Heritability of rhizobacteria-mediated induced systemic resistance and basal resistance in arabidopsis

Selected strains of non-pathogenic rhizobacteria have the ability to trigge r an induced systemic resistance (ISR) response in plants. In Arabidopsis, rhizobacteria-mediated ISR has been extensively studied, using Pseudomonas fluorescens WCS417r as the inducing agent and P. syringae pv. tomato DC3000 (Pst) as the challenging pathogen. To investigate how far expression of IS R depends on the level of basal resistance, 10 different Arabidopsis ecotyp es were screened for their potential to express WCS417r-mediated ISR and ba sal resistance against Pst. Two Arabidopsis ecotypes, RLD and Wassilewskija (Ws), were found to be blocked in their ability to express ISR. This ISR-n oninducible phenotype correlated with a relatively low level of basal resis tance against Pst. Genetic analysis of crosses between the ISR-inducible ec otypes Columbia (Col) and Landsberg erecta (Ler), on the one hand, and the non-inducible ecotypes RLD and Ws, on the other hand, revealed that ISR ind ucibility and basal resistance against Pst were inherited as monogenic domi nant traits that are genetically linked. Neither ISR inducibility, nor basa l resistance against Pst was complemented in the F-1 progeny of a cross bet ween RLD and Ws, indicating that both ecotypes are affected in the same loc us. This locus, designated ISR1, was mapped between markers Ein3 and GL1 on chromosome III. Interestingly, ecotypes RLD and Ws also failed to express ISR against the oomycetous pathogen Peronospora parasitica, but they were n ot affected in their level of basal resistance against this pathogen. Thus, the ISR1 locus controls the expression of ISR against different pathogens but basal resistance only against Pst and not against P. parasitica. Like e cotypes RLD and Ws, ethylene-insensitive mutants showed the isr1 phenotype in that they were unable to express WCS417r-mediated ISR and show enhanced susceptibility to Pst infection. Analysis of ethylene responsiveness of RLD and Ws revealed that both ecotypes exhibit reduced sensitivity to ethylene . Therefore, it is proposed that the Arabidopsis ISR1 locus encodes a compo nent of the ethylene-response pathway that plays an important role in ethyl ene-dependent resistance mechanisms.