Metabolomic Analysis of Citrus Infection by ‘<i>Candidatus</i> Liberibacter’ Reveals Insight into Pathogenicity

Huanglongbing (HLB), considered the most serious citrus disease in the world, is associated with the nonculturable bacterium ‘<i>Candidatus</i> Liberibacter asiaticus’ (Las). Infection of citrus by this pathogen leads to reduced plant vigor and productivity, ultimately resulting in death of the infected tree. It can take up to two years following initial infection before outward symptoms become apparent, making detection difficult. The existing knowledge gap in our understanding of Las and its pathogenesis leading to HLB has stymied development of treatments and methods to mitigate the pathogen’s influence. To evaluate the influence of Las on fruit quality in both symptomatic and asymptomatic fruit, and gain further insight into the pathogenesis of the disease, a ¹H NMR metabolomics investigation, complemented with physicochemical and analyte-specific analyses, was undertaken. Comparison of the juice obtained from oranges gathered from Las⁺ (symptomatic and asymptomatic) and Las^– (healthy) trees revealed significant differences in the concentrations of sugars, amino and organic acids, limonin glucoside, and limonin. This study demonstrates differing metabolic profiles in the juice of oranges from Las⁺ and Las^– and proposes how Las may be able to evade citrus defense responses