Citrus vascular proteomics highlights the role of peroxidases and serine proteases during Huanglongbing disease progression

Huanglongbing (HLB) is the most devastating and widespread citrus disease. All commercial citrus varieties are susceptible to the HLB-associated bacterium, Candidatus Liberibacter asiaticus (CLas), which resides in the phloem. The phloem is part of the plant vascular system and is involved in sugar transport. To investigate the plant response to CLas, we enriched for proteins surrounding the phloem in an HLB susceptible sweet orange variety, Washington navel (Citrus sinensis (L) Osbeck). Quantitative proteomics revealed global changes in the citrus proteome after CLas inoculation. Plant metabolism and translation were suppressed, while defense-related proteins including proteases and protease inhibitors were induced in the vasculature. Specific serine proteases were upregulated in Washington navel in the presence of CLas based on quantitative proteomics. Subsequent activity-based protein profiling revealed genotype-specific variation in serine protease activity in other C. sinensis sweet orange varieties. Plant peroxidases involved in extracellular ROS production or scavenging were induced in vascular enriched tissues based on quantitative proteomics. While peroxidase transcript accumulation was induced in CLas infected sweet orange varieties, peroxidase enzymatic activity varied between varieties, supporting the genotype-specific regulation of enzymatic activity. The observations in the current study highlight dynamic global reprogramming of the citrus vascular proteome and genotype-specific regulation of different enzyme classes in response to CLas infection. These results open an avenue for further investigation of diverse responses to HLB across different citrus genotypes.