Novel insights into the infection mechanism of oomycete Pythium spp. in the host Arabidopsis thaliana

Abstract

Phytopathogenic Pythium spp. cause seedling damping-off of a wide range of plant species worldwide and are traditionally considered necrotrophs. In this study, novel pathogenicity was discovered involving the oomycete Pythium cryptoirregulare and the model plant Arabidopsis thaliana. This pathogen was isolated from A. thaliana seedlings that were showing damping-off symptoms and was later identified as Pythium cryptoirregulare based on morphological and molecular characterization alongside reference species P. irregulare and P. ultimum var. ultimum. To examine its infection strategy, A. thaliana was inoculated with P. cryptoirregulare and studied using a microscopy approach. Viable colonized cells were observed based on neutral red uptake and the ability to undergo cell plasmolysis after infection. This biotrophic interaction contradicts the previously reported necrotrophic lifestyle of Pythium spp., which is characterized by killing the host cells prior to colonization. In addition, inhibition of root growth was detected prior to colonization by P. cryptoirregulare, suggesting that P. cryptoirregulare secreted growth inhibitors. Potentially, these inhibiting metabolites facilitate infection by delaying plant development and, thereby, extending the seedling stage that is targeted by this pathogen. Notably, P. cryptoirregulare culture filtrates disturbed transport and distribution of auxins, indicated by altered GFP expression in the A. thaliana lines PIN1-GFP, PIN2-GFP, PIN3-GFP, PIN7-GFP and DR5::GFP which visualize the auxin efflux. This disturbance was further confirmed by a reduced inhibitory effect on the auxin-insensitive A. thaliana mutants axr1-3, axr4-2, and aux1-7. Metabolic activity assay results suggested that P. cryptoirregulare secretes auxin-related metabolites that are involved in reprogramming plant growth. Overall, the characterization of P. cryptoirregulare as a novel pathogen on A. thaliana gives new insights into understanding the pathogenic mechanisms and interactions between oomycetes and plants.