Induction of systemic resistance by Pseudomonas fluorescens Pf1 against Xanthomonas oryzae pv. oryzae in rice leaves

When lower leaves of rice plants were inoculated with powder formulation of a saprophytic strain of Pseudomonas flurorescens, Pf1, upper leaves, in addition to the inoculated lower leaves, showed resistance to the rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae. When the leaves were challenge-inoculated with X. oryzae pv. oryzae 4 days after P. fluorescens application on lower leaves, the disease intensity in upper leaves decreased from 6.7 to 1.1. When rice seeds were treated with the formulation of P. fluorescens Pf1 and sown, 30-day-old seedlings showed resistance to X. oryzae pv. oryzae and the disease intensity decreased from 6.8 to 1.2. The induced resistance was transient; leaves sprayed with P. fluorescens Pf1 at 30 days after treatment and leaves of 60-day-old seedlings from P. fluorescens-treated seeds did not show resistance to the pathogen. In field trials, seed treatment followed by foliar application of the powder formulation of P. fluorescens Pf1 effectively controlled lice bacterial blight and increased the yield. in the induced resistant leaves a sharp increase in lignification and activities of peroxidase, phenylalanine ammonialyase and 4-coumarate: CoA ligase was observed when the leaves were challenge-inoculated with X. oryzae pv. oryzae. An approximately threefold increase in lignin content, peroxidase activity and phenylalanine ammonia-lyase activity and a fivefold increase in 4-coumarate: CoA ligase activity were observed 5 days after challenge inoculation with X. oryzae pv. oryzae in rice leaves pretreated with P. fluorescens for 5 days. A similar increase in defense-related activities was not observed in susceptible interactions or in P. fluorescens-treated plants at later stages of interactions when no resistance to the pathogen was observed.