Differential inducible defense mechanisms against bacterial speck pathogen in Arabidopsis thaliana by plant-growth-promoting-fungus Penicillium sp. GP16-2 and its cell free filtrate

Although a wealth of information is available regarding resistance induced by plant growth-promoting rhizobacteria (PGPR), not much is known about plant growth-promoting fungi (PGPF). Hence, the goal of the present research was to provide more information on this matter. In Arabidopsis thaliana L., root colonizing PGPF Penicillium sp. GP16-2 or its cell free filtrate (CF) elicited an induced systemic resistance (ISR) against infection by Pseudomonas syringae pv. tomato DC3000 (Pst), leading to a restriction of pathogen growth and disease development. We demonstrate that signal transduction leading to GP16-2-mediated ISR requires responsiveness to JA and ET in a NPR1-dependent manner, while CF-mediated ISR shows dispensability of SA, JA, ET and NPR1-dependent signaling (at least individually). In addition, root colonization by GP16-2 is not associated with a direct effect on expression of known defense-related genes, but potentiates the activation of JA/ET-inducible ChitB, which only becomes apparent after infection by Pst. However, CF-mediated ISR was partly associated with the direct activation of marker genes responsive to both SA and JA/ET signaling pathways and partly associated with priming, leading to activation of JA-/ET-inducible ChitB and Hel genes. These suggest that CF may contain one or more elicitors that induce resistance by way where at least SA, JA and ET may play a role in defense signaling in Arabidopsis. Therefore, defense gene changes and underlying signaling pathways induced by Penicillium sp. GP16-2 root colonization and its CF application are not the same and only partially overlap.