Auxin-Mediated Redox Control of the Ubiquitin-Proteasome System: A Key Mechanism for Plant Growth and Development

In plants, the ubiquitin–proteasome system (UPS) plays a central role in hormonal regulation, including the action of the phytohormone auxin, which orchestrates numerous aspects of growth and development. Auxin modulates redox metabolism and promotes the accumulation of nitric oxide (NO) in various tissues and physiological contexts. NO functions as a redox signaling molecule, exerting its effects in part through the reversible oxidation of cysteine residues via a post-translational modification known as S-nitrosylation. Recent findings highlight a dynamic interplay between S-nitrosylation and the ubiquitination machinery, shaping critical aspects of auxin-mediated plant responses. In this review, we summarize current knowledge on redox regulation of UPS components involved in auxin-mediated pathways and propose new perspectives on the integration of hormonal and redox signaling in plants. We describe and discuss the complexity of the latest evidence supporting the role of NO as a second messenger in auxin signaling, with S-nitrosylation acting as a regulatory mechanism that fine-tunes the UPS to control developmental outcomes. We focused on the direct effects of NO that include S-nitrosylation of specific cysteine residues of substrates, adaptors, and substrate receptors belonging to different CULLIN1- and CULLIN4-based E3 ubiquitin ligase complexes.