Impact of Huanglongbing Pathogen Infection on the Amino Acid Composition in Both Citrus Plants and the Asian Citrus Psyllid

The Asian citrus psyllid (ACP) is the main vector of the pathogen Liberibacter asiaticus (Las), which is the causal agent of citrus Huanglongbing disease. Feeding by both ACP nymphs and adults on host plants allows them to obtain nutrition. Therefore, the nutritional content within the plant phloem is of much importance for the development and reproduction of ACP. The infection by pathogenic microbiomes may affect the amino acid contents of their host plants and then indirectly affect the biology of sap-feeding insects. In this study, we investigated the amino acid contents and their proportions in both Las-infected and Las-free citrus plants, ACP adults, and also in honeydew produced by ACP nymphs. Results showed that infection by Las had a large impact on the amino acid species and proportion in all the tested target plants, ACP adults, and in the honeydew of ACP nymphs. The content of total amino acids in Las-infected citrus was much higher than that of Las-free citrus. However, Las infection significantly reduced the proportion of essential amino acids (EAAs) in these plants. When feeding on Las-infected citrus plants, ACP adults absorbed less total amino acids than those adults feeding on healthy plants, but the proportion of EAAs was significantly higher when they fed on Las-infected citrus plants. The proportion of EAAs also significantly increased in the honeydew secreted by ACP nymphs that fed on Las-infected citrus plants. However, EAA detection in the honeydew of ACP nymphs indicated that the utilization rate of EAAs by Las positive ACP nymphs was reduced. Our study has revealed that Las infection significantly affects the contents, proportion, and utilization efficiency of different amino acids in citrus plants, ACP adults, and nymphs, leading to a developmental pattern of ACP that is more conducive to Las transmission.