Regulation of CRL neddylation dynamics in Arabidopsis thaliana

Cullin-RING ubiquitin ligases (CRL1 to 4 in plants) control the stability of regulators of essential cellular processes, including hormone signalling and abiotic stress response. CRL activity is switched on by the conjugation of the ubiquitin-like modifier NEDD8 to their cullin subunit (neddylation), which is catalysed by an enzymatic cascade which includes AXR1, and switched off by NEDD8 deconjugation (deneddylation), which is catalysed by the COP9 signalosome (CSN). Both neddylation and deneddylation are critical for CRL function and plant viability, as mutations disrupting these processes result in developmental arrest. Previously, we reported a reversible switch in the ratio of NEDD8-CUL1 to CUL1 (the CRL1 subunit) during Arabidopsis seed maturation and germination, with total CUL1 levels remaining constant. This switch is severely impaired in csn mutants, which accumulate neddylated cullins, suggesting that Arabidopsis cells actively modulate this ratio to sustain postembryonic development. Here, we show that, akin to seed maturation, high salinity induces a rapid, transient accumulation of NEDD8-CUL1. Interestingly, both csn and axr1 mutations, which have opposite effects on NEDD8-CUL1 levels, are salt sensitive, indicating that a reversible switch of NEDD8-CUL1 to CUL1 is crucial to ensure a prompt salt stress response. Indeed, simultaneous impairment of both cullin neddylation and deneddylation exacerbates salt sensitivity. We are currently investigating whether the salt-induced accumulation of NEDD8-CUL1 is a result of increased CRL activity or decreased CSN activity. Interestingly, water deprivation similarly triggers NEDD8-CUL1 accumulation across various Arabidopsis ecotypes, indicating that this mechanism might be a generalized plant response to osmotic stress. Understanding this process could unveil new strategies to enhance plant resilience in the face of environmental changes.