picA, a novel plant-inducible locus on the Agrobacterium tumefaciens chromosome

We used the transposon Mu dI1681 to identify genes on the Agrobacterium tumefaciens chromosome that are inducible by extracts from carrot roots. One such locus (picA, for plant inducible chromosomal), harbored by A. tumefaciens At156, was inducible 10- to 50-fold by these extracts. Mutation of picA had no detectable effect upon bacterial growth or virulence under laboratory assay conditions. However, A. tumefaciens cells harboring a mutated picA locus aggregated into long "ropes" when incubated with pea root tip cells. Such aggregation was not displayed by the parental strain A. tumefaciens A136. A preliminary characterization of the inducing compound in the carrot root extract suggests that the active substance is an acidic polysaccharide that is most likely derived from the pectic portion of the plant cell wall.