Elicitation of systemic resistance against the bacterial speck pathogen in Arabidopsis thaliana by culture filtrates of plant growth-promoting fungi

Abstract

The potential for mediation of induced systemic resistance (ISR) and the mechanisms used by nonpathogenic beneficial microbes to mediate ISR are currently under intense investigation but have been overlooked in studies examining plant growth promoting fungi (PGPF) or their metabolites. Here, we examined whether culture filtrates (CFs) of three well-characterized PGPF isolates, which demonstrated different responses in hypersensitive reaction-like cell death and β-glucuronidase reporter gene expression of PR-1a and PDF 1.2 constructs, had the ability to induce systemic resistance against the bacterial leaf speck pathogen Pseudomonas syringae pv. tomato DC3000 (Pst) in Arabidopsis thaliana. Arabidopsis plants treated with CFs of Phoma sp. (GS6-2 and GS7-3) and sterile fungus (GU23-3) elicited ISR against the Pst pathogen, resulting in a reduction in the number of symptomatic leaves, restriction of disease severity, and suppression of pathogen proliferation, with the best results obtained with Phoma sp. To determine the mechanism used to induce systemic resistance, Arabidopsis genotypes expressing the bacterial salicylate hydroxylase gene or containing disruptions in the nonexpressor of pathogenesis-related genes 1 (NPR 1), jasmonic acid JA, and ethylene (ET) signaling were screened. We demonstrate that signal transduction leading to GS6-2-mediated ISR requires salicylic acid (SA) accumulation, whereas CF of GU23-3-mediated ISR stimulates a pathway dependent on JA signaling. However, GS7-3-mediated ISR does not require SA, JA, ET, or NPRl signaling. Our results suggest that ISR mediated by three PGPF isolates studied were divergent in the biochemical pathways affected.