The term "nonhost resistance" (NHR) describes the phenomenon that an entire plant species is resistant to all genetic variants of a non-adapted pathogen species. In nature, NHR represents the most robust form of plant immunity and is therefore of scientific as well as economic importance. Due to its highly complex nature, NHR has previously not been studied in detail. Recently, the establishment of model interaction systems utilizing Arabidopsis and non-adapted powdery mildews allowed the identification of several key components and conceptual conclusions. It is now generally accepted that NHR of Arabidopsis to powdery mildews comprises two distinct layers of defence: pre-invasion entry control at the cell periphery and post-invasion resistance based on cell death execution. The timely production and localised discharge of toxic compounds at sites of fungal attack appear to be pivotal for entry control. This process requires proteins involved in secretion and trans-membrane transport, synthesis and activation of indolic glucosinolates as well as gene regulation and post-translational protein modification. Post-invasion defence relies on lipase-like proteins and salicylic acid signalling. To what extent pathogen-associated molecular pattern- or effector-triggered immunity contribute to NHR remains to be investigated and is likely to depend on the model system studied.
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