The ubiquitin code of receptor kinases in plants.

Ubiquitination is a central mechanism that regulates receptor kinases (RKs) in plants, where the ubiquitin code controls RK stability, endocytosis, and kinase activity, ensuring precise signaling during development and immunity. As transmembrane signaling hubs, RKs are dynamically controlled by E3 ubiquitin ligases, whose activity is itself regulated by RK phosphorylation, forming intricate feedback loops. Ubiquitination directs RKs toward degradation via either the endocytic-vacuolar or 26S proteasome pathways, with emerging evidence suggesting functional interplay between these routes. Beyond proteolysis, ubiquitination can also directly suppress RK activity. Phosphorylation of E3 ligases by activated RKs or their co-receptors modulates ligase activity, substrate binding, and ubiquitin chain linkage, enabling dynamic signal regulation. This reciprocal control establishes a sophisticated network that maintains receptor homeostasis and signaling fidelity. Despite significant progress, key questions remain about degradation pathway integration, structural mechanisms of E3-substrate-E2 complexes, and crosstalk with other post-translational modifications. Elucidating these regulatory circuits will deepen our understanding of RK-mediated cellular signaling and provide strategies to enhance crop resilience and symbiotic efficiency.