Metabolic alterations in the nymphal instars of Diaphorina citri induced by Candidatus Liberibacter asiaticus, the putative pathogen of huanglongbing

Abstract

Currently, huanglongbing is the most damaging disease of citrus causing huge economic losses. The disease is caused by the Gram-negative bacterium Candidatus Liberibacter asiaticus (CLas). The pathogen is transmitted in a persistent propagative circulative manner within its vector, the Asian citrus psyllid, Diaphorina citri. Exploring the metabolic alteration in the vector may lead to a better understanding of the nutritional needs of CLas and to designing an artificial medium for culturing the pathogen. It has been shown that the nymphal stages have a greater role in transmission mainly because they feed on plants more actively than adults. In this study, we carried out an untargeted comparative metabolomic analysis for healthy and CLas-infected 4th / 5th instar nymphs. The metabolic analysis was performed using trimethylsilylation and methyl chloroformate derivatization followed by Gas Chromatography-Mass Spectrometry (GC-MS). Overall, the changes in the nymph metabolism due to the infection with CLas were more pronounced than in adults, as we previously published. Nymphs reared on CLas-infected Valencia sweet orange were higher in many metabolites, mainly those of the TCA cycle, C16 and C18 fatty acids, glucose, sucrose, L-proline, L-serine, pyroglutamic acid, saccharic acid, threonic acid and myo-inositol than those reared on healthy plants. In contrast, CLas-infected nymphs were lower in putrescine, glycine, L -phenylalanine, L -tyrosine, L -valine, and chiro-inositol. The information provided from this study may contribute in acceleration of the availability of CLas in culture and consequent screening of antibacterial compounds to discover a definitive solution for huanglongbing.

Friend or foe? Relationship between ‘Candidatus Liberibacter asiaticus’ and Diaphorina citri

“…The enzymes that catalyze the first and the second step in fatty acid β-oxidation and the production of acetyl-CoA, as well as enzymes involved in the citric acid cycle, were induced in Las-positive D. citri adult. Citric acid was induced in both Las-positive D. citri nymphs and adults Killiny and Jones 2018). Succinate dehydrogenase and succinate semialdehyde dehydrogenase enzymes were also enhanced in Las-positive D. citri ).…”

“…Las-positive D. citri adults have less hexamerin, an amino acid-storage protein, which suggests that Las may manipulate free amino acid viability (Kruse et al 2017). Moreover, the concentrations of L-proline, the main energy source of insects (Teulier et al 2016), and L-aspartic acid were lower in Las-positive D. citri compared with the uninfected control Killiny and Jones 2018). The decrease in both amino acid concentrations may result from their oxidation to supplement the TCA cycle intermediates, which have higher demand under the energetic stress caused by Las infection.…”

“…The increase in glycolysis rate is supported by increase in the gene expression of enzymes implicated in glycolysis such as hexokinase and pyruvate carboxylase . Interestingly, unlike Laspositive adults, Las-positive D. citri nymphs have a reduction of glycine content, suggesting that Las should use this amino acid during nymphal stages (Killiny and Jones 2018).…”

“…Transcriptomic, proteomic and metabolomic studies suggest that not only glycolysis but also the TCA cycle is changed by Las in both nymphal and adult life stages Killiny and Jones 2018). The activation of TCA cycle in Las-positive D. citri indicated that it is under nutrient and energy stress.…”

Infection with phytopathogenic bacterium inhibits melatonin biosynthesis, decreases longevity of its vector, and suppresses the free radical‐defense

“…The full list of expressed genes, names, accession numbers, and primers are available in Table S2 and uninfected D. citri adults; 56 however, it was detected as a trace in D. citri nymphs reared on healthy or CLas-infected Valencia sweet orange flushes without any significant differences between them. 66 Another probability for the reduction of melatonin is due to the downregulation of the enzymes involved in melatonin biosynthesis. Our gene expression results showed that the expression levels of all enzymes involved in melatonin biosynthesis were decreased in CLas-infected D. citri, including DcTPH-1, DcTPH-2, DcAAAD, DcSNAT, and DcASMT.…”