Comparative metabolomic analysis of the phloem sap of nine citrus relatives with different degrees of susceptibility to Huanglongbing disease

Citrus Huanglongbing (HLB) disease, also known as “citrus greening”, is currently considered the most devastating citrus disease due to its rapid spread, and high severity. Presently, research efforts are focused on searching for either curative treatments or resistant cultivars to combat the HLB-associated bacterium ‘Candidatus Liberibacter asiaticus’ (CLas).

Metabolomics can help to unravel the mechanisms supporting the potential tolerance/resistance of citrus relatives. Herein, we carried out a metabolomic analysis to determine whether the level of resistance of nine citrus-related genotypes is influenced by their pre-existing metabolic background before infection. For this purpose, the healthy phloem of nine Citrinae genotypes previously categorized according to their different responses to HLB was analyzed. A total of 53 different metabolites were targeted, including amino acids, organic and inorganic acids, and sugars. Interestingly, we observed that resistant and partially resistant genotypes exhibited higher accumulations of organic acids such as quinic acid and citric acid. In contrast, the amount of total sugars showed a clear upward trend in the susceptible genotypes. Notably, within this last group of metabolites, sugar acids increase in both partially resistant and resistant accessions, being more evident in the resistant group.

Alterations potentially linked to resistance levels were detected in specific amino acids belonging to the aspartate and glutamate families. Notably, only lysine levels exhibited a significant increase in the susceptible cultivars. The evaluation of five genes associated with lysine catabolism by RT-qPCR revealed differences in transcript abundance between resistant and susceptible samples suggesting a potential key role in plant defence. These findings open a new avenue to identify metabolites and/or substances that could aid in developing resistance strategies to this devastating disease.