Suppression of DRR1 results in the accumulation of insoluble ubiquitinated proteins,which impairs drought stress tolerance

Drought stress has detrimental effects on plants.Although the abscisic acid(ABA)-mediated drought response is well established,defensive mechanisms to cope with dehydration-induced proteotoxicity have been rarely studied.DRR1 was identified as an Arabidopsis drought-induced gene encoding an ER-localized RING-type E3 Ub ligase.Suppression of DRR1 markedly reduced tolerance to drought and proteotoxic stress without altering ABA-mediated germination and stomatal movement.Proteotoxicityand dehydration-induced insoluble ubiquitinated protein accumulation was more obvious in DRR1 loss-of-function plants than in wild-type plants.These results suggest that DRR1 is involved in an ABA-independent drought stress response possibly through the mitigation of dehydration-induced proteotoxic stress.

PUB22 and PUB23 U-box E3 ubiquitin ligases negatively regulate 26S proteasome activity under proteotoxic stress conditions

The mechanism regulating proteasomal activity under proteotoxic stress conditions remains unclear.Here,we showed that arsenite-induced proteotoxic stress resulted in upregulation of Arabidopsis homologous PUB22 and PUB23 U-boxE3 ubiquitin ligases and that pub22 pub23 double mutants displayed arsenite-insensitive seed germination and root growth phenotypes.PUB22/PUB23 downregulated 26 S proteasome activity by promoting the dissociation of the 19 S regulatory particle from the holo-proteasome complex,resulting in intracellular accumulation of UbG76 VGFP,an artificial substrate of the proteasome complex,and insoluble poly-ubiquitinated proteins.These results suggest that PUB22/PUB23 play a critical role in arsenite-induced proteotoxic stress response via negative regulation of 26 S proteasome integrity.