A plant pathogenic bacterium exploits the tricarboxylic acid cycle metabolic pathway of its insect vector

Abstract

Huanglongbing in citrus is caused by a phloem-limited, uncultivable, gram-negative α-proteobacterium, Candidatus Liberibacter asiaticus (CLas). CLas is transmitted by the phloem-sucking insect, Diaphorina citri (Hemiptera: Liviidae), in a persistent, circulative, and propagative manner. In this study, we investigated the metabolomic and respiration rates changes in D. citri upon infection with CLas using gas chromatography-mass spectrometry (GC-MS) and gas exchange analysis. The level of glycine, _L-serine, _L-threonine, and gamma-amino butyric acid were higher in CLas-infected D. citri, while _L-proline, _L-aspartic acid, and _L-pyroglutamic acid were lower in CLas-infected D. citri compared with the control. Citric acid was increased in CLas-infected D. citri, whereas malic and succinic acids were reduced. Interestingly, most of the reduced metabolites such as malate, succinate, aspartate, and _L-proline are required for the growth of CLas. The increase in citric acid, serine, and glycine indicated that CLas induced glycolysis and the tricarboxylic acid cycle (TCA) in its vector. In agreement with the GC-MS results, the gene expression results also indicated that glycolysis and TCA were induced in CLas-infected D. citri and this was accompanied with an increases in respiration rate. Phosphoric acid and most of the sugar alcohols were higher in CLas-infected D. citri, indicating a response to the biotic stress or cell damage. Only slight increases in the levels of few sugars were observed in CLas-infected D. citri, which indicated that sugars are tightly regulated by D. citri. Our results indicated that CLas induces nutrient and energetic stress in its host insect. This study may provide some insights into the mechanism of colonization of CLas in its vector.

Keywords: Candidatus Liberibacter asiaticus; Diaphorina citri; GC-MS; Huanglongbing; tricarboxylic acid cycle.

Figure 1.

A representative gas chromatography-mass spectrometry (GC-MS) chromatogram of trimethylsilyl (TMS) derivatives detected in the extract of healthy and Candidatus Liberibacter asiaticus-infected Diaphorina citri, with a magnification of selected peaks.

Figure 2.

Principal components analysis (A) and its associated loading plot (B) showing the distribution of uninfected and Candidatus Liberibacter asiaticus-infected Diaphorina citri using the concentrations of the main metabolites groups. Principal components analysis (C) and its associated loading plot (D) showing the distribution of uninfected and Candidatus Liberibacter asiaticus- infected Diaphorina citri using the concentrations of individual metabolites groups (n = 20). In panel (D), Some of the detected compounds names have been deleted from the loading plot for better presentation.

Figure 3.

Differential gene expression patterns of expressed genes involved in TCA cycle of CLas-infected and uninfected D. citri. The complete list of expressed genes is available Table 2.

Figure 4.

Respiration of CLas-infected and uninfected D. citri recorded over 10 min. Gray bands represent 95% confidence intervals (n = 6), and lines represent single representative samples.