Ubiquitination-driven proteostasis remodeling underlies oxidative stress and actin degradation in citrus...

Citrus huanglongbing, caused by Candidatus Liberibacter asiaticus (CLas), is associated with severe metabolic disruption, chronic immune activation, and progressive decline of citrus trees, but the posttranslational mechanisms underlying these responses remain poorly understood. Here, we combined ubiquitinated peptide immunoaffinity enrichment with 4D label-free quantitative proteomics to profile protein abundance and protein ubiquitination in infected sweet orange leaves. Infection caused extensive proteome remodeling and a strong global increase in protein ubiquitination. Integrated analysis revealed a predominant group of proteins with reduced abundance and increased ubiquitination, pointing to ubiquitination-associated depletion of proteins involved in chloroplast function, redox homeostasis, and primary metabolism. The degradation machinery itself was also remodeled, including widespread ubiquitination of proteasome subunits and increased proteasome-dependent bulk protein turnover. Physiological and biochemical analyses showed that infection suppressed the hydrogen peroxide-scavenging system, with reduced abundance and increased ubiquitination of catalase, accompanied by elevated hydrogen peroxide accumulation. We also identified actin as a major downstream target of CLas-induced proteostasis remodeling. Actin proteins underwent enhanced ubiquitination and degradation in infected tissues, accompanied by actin filament disruption, and CsACT7 showed 26S proteasome-dependent degradation. Seven CLas-upregulated ubiquitination sites acted cooperatively to promote CsACT7 degradation, as simultaneous substitution of all 7 lysines with alanine strongly stabilized CsACT7 and reduced CLas accumulation in citrus hairy roots. Together, these findings establish ubiquitination-dependent proteome remodeling as a central feature of the citrus response to CLas infection and link ubiquitination-mediated posttranslational regulation to redox imbalance, cytoskeletal disruption, and disease susceptibility.