Figure 1. Extracellular and intracellular receptor proteins can promote disease resistance or susceptibility to fungal pathogens.

Cell-surface receptors are embedded in the plasma-membrane while intracellular receptors are localized to the cytosol. The cell-surface receptors are grouped into subfamilies based on their extracellular domain/s. Lysin-motif receptor-like proteins and receptor-like kinases (LysM-RLP/RLKs) can detect chitin oligomers released from the fungal cell wall to activate defense responses that promote disease resistance against biotrophic, hemibiotrophic, and necrotrophic fungi without leading to plant cell death. Leucine-rich repeat (LRR) receptors can detect secreted proteins (effectors) from pathogenic fungi and mediate downstream immune signaling by forming receptor complexes with other LRR-RLKs (SOBIR1 and BAK1). Notably, the LRR-RLP/RLKs that detect effectors from necrotrophic fungi and promote disease resistance do not lead to plant cell death. Likewise, wall-associated kinase (WAK) receptor proteins activate responses that typically promote disease resistance. However, Snn1 from wheat detects a necrotrophic effector (Tox1) and triggers a cell-death response that supports the growth of the invading necrotroph. Inside the cell, intracellular receptors detect effectors and typically promote disease resistance by initiating a localized cell-death response that restricts the growth of biotrophic and hemibiotrophic fungi. However, multiple necrotrophic fungi use effectors to manipulate intracellular receptors into activating cell death, ultimately promoting disease susceptibility. Only the receptors discussed in this review are depicted in the figure.