Publications

4492

Diaphorina citri (Hemiptera: Liviidae) is the main vector for the bacterium ‘ Candidatus Liberibacter asiaticus’, which is associated with citrus greening, also known as Huanglongbing. D. citri transmits ‘ Ca. L. asiaticus’ during its feeding on citrus phloem sap. Transmission occurs in a circulative, propagative, and persistent manner. ‘ Ca. L. asiaticus’ has a small genome (1.2 Mb). Therefore, it acquires most of its nutrients and energetic nucleotides from its hosts. The objective of this study was to assess the effect of ‘ Ca. L. asiaticus’ infection on the level of the free fatty acids in its vector. The fatty acids were extracted from adult D. citri using ethyl acetate, derivatized with boron trifluoride-methanol, and analyzed using gas chromatography-mass spectrometry. Nine fatty acids were identified in the D. citri extracts. Oleic acid was the most predominant fatty acid, followed by stearic and palmitic acid, whereas the rest of the fatty acids were present in low amounts. In general, the levels of the detected fatty acids in ‘ Ca. L. asiaticus’-infected D. citri were lower than those found in healthy psyllids. Our findings showed that the reduction of fatty acids in ‘ Ca. L. asiaticus’-infected psyllids resulted from the higher activity of β-oxidation to generate acetyl-coenzyme A, which causes more production of ATP. Our results indicated that ‘ Ca. L. asiaticus’ may enhance the β-oxidation of fatty acids in its vector insect to fulfill its nutrient and energetic nucleotide requirements.

Citrus Huanglongbing, caused by ‘ Candidatus Liberibacter asiaticus’ (CLas), is the most devastating citrus disease worldwide. The CLas genome is much smaller than those of its relatives, such as Sinorhizobium meliloti, due to its reductive evolution. Because CLas has not been cultured in artificial media, despite some progress in co-cultivating, and because genetic manipulation of CLas remains impossible, the understanding of CLas biology is very limited. Usually, 10% of total genes in bacteria are regulatory genes, but only 2% of CLas genes encode transcriptional factors. Here, 20 transcriptional regulators were predicted, including nine genes ( lsrB, ldtR, rem, visR, visN, ctrA, mucR, pelD, and atoC) directly or indirectly involved in regulating motility, and five genes ( rpoH, prbP, phrR, rirA, and lsrB) involved in oxidative stress response. We demonstrated that rem, lsrB, and visNR of CLas can complement the corresponding mutants of S. meliloti in their reduced motility. We further investigated traits controlled by Rem in S. meliloti and CLas using RNA sequencing analyses of rem mutant versus complementation strains with remSmc or remLas. Transcriptomic analysis showed that RemLas significantly regulates the expression of genes in S. meliloti, which was used to infer its regulation of CLas genes by identification of homologous genes. We found that Rem is involved in regulating motility, chemotaxis, transporters, and oxidative phosphorylation in S. meliloti and regulating flagellar and transporter genes in CLas. Among the 39 putative RemLas-regulated genes in CLas, 16 contain the Rem-binding motif, including 10 genes involved in flagellar assembly. Taken together, this study offers valuable insights regarding CLas regulatory genes, with many of them involved in regulating motility and oxidative stress response. The regulation of flagellar genes by Rem in CLas unravels critical information regarding motility in CLas infection of hosts.

AbstractThe severe Asiatic form of huanglongbing (HLB), caused by “Candidatus Liberibacter asiaticus” (CLas), threatens global citrus production via the citrus psyllid, Diaphorina citri. Culturing challenges of CLas necessitate reducing D. citri populations for disease management. CLas boosts the fecundity of CLas‐positive (CLas+) D. citri and fosters its own proliferation by modulating the insect host's juvenile hormone (JH), but the intricate endocrine regulatory mechanisms remain elusive. Here, it is reported that the D. citri ecdysis‐triggering hormone (DcETH) and its receptor DcETHR play pivotal roles in the reciprocal benefits between CLas and D. citri within the ovaries, influencing energy metabolism and reproductive development in host insects; miR‐210, negatively regulates DcETHR expression, contributing to this symbiotic interaction. CLas infection reduces 20‐hydroxyecdysone (20E) levels and stimulates DcETH release, elevating JH production via DcETHR, enhancing fecundity and CLas proliferation. Furthermore, circulating JH levels suppress 20E production in CLas+ ovaries. Collectively, the orchestrated functional interplay involving 20E, ETH, and JH increases energy metabolism and promotes the fecundity of CLas+ D. citri and CLas proliferation. These insights not only broaden the knowledge of how plant pathogens manipulate the reproductive behavior of insect hosts but also offer novel targets and strategies for combatting HLB and D. citri.

A novel, strictly anaerobic, slightly alkaliphilic, halotolerant, peptide- and amino acid-utilizing bacterial strain, SD1T, was isolated from a hypersaline lake in Western Australia. The strain stained Gram-negative and was a motile, spore-forming rod. The strain grew between 15 and 50 °C (optimum 40 °C), 1–15% w/v sodium chloride (optimum 5%) and pH 6.0–10.0 (optimum 9.0). Major fatty acids included anteiso-C15 : 0 (24.9%), C14 : 0 dimethyl acetyl (13.2%), anteiso-C15 : 0 dimethyl acetyl (11.5%) and iso-C15 : 0 (10.4%). The DNA G+C content was 30.3 mol%. The isolate did not grow using any tested sugars but grew well on arginine and glycine. It is capable of using elemental sulfur and thiosulfate as alternate electron acceptors, but not sulfide, sulfate, nitrate or nitrite. 16S rRNA gene similarity indicates that the isolate is related to Sporosalibacterium tautonense MRo-4T (94.33% identity). SD1T showed 76.18%–76.31% average nucleotide identity with other strains within the family Thermohalobacteraceae. Phylogenetics, based on the 16S rRNA gene and whole-genome sequence, as well as phenotypic analysis, differentiates the isolate from close neighbors. We propose that SD1T represents a novel species in a new genus, which we have named Dethiothermospora halolimnae gen. nov., sp. nov., type strain SD1T (DSM 117405T = TSD-443T). From this work, we also propose repositioning of the genus Anaeromonas to the family Thermohalobacteraceae.