Abstract
The Asian citrus psyllid (ACP) Diaphorina citri is the main vector of the pathogen Candidatus Liberibacter asiaticus (CLas), 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 CLas-infected and CLas-free citrus plants, ACP adults, and also in honeydew produced by ACP nymphs. Results showed that infection by CLas 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 CLas-infected citrus was much higher than that of CLas-free citrus. However, CLas infection significantly reduced the proportion of essential amino acids (EAAs) in these plants. When feeding on CLas-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 CLas-infected citrus plants. The proportion of EAAs also significantly increased in the honeydew secreted by ACP nymphs that fed on CLas-infected citrus plants. However, EAA detection in the honeydew of ACP nymphs indicated that the utilization rate of EAAs by CLas positive ACP nymphs was reduced. Our study has revealed that CLas 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 CLas transmission.
Highlights
Huanglongbing (HLB), commonly known as citrus greening disease, is caused by three species of the fastidious, phloemresiding, gram-negative bacteria “Candidatus Liberibacter asiaticus” (CLas; Bové, 2006), “Ca. L. africanus” (CLaf; Garnier et al, 2000), and “Ca. L. americanus” (CLam; Texeira et al, 2005)
We selected the same species of amino acids detected in the phloem sap of CLas-infected and CLas-free citrus plants for analysis and found that the proportion of essential amino acids (EAAs) in the CLas-infected citrus plants was significantly lower than that of the CLas-free citrus plants (t = −18.395, df = 2, P = 0.003) (Figure 1B), which means the infection of CLas significantly reduced the proportion of EAAs in the CLas-infected citrus plants (t = −18.395, df = 2, P = 0.003), but no significant difference was found between the non-essential amino acids (NEAAs) proportions of CLas infection and CLas-free citrus plants (t = −2.014, df = 2, P = 0.182)
The results of our study showed that CLas infection remarkably increased the mass proportions of Thr, His, Arg, and Pro in the Asian citrus psyllid (ACP) adults that fed on CLas-infected citrus plants, which indicated that the digestion and utilization efficiency of Pro by ACP adults was improved by CLas infection
Summary
Huanglongbing (HLB), commonly known as citrus greening disease, is caused by three species of the fastidious, phloemresiding, gram-negative bacteria “Candidatus Liberibacter asiaticus” (CLas; Bové, 2006), “Ca. L. africanus” (CLaf; Garnier et al, 2000), and “Ca. L. americanus” (CLam; Texeira et al, 2005). It is one of the most economically destructive diseases of the citrus industry worldwide (Wang et al, 2016). The fruit coloration starts at the peduncular end (color inversion) and results in fruit loss and eventual tree death (Roistacher, 1996; Bové, 2006)
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