Quantification of the chemical profile induced in healthy Citrus sinensis and C. limonia by inoculation with Xylella fastidiosa

Xylella fastidiosa is transmitted by xylem-feeding leafhoppers (Cicadellidae), and colonizes the xylem of plants, causing diseases such as citrus variegated chlorosis (CVC). Rooted Citrus limonia trees, however, may be grown in orchards in the presence of high disease and insect pressure without showing foliar symptoms of the disease. This information encouraged us to undertake phytochemical studies of C. sinensis L. Osbeck grafted onto C. limonia. The graft produces a considerable number of compounds that are not common in either of the seedling plants. The chemical profiles of scion and rootstock differ notably for the absence, in the former, of flavonoids and coumarins containing C₅ prenyl groups attached to the carbon atoms of aromatic and heterocyclic systems or to oxygen [1]. C. sinensis grafted onto C. limonia, symptomatic and asymptomatic for X. fastidiosa, was examined to determine if secondary metabolites in the plant were produced or accumulated as a chemical defense. This study revealed that the flavonoid hesperidin could be present at higher concentrations in symptomatic plants than in asymptomatic ones. Thus, we used HPLC and HPLC-ESI-MS/MS by the selected reaction monitoring (SRM) mode, to develop a rapid and sensitive method for detecting hesperidin and rutin, and the coumarins xanthyletin and seselin in all the aerial parts of plants. The amount of hesperidin was 52% higher in the leaves, and 25% higher in the scion stem of the symptomatic plant than in the asymptomatic one, strongly suggesting that hesperidin plays a role in plant-pathogen interaction, probably as a phytoanticipin. On the other hand, the amount of the coumarin xanthyletin in leaves decreased by 80% and unvaryingly occurred in only small amounts in scion stem, strongly indicating that xanthyletin is recycled within the plant enabling cinnamic acid to be incorporated into flavanone hesperidin.

References: [1] Ribeiro, A.B. et al. (2008)J. Agric. Food Chem. 56:7815–7822.