A House Divided: Unifying the Classification of Plant Pathogen Effectors

Plant pathogens deploy diverse effectors to interfere with host defense systems and create favorable conditions for infection. As effector biology has advanced, the terminology used to describe these effectors has become increasingly complex, reflecting differences in secretion pathways, molecular features, and subcellular localization across pathogens. This inconsistency complicates comparative studies and limits effective communication across research groups. Here, we propose a streamlined and broadly applicable classification framework based on the functional destination of effectors within host plants, dividing them into two categories: extracellular and intracellular effectors. Extracellular effectors operate beyond the host plasma membrane, acting in apoplast, cell wall, and xylem, whereas intracellular effectors act within host cells, targeting the cytoplasm, nucleus, and other organelles. This approach provides a common terminology applicable to effectors from fungi, Oomycetes, bacteria, nematodes, and viruses, while broadly reflecting the spatial organization of plant immune surveillance. Moreover, it establishes a practical foundation for the systematic organization and management of effector databases. Together, this classification facilitates standardized effector annotation, comparative analyses, and interdisciplinary understanding of molecular plant-pathogen interactions.