The rise of necrotrophic effectors

Many biotrophic pathogens, which require living plant tissues for colonization, interact with their hosts in a gene-for-gene (GFG) manner whereby plant disease resistance (R) genes recognize products of specific avirulence (Avr) genes in pathogens leading to disease resistance (Flor, 1956). In these interactions, the absence of either the R gene, or the corresponding Avr gene results in disease. GFG interactions have also been well documented in the pathosystem involving the necrotrophic fungal pathogen Parastagonospora nodorum (also known as Septoria, Stagonospora, Leptosphaeria and Phaeosphaeria nodorum) and wheat. However, in this case, the interactions lead to increased disease when Parastagonospora nodorum expresses specific necrotrophic effector (NE) genes and wheat contains the corresponding dominant sensitivity genes. This type of interaction is often described as an ‘inverse’ GFG (Fig. 1).