Functional paradigms of PUB-RLK signaling modules in plant growth and biotic stress adaptation

Plant receptor-like kinases (RLKs) form a large superfamily of cell-surface receptors that are central to growth, development, and environmental adaptation. To maintain homeostasis and coordinate the allocation of resources between growth and defense, the output of RLK signaling must be tightly regulated. One key mechanism is the selective degradation of RLKs via the ubiquitin-proteasome system, frequently mediated by Plant U-box (PUB) E3 ubiquitin ligases. Emerging evidence highlights a reciprocal interplay between RLK-mediated phosphorylation and PUB-driven ubiquitination; however, a cohesive framework to interpret these reciprocal interactions is still lacking.

This review synthesizes recent advances to establish a functional framework for PUB-RLK signaling modules. We aim to dissect the molecular basis of their bidirectional regulation and classify these interactions into mechanistically distinct paradigms. The proposed framework illustrates how PUB-RLK modules act as critical regulatory hubs that integrate external signals with internal cellular programs, especially in modulating the trade-off between plant growth and biotic defense. Here, we primarily focus on biotic defense signaling, where PUB-RLK modules are best characterized, while briefly touching on emerging links to broader stress tolerance.

We define three core paradigms for PUB-RLK module function. First, negative feedback loops, where ligand-activated RLKs phosphorylate PUB ligases, which in turn ubiquitinate the activated RLK to attenuate signaling, forming a self-limiting circuit. Second, basal inhibition and release, where PUB ligases constitutively degrade or inactivate RLKs, maintaining signaling quiescence until ligand perception disrupts this inhibition. Third, signal relay mechanism, where activated RLKs phosphorylate PUBs, reprogramming their substrate specificity to target downstream signaling components, thus propagating the response. We conclude by discussing the phylogenetic relationships of these PUBs and their potential as molecular targets for engineering crops with improved stress resilience without compromising growth.