A Targeted Mass Spectrometry-Based Metabolomics Approach toward the Understanding of Host Responses to Huanglongbing Disease

Candidatus Liberibacter asiaticus (CLas) is the major culprit of Huanglongbing (HLB), the most destructive citrus disease worldwide. The polymerase chain reaction (PCR) is the most common method for detecting the presence of CLas in the tree. However, due to the uneven distribution of bacteria and a minimum bacterial titer requirement, an infected tree may test false negative. Thus, our current study profiled primary and secondary metabolites of CLas-free leaves harvested from a citrus undercover protection system (CUPS) to prevent a misjudgment of CLas infection. Functional enrichment analysis revealed several metabolic pathways significantly affected by CLas infection, mainly biosynthesis of amino acids and secondary metabolites. Comparisons of CLas-infected metabolite alterations among oranges, mandarins, and grapefruits revealed that host responses to CLas were different. The metabolite signature highlighted in this study will provide a fuller understanding of how CLas bacteria affect the biosynthesis of primary and secondary metabolites in different hosts.