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, Liberibacter asiaticus (Las), 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 Las, we enriched for proteins surrounding the phloem in an HLB susceptible sweet orange variety, Washington navel ( (L) Osbeck). Quantitative proteomics revealed global changes in the citrus proteome after Las inoculation. Plant metabolism and translation were suppressed, whereas defense-related proteins such as peroxidases, proteases and protease inhibitors were induced in the vasculature. Transcript accumulation and enzymatic activity of plant peroxidases in CLas infected sweet orange varieties under greenhouse and field conditions were assessed. Although peroxidase transcript accumulation was induced in Las infected sweet orange varieties, peroxidase enzymatic activity varied. Specific serine proteases were up-regulated in Washington navel in the presence of Las based on quantitative proteomics. Subsequent activity-based protein profiling revealed increased activity of two serine proteases, and reduced activity of one protease in two sweet orange varieties under greenhouse and field conditions. The observations in the current study highlight global reprogramming of the citrus vascular proteome and differential regulation of enzyme classes in response to Las infection. These results open an avenue for further investigation of diverse responses to HLB across different environmental conditions and citrus genotypes.