Tête à tête inside a plant cell: establishing compatibility between plants and biotrophic fungi and oomycetes

'Compatibility' describes the complementary relationship between a plant species and an adapted pathogen species that underlies susceptibility and which ultimately results in disease. Owing to elaborate surveillance systems and defence mechanisms on the plant side and a common lack of adaptation of many microbial pathogens, resistance is the rule and compatibility the exception for most plant-microbe combinations. While there has been major scientific interest in 'resistance' in the past decade, which has revealed many of its underlying molecular components, the analysis of 'compatibility', although intimately intertwined with 'resistance', has not been pursued with a similar intensity. Various recent studies, however, provide a first glimpse of the pivotal players and potential molecular mechanisms essential for compatibility in both the plant and parasite partners. In this review we highlight these findings with a particular emphasis on obligate biotrophic and hemibiotrophic fungal and oomycete pathogens and discuss novel strategies that might help to uncover further the molecular principles underlying compatibility to these highly specialized pathogens.