Improved resistance to pathogens through the induced release of damage-associated molecular patterns

Oligogalacturonides (OGs), oligomers of α-1,4–linked galacturonic acid, are a well-known class of damage-associated molecular patterns (DAMPs) that are recognized by plants as signals of an altered (damaged) self and activate the plant immune response. The accumulation of OGs in vivo is favored by the interaction of microbial polygalacturonases (PGs) with plant-derived inhibitors (PGIPs). Transgenic Arabidopsis plants conditionally expressing a chimeric protein made by a fusion between a PG of Fusarium phyllophilum and a plant-derived PGIP, named “OG machine” (OGM), have been generated, allowing the release on command of OGs in planta and the consequent activation of defense-related responses. A massive and prolonged accumulation of OGs provokes a reduced growth and eventually the senescence of leaves and a cell death resembling the typical hypersensitive response. Expression of the OGM has been obtained in genetic backgrounds defective in key elements involved in immunity. We observed that phenotypes of the OGM plants, i.e. inhibition of seedling root growth and senescence of adult leaves, are recovered, at different extents, in mutants defective in EDS1, a transduction element necessary for salicylic acid accumulation during Effector-Triggered Immunity (ETI), and in RBOHD, a NADPH oxidase involved in the formation of reactive oxygen species (ROS). Root growth is recovered also in plants expressing NahG, a bacterial transgene that leads to reduced level of salicylic acid. Moreover, a cross between DR5::GUS transgenic plants (which express the GUS reporter gene under the control of an IAA inducible promoter) and the OGM was generated, in order to understand whether the OGs can antagonize auxin also when they are released in planta and not only when they are exogenously applied.