ABSTRACT
BACKGROUND: Necrotizing pathogens pose an immense economic and ecological threat to trees and forests, but the molecular analysis of these pathogens is still in its infancy because of lacking model systems. To close this gap, we developed a reliable bioassay for the widespread necrotic pathogen Botrytis cinerea on poplars (Populus sp.), which are established model organisms to study tree molecular biology. RESULTS: Botrytis cinerea was isolated from Populus x canescens leaves. We developed an infection system using fungal agar plugs, which are easy to handle. The method does not require costly machinery and results in very high infection success and significant fungal proliferation within four days. We successfully tested the fungal plug infection on 18 poplar species from five different sections. Emerging necroses were phenotypically and anatomically examined in Populus x canescens leaves. We adapted methods for image analyses of necrotic areas. We calibrated B. cinerea DNA against Ct-values obtained by quantitative real-time polymerase chain reaction and measured the amounts of fungal DNA in infected leaves. Increases in necrotic area and fungal DNA were strictly correlated within the first four days after inoculation. Methyl jasmonate pretreatment of poplar leaves decreased the spreading of the infection. CONCLUSIONS: We provide a simple and rapid protocol to study the effects of a necrotizing pathogen on poplar leaves. The bioassay and fungal DNA quantification for Botrytis cinerea set the stage for in-depth molecular studies of immunity and resistance to a generalist necrotic pathogen in trees.
ABSTRACT
Ectomycorrhizal fungi (EMF) grow as saprotrophs in soil and interact with plants, forming mutualistic associations with roots of many economically and ecologically important forest tree genera. EMF ensheath the root tips and produce an extensive extramatrical mycelium for nutrient uptake from the soil. In contrast to other mycorrhizal fungal symbioses, EMF do not invade plant cells but form an interface for nutrient exchange adjacent to the cortex cells. The interaction of roots and EMF affects host stress resistance but uncovering the underlying molecular mechanisms is an emerging topic. Here, we focused on local and systemic effects of EMF modulating defenses against insects or pathogens in aboveground tissues in comparison with arbuscular mycorrhizal induced systemic resistance. Molecular studies indicate a role of chitin in defense activation by EMF in local tissues and an immune response that is induced by yet unknown signals in aboveground tissues. Volatile organic compounds may be involved in long-distance communication between below- and aboveground tissues, in addition to metabolite signals in the xylem or phloem. In leaves of EMF-colonized plants, jasmonate signaling is involved in transcriptional re-wiring, leading to metabolic shifts in the secondary and nitrogen-based defense metabolism but cross talk with salicylate-related signaling is likely. Ectomycorrhizal-induced plant immunity shares commonalities with systemic acquired resistance and induced systemic resistance. We highlight novel developments and provide a guide to future research directions in EMF-induced resistance.
