Abstract
Molecular studies focusing on the interface between microbes and plant hosts have provided major insights into the basis underlying pathogenesis, symbiosis and plant defence and resistance mechanisms. A more recent focus on microbes, facilitated by the generation of complete genome sequences, has uncovered the sheer number of protein effectors microbes deliver in this interface as well as inside host cells to manipulate the plant immune system. Although studies on the characterization and roles of bacterial effectors are further advanced, in this review we focus on the current knowledge of fungal and oomycete effectors and their roles. Examples are given of effectors disarming plant defence enzymes, such as the apoplastic effector AVR2 from Cladosporium fulvum which inhibits the tomato defence cysteine protease. Other effectors interfere with the perception by the host of microbes exposing molecular determinants such as Phytophthora infestans INF1 protein. Many effectors alter gene expression induced by the host during defence, exemplified in fungi by Ustilago maydis Pit2 suppressing maize defence genes. Effectors recognized by resistance gene products, either directly or indirectly, and eliciting defence, represent the classical avirulence genes and almost 50 have now been cloned from fungi and oomycetes. Evolutionary adaptations and arms races have produced diversification in both pathogen and host, and in pathogens, are the cause of breaking crop resistance in agricultural settings. Molecular insight provides valuable information for applications. For example, some effectors are crucial for pathogenesis, thereby revealing targets for disease control and others interact with host resistance gene products and could be used to screen germplasm for novel sources of disease resistance. Variation among effectors will likely yield diagnostic tools for pathogen race identification. The study of model systems is providing insight into avenues by which other, major plant diseases can potentially be controlled.