemotaxis in Agrobacterium tumefaciens. Several A. tumefaciens strains, including two lacking Ti plasmids, were attracted to root exudates from isolated root cap cells or from excised root tips of pea. Only 300 sloughed pea cells were required to elicit chemotaxis, and the relative distance moved by the bacteria increased with increasing numbers of plant cells up to approximately 4,000 cells. Tn5 mutagenesis was used to generate strains that did not respond chemotactically to root exudates. The mutants included several categories: nonmotile mutants, slow-migrating mutants, and mutants with normal motility that were not attracted to root exudates. In addition, one Tn5 mutant was attracted to excised root tips but not to isolated root cap cells from pea. A quantitative root cap cell binding assay was used to demonstrate that all of the isolated mutants exhibit wild-type binding levels.