Stimulation of Agrobacterium tumefaciens virulence with indole‐3‐acetic acid

The phytopathogen Agrobacterium tumefaciens incites the production of crown‐gall on a wide range of dicotyledonous plants. Gall formation is dependent upon indole‐3‐acetic acid (IAA) and cytokinin production by the transformed plant cells. Upon incubation of Agrobacterium tumefaciens C58 with the plant hormone indole‐3‐acetic acid (IAA), bacterial virulence on cucumber plants was stimulated up to tenfold. Stimulation was maximized after exposure of bacteria to 50 or 100 μg ml^‐1 IAA for 3 h. This was shown to be at the early log phase of bacterial growth. The authors suggest that the excretion of IAA by the transformed plant cells stimulates bacterial virulence mechanism(s) encoded by the Ti plasmid, the chromosome, or both.