Publications

4606

Citrus Huanglongbing (HLB; yellow shoot disease) is associated with an unculturable α-proteobacterium “Candidatus Liberibacter asiaticus” (CLas). HLB was found in southern California in 2012, and the current management strategy is based on suppression of the Asian citrus psyllid (Diaphorina citri) that transmits CLas and removal of confirmed CLas-positive trees. Little is known about Asian citrus psyllid-associated bacteria and citrus-associated bacteria in the HLB system. Such information is important in HLB management, particularly for accurate detection of CLas. Recent advancements in next-generation sequencing technology provide new opportunities to study HLB through genomic DNA sequence analyses (metagenomics). In this study, HLB-related bacteria in Asian citrus psyllid and citrus (represented by leaf midrib tissues) samples from southern California were analyzed. A metagenomic pipeline was developed to serve as a prototype for future bacteriomic research. This pipeline included steps of next-generation sequencing in Illumina platform, de novo assembly of Illumina reads, sequence classification using the Kaiju tool, acquisition of bacterial draft genome sequences, and taxonomic validation and diversity evaluation using average nucleotide identity. The identified bacteria in Asian citrus psyllids and citrus together included Bradyrhizobium, Buchnera, Burkholderia, “Candidatus Profftella armature,” “Candidatus Carsonella ruddii,” CLas, Mesorhizobium, Paraburkholderia, Pseudomonas, and Wolbachia. The whole genome of a CLas strain recently found in San Bernardino County was sequenced and classified into prophage typing group 1 (PTG-1), one of the five known CLas groups in California. Based on sequence similarity, Bradyrhizobium and Mesorhizobium were identified as possible source that could interfere with CLas detection using the 16S rRNA gene-based PCR commonly used for HLB diagnosis, particularly at low or zero CLas titer situation.

Summary Large amounts of detrital organic matter and osmolytes accumulate in the sediments of hadal trenches (>6000 m depth) due to the funnelling effect. It is still unknown whether there are novel active microbes that depend on specific carbon sources in extreme and isolated environments. In this study, we present a novel active bacterial phylum, Candidatus Tianyabacteria in the FCB superphylum, which was enriched in sediments collected from the Challenger Deep. Genome binning resulted in high‐quality Ca . Tianyabacteria genomes representing two Ca . Tianyabacteria lineages (L1 and L2) in sediments 0–21 cm below the surface (cmbsf); L1 tends to be abundant in the upper layers (0–9 cmbsf), and L2 seems to be more prevalent in the deeper layers (12–21 cmbsf). Gene annotation and transcriptomics results indicate that the two lineages might import and catalyse amino acids and myo‐inositol into central carbon metabolism for a heterotrophic lifestyle. Probably due to differences in environmental oxygen levels, the L2 genomes harbour gene clusters responsible for denitrification and fermentation, while the L1 genomes encode octahaem cytochrome c and multicopper oxidase using unknown substrates. The Ca . Tianyabacteria are thus novel heterotrophic organisms that participate in processes of carbon, nitrogen and organic osmolyte cycling in hadal sediments.

Streptomycin (STR) has been used to control citrus huanglongbing (HLB) caused by ‘Candidatus Liberibacter asiaticus’ (CLas) via foliar spray. Here, we studied the residue dynamics of STR and its effect on CLas titers in planta applied by foliar spray and trunk injection of 3-year-old citrus trees that were naturally infected by CLas in the field. After foliar spray, STR levels in leaves peaked at 2 to 7 days postapplication (dpa) and gradually declined thereafter. The STR spray did not significantly affect CLas titers in leaves of treated plants as determined by quantitative PCR. After trunk injection, peak levels of STR were observed 7 to 14 dpa in the leaf and root tissues, and near-peak levels were sustained for another 14 days before significantly declining. At 12 months after injection, moderate to low or undetectable levels of STR were observed in the leaf, root, and fruit, depending on the doses of STR injected, with a residue level of 0.28 µg/g in harvested fruit at the highest injection concentration of 2.0 µg/tree. CLas titers in leaves were significantly reduced by trunk injection of STR at 1.0 or 2.0 g/tree, starting from 7 dpa and throughout the experimental period. The reduction of CLas titers was positively correlated with STR residue levels in leaves. The in planta minimum effective concentration of STR needed to suppress the CLas titer to an undetectable level (cycle threshold ≥36.0) was 1.92 µg/g fresh weight. Determination of the in planta minimum effective concentration of STR against CLas and its spatiotemporal residue levels in planta provides the guidance to use STR for HLB management.

AbstractBACKGROUNDDiaphorina citri is a vector of ‘Candidatus Liberibacter asiaticus’ (CLas), which is associated with citrus huanglongbing (HLB). In this study, the adaptability and CLas titres of D. citri adults on three weed species, namely, Ageratum conyzoides, Solanum nigrum and Praxelis clematidea, which are widely distributed in citrus orchards in China, were determined.RESULTSD. citri selected S. nigrum preferentially over the other weed species. The longest survival times of D. citri adults on A. conyzoides, S. nigrum and P. clematidea were 48, 41 and 11 days, respectively. On S. nigrum, the survivorship of females was significantly higher than that of males. The CLas titres of D. citri adults feeding on the three weeds for 1 week increased significantly during autumn but did not change significantly during spring. Conversely, the titres of adults on citrus increased significantly during spring but did not change significantly during autumn.CONCLUSIOND. citri exhibited different adaptabilities to the three weed species, but the CLas titres of D. citri adults did not decrease on any of the weeds. These results suggested that the three weed species could help dispersing of D. citri and transmission of CLas while ideal host conditions were scarce or absent. © 2021 Society of Chemical Industry.

We report the complete genome sequence and annotation of “ Candidatus Nardonella dryophthoridicola” strain NardRF, obtained by sequencing its host bacteriome, Rhynchophorus ferrugineus , using Oxford Nanopore technology.