Proteomic analysis highlights the role of detoxification pathways in increased tolerance to Huanglongbing disease

Abstract

Background: Huanglongbing (HLB) disease is still the greatest threat to citriculture worldwide. Although there is not any resistance source in the Citrus germplasm, a certain level of moderated tolerance is present. A large-scale analysis of proteomic responses of Citrus may help: 1) clarifying physiological and molecular effects of disease progression, 2) validating previous data at transcriptomic level, and 3) identifying biomarkers for development of early diagnostics, short-term therapeutics and long-term genetic resistance.

Results: In this work we have conducted a proteomic analysis of mature leaves of two Citrus genotypes with well-known differing tolerances to HLB: Navel orange (highly susceptible) and Volkameriana (moderately tolerant). Pathway enrichment analysis showed that amino acid degradation processes occurred to a larger degree in the Navel orange. No clear differences between the two genotypes were observed for primary metabolic pathways. The most important finding was that four glutathione-S-transferases were upregulated in Volkameriana and not in Navel orange. These proteins are involved in radical ion detoxification.

Conclusions: Upregulation of proteins involved in radical ion detoxification should be considered as an important mechanism of increased tolerance to HLB.

Keywords: Candidatus liberibacter; Citrus; Huanglongbing; Proteome; Proteomic; iTRAQ.

Fig. 1

Principal component analysis of differentially regulated proteins of four types of leaf tissues (Control Navel orange, Control Volkameriana, Infected Volkameriana and Infected Navel orange). Proteins that contribute highly to the separation of the the four samples are numbered and listed next to each graph

Fig. 2

Gene set enrichment analysis using Pageman web-tool. Upregulated and downregulated pathways at proteomic level in infected Volkameriana (V) and infected Navel orange (N) in comparison to respective healthy controls

Fig. 3

Proteins involved in primary metabolic responses to HLB. A) Sucrose and starch metabolism; B) Raffinose metabolism; C) TCA cycle; D) Oxidative pentose phosphate. Proteins that were differentially expressed between control and infected trees are indicated by colors, based on their pattern of expression in the two genotypes (see color key). Each colored square represents expression change in a protein catalyzing a step in the pathway. More than one square grouped together indicates different members of the same protein family found to be differentially regulated

Fig. 4

Biotic stress–related proteins altered in response to HLB disease in both Citrus genotypes. Proteins that were differentially expressed between control and infected trees are indicated by colors, based on their pattern of expression in the two genotypes (see color key). Each colored square represents expression change in a protein associated with regulatory and enzymatic functions

Fig. 5

Global view of proteomic changes in Citrus leaves (Volkameriana and Navel orange) in response to CaLas infections. Proteins, pathways, and cell functions that were differentially expressed are indicated by colors, based on their pattern of expression in the two genotypes. Each colored square represents expression change in a protein associated with regulatory and enzymatic functions. More than one rectangle grouped together indicates different members of the same protein family were found to be differentially regulated