Colonization of Arabidopsis roots by Pseudomonas fluorescens primes the plant to produce higher levels of ethylene upon pathogen infection

Plants develop an enhanced defensive capacity against a broad spectrum of plant pathogens after colonization of the roots by selected strains of non-pathogenic, fluorescent Pseudomonas spp. In Arabidopsis thaliana, this rhizobacteria-induced systemic resistance (ISR) functions independently of salicylic acid but requires responsiveness to the plant hormones jasmonic acid and ethylene. Leaves of plants of which the roots are colonized by ISR-inducing Pseudomonas fluorescens WCS417r bacteria show an enhanced capacity to convert the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) to ethylene. Here we show that this enhanced ACC-converting capacity leads to a potentiated expression of the ethylene-responsive genes PDF1·2 and HEL after treatment of the leaves with 1 mM ACC, and a significantly higher level of ethylene emission after challenge inoculation with the bacterial pathogen Pseudomonas syringae pv. tomato DC3000/avrRpt2. P. fluorescens WCS374r bacteria that are unable to induce ISR against P. syringae pv. tomato DC3000 in Arabidopsis likewise enhanced the in vivo ACC oxidase acitivity in Col-0 plants. Moreover, the ISR-compromised mutants jar1-1 and npr1-1 also showed a significant increase in their ability to convert ACC to ethylene after treatment of the roots with P. fluorescens WCS417r. These results suggest that the induction of an enhanced ACC-converting capacity is a general response of plants to P. fluorescens bacteria and that this response does not contribute to ISR against P. syringae pv. tomato DC3000 in Arabidopsis. Nevertheless, P. fluorescens strains clearly prime the plant to produce more ethylene upon pathogen infection. The increased capacity for ethylene production might contribute to an enhanced defensive capacity against pathogens that are sensitive to ethylene-dependent defense responses.