| Plants are constantly exposed to a wide variety of pathogens. However, they are susceptible to only a few pathogens they come in contact with. This indicates that plants are capable of defending themselves against the majority of pathogens. This type of general defense is often referred to as nonhost defense. More rigorously, nonhost resistance occurs when every member of a plant species is resistant to every member of a pathogen species. Nonhost resistance is thought to be multigenic, non-specific, and durable. In contrast, the resistance of certain genotypes of an otherwise susceptible species to a pathogen is termed host resistance. This type of resistance often follows gene-for-gene interactions and is typically short-lived in the field.; Although much is known about gene-for-gene resistance, very little research has been carried out to elucidate the molecular basis of nonhost resistance. In this work, we used the barley powdery mildew pathogen Blumeria graminis f.sp. hordei as a nonhost pathogen on the model plant Arabidopsis thaliana. A cytological characterization of the interaction revealed that resistance occurs predominantly at the penetration stage of infection and this resistance is accompanied by widespread deposition of callose, a cell wall polymer. A screen for loss of nonhost resistance identified loci affecting callose deposition, including the callose synthase responsible for depositing callose in response to pathogens.; Cloning of one of these loci revealed the involvement of an ATP binding cassette transporter in nonhost resistance. While plants mutated for this transporter allow increased penetration by two different nonhost pathogens, they are resistant to the host pathogen powdery mildew. This resistance is associated with cell death and dependent on the salicylic acid plant defense pathway.; Identifying plant factors involved in the initial stages of nonhost resistance increases our understanding of multi-layered basal defenses and provides a starting point from which to identify subsequent layers. |
