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

Abstract

Abstract 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 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 displayed a trend toward an average percentage increase in both partially resistant and resistant accessions, being more evident in the resistant group. Changes potentially associated with the level of resistance were observed in certain amino acids within the aspartate and glutamate families. However, only lysine levels were significantly higher in the susceptible samples. The evaluation of five genes associated with lysine catabolism by RT-qPCR revealed differences in transcript abundance between resistant and susceptible samples. These findings open a new avenue of opportunity for identifying metabolites and/or substances that could aid in developing resistance strategies to this devastating disease.