Citrus Vascular Proteomics Highlights the Role of Peroxidases and Serine Proteases during Huanglongbing Disease Progression

Abstract

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 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. While peroxidase transcript accumulation was induced in CLas infected sweet orange varieties, peroxidase enzymatic activity varied. Specific serine proteases were upregulated in Washington navel in the presence of CLas based on quantitative proteomics. Subsequent activity-based protein profiling revealed increased activity of two serine proteases, and reduced activity of one protease in two C. sinensis 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 CLas infection. These results open an avenue for further investigation of diverse responses to HLB across different environmental conditions and citrus genotypes.

Insights into the mechanism of Huanglongbing tolerance in the Australian finger lime (Citrus australasica)

“…SPs have various functions in plant cells, including functions in the pathogen response (Tornero et al, 1997;Jordáet al, 1999). Specific citrus SPs were detected, suggesting that protease enzymatic activity is finely tuned during CaLas infection (Franco et al, 2020). The overexpression of XSP1 in finger lime suggests that there is specific mechanism underlying the defense against CaLas infection in finger lime tissues.…”

Citrus Huanglongbing is a pathogen-triggered immune disease that can be mitigated with antioxidants and gibberellin

“…Exudates of phloem-enriched bark tissues were extracted from stems following the procedure described elsewhere 74 . Stems were collected from small branches of spring spouts with mildly symptomatic leaves.…”

Metabolomic Analysis Revealed Distinct Physiological Responses of Leaves and Roots to Huanglongbing in a Citrus Rootstock

“…The markedly elevated activities of all three antioxidant enzymes were observed by Pitino et al [ 39 ]. Franco et al [ 41 ] found that in sweet orange Harmlin and N-33, Huanglongbing induced the accumulation of peroxidase transcripts, but POD enzyme activities exhibited different alterations: the enzyme activity was downregulated after infection in Harmlin, while the enzyme activity was upregulated after infection in N-33 [ 41 ]. In the tolerant cultivar Kaffir lime, Cu/Zn-SOD and POD4 were upregulated, while they were downregulated in C. sinensis [ 19 ].…”