CORONATINE INSENSITIVE 1-mediated repression of immunity-related genes in Arabidopsis roots is overcome upon infection with Verticillium longisporum

Verticillium longisporum is a soil-borne fungal pathogen causing vascular disease predominantly in Brassicaceae. We have reported previously that the receptor of the plant defense hormone jasmonoyl-isoleucine (JA-Ile), CORONATINE INSENSITIVE 1 (COI1), is required in roots for efficient proliferation of the fungus in the shoot implicating a mobile root-borne signal that influences the outcome of the disease in shoots. To explore the underlying mechanism we compared the root transcriptome of coi1 with the transcriptomes of three susceptible genotypes (wild-type, mutants deficient in JA-Ile and salicylic acid (SA) synthesis). At 10 days after infection, genes related to either xylem formation or plant immunity were induced independently of JA-Ile and SA. The biggest difference between the transcriptomes was due to 316 immunity-related genes that were pre-induced in coi1. Interfering with the expression of a subgroup of these genes partially suppressed the coi1 phenotype. We therefore hypothesize that mobile defense compounds secreted into the xylem and being transported with the transpiration stream confer tolerance to the shoot. We furthermore report that 149 of the COI1-repressed genes are induced in WT upon infection reaching similar levels as in mock-treated coi1. The majority of these were not further induced in coi1, indicating that COI1 is required for infection-induced expression.