Publications

4524

Huanglongbing (HLB), which is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), is a devastating disease that affects citrus trees worldwide. Because of the pervasiveness of the bacteria and psyllid vector, the disease is considered endemic in Florida. Although the effects of CLas on tree growth and physiology have been investigated for decades, most studies compared infected and noninfected trees under greenhouse conditions. This study used newly planted field-grown ‘Valencia’ sweet orange (Citrus sinensis) trees on two different rootstocks to monitor the distribution and accumulation of CLas in aboveground and belowground tissues following natural psyllid colonization and assess tree physiological responses and biomass reductions under HLB-endemic conditions. Trees were transplanted into the field with individual protective covers (IPCs), which are used to exclude psyllids and prevent infection. Openings were cut in the IPCs of half of the trees; to promote infection, these IPCs were temporarily removed during the main vegetative flushing period when psyllid populations were high. All trees that were exposed to psyllids became infected and displayed the symptoms typically associated with HLB. Throughout the study, higher levels of CLas were detected in the leaves compared with those in the fibrous roots. Trees that were not exposed to psyllids remained noninfected and healthy. After 18 months, a subset of trees was excavated to assess biomass differences between infected and noninfected trees. Infected trees had root system reductions of 37% and shoot system reductions of 20%, thereby significantly reducing the belowground-to-aboveground biomass ratio. Fibrous root loss was 49% and more severe than the loss of the rest of the root tissue. This study is the first to demonstrate the full extent of damage caused by CLas infection under natural HLB-endemic conditions. The results confirm previous observations that suggested fibrous root loss as one of the major consequences of infection and colonization with CLas. They also reinforce the benefits of using IPCs to prevent infection of young citrus trees during the first years of growth in the field.

A new disease syndrome of zucchini squash was observed in Southern Italy, in 2018 and again in 2020. Affected plants were severely stunted and leaves were bent downwards, small, stiff, thick, leathery, and had interveinal chloroses. In addition, flowers were virescent and fruits were deformed and often cracked. Disease incidence was 20 and 30% in two different zucchini cultivations in Campania region (Southern Italy). Tomato yellow leaf curl New Delhi virus (ToLCNDV) was detected in eight samples, by loop-mediated isothermal amplification–based (LAMP) kit and by PCR and Sanger sequencing of the AV1 gene. Phytoplasmas were detected in the same samples using nested PCR assays with primer pairs P1/P7 and R16F2n/R16R2. Phytoplasma associations in plant samples were confirmed using specific primers for the multilocus genes SecY, tuf and rp. Sequence comparison of multilocus genes and phylogenetic analyses of the 16S rDNA gene confirmed the association of a phytoplasma strain closely related to ‘Candidiatus Phytoplamsa asteris’. This is the first report of mixed infections of ToLCNDV and a putative ‘Ca. Phytoplamsa asteris’ strain in zucchini, associated with a new Squash-Phytoplasma-Begomovirus (SqPB) disease syndrome.

Abstract Backgroud The genus Mesorhizobium is shown by phylogenomics to be paraphyletic and forms part of a complex that includes the genera Aminobacter, Aquamicrobium, Pseudaminobacter and Tianweitania. The relationships for type strains belong to these genera need to be carefully re-evaluated. Results The relationships of Mesorhizobium complex are evaluated based on phylogenomic analyses and overall genome relatedness indices (OGRIs) of 61 type strains. According to the maximum likelihood phylogenetic tree based on concatenated sequences of 539 core proteins and the tree constructed using the bac120 bacterial marker set from Genome Taxonomy Database, 65 type strains were grouped into 9 clusters. Moreover, 10 subclusters were identified based on the OGRIs including average nucleotide identity (ANI), average amino acid identity (AAI) and core-proteome average amino acid identity (cAAI), with AAI and cAAI showing a clear intra- and inter-(sub)cluster gaps of 77.40–80.91% and 83.98–86.16%, respectively. Combined with the phylogenetic trees and OGRIs, the type strains were reclassified into 15 genera. This list includes five defined genera Mesorhizobium, Aquamicrobium, Pseudaminobacter, Aminobacterand Tianweitania, among which 40/41 Mesorhizobium species and one Aminobacter species are canonical legume microsymbionts. The other nine (sub)clusters are classified as novel genera. Cluster III, comprising symbiotic M. alhagi and M. camelthorni, is classified as Allomesorhizobium gen. nov. Cluster VI harbored a single symbiotic species M. albiziae and is classified as Neomesorhizobium gen. nov. The remaining seven non-symbiotic members were proposed as: Neoaquamicrobium gen. nov., Manganibacter gen. nov., Ollibium gen. nov., Terribium gen. nov., Kumtagia gen. nov., Borborobacter gen. nov., Aerobium gen. nov.. Furthermore, the genus Corticibacterium is restored and two species in Subcluster IX-1 are reclassified as the member of this genus. Conclusion The Mesorhizobium complex are classified into 15 genera based on phylogenomic analyses and OGRIs of 65 type strains. This study resolved previously non-monophyletic genera in the Mesorhizobium complex.

Two Gram-negative, aerobic, rod-shaped bacterial strains, 7MK25T and 6Y81T, were isolated from forest soil of Dinghushan Biosphere Reserve, Guangdong Province, PR China. Based on the results of 16S rRNA gene sequence analysis, strain 7MK25T showed the highest similarity (93.6 %) to Methyloferula stellata AR4T, followed by Bosea thiooxidans DSM 9653T (93.3 %). Strain 6Y81T had the highest similarity of 97.9 % to Lichenibacterium minor RmlP026T, followed by Lichenibacterium ramalinae RmlP001T (97.2 %). Phylogenomic analysis using the UBCG and PhyloPhlAn methods consistently showed that strain 7MK25T formed a sister clade to Boseaceae, while strain 6Y81T formed an independent clade within the genus Lichenibacterium, both in the order Hyphomicrobiales. The digital DNA–DNA hybridization and average nucleotide identity values between strains 7MK25T, 6Y81T and their close relatives were in the ranges of 19.1–29.9 % and 72.5–85.5 %, respectively. The major fatty acids of 7MK25T were summed feature 8 (C18 : 1 ω7c/C18 : 1 ω6c), C19 : 0 cyclo ω8c, C16 : 0 and C17 : 0 cyclo, while those of 6Y81T were summed feature 8 (C18 : 1 ω7c/C18 : 1 ω6c), C16 : 0 and C16 : 0 3-OH. Strains 7MK25T and 6Y81T took diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylcholine as their dominant polar lipids, and Q-10 as their major respiratory quinone. On the basis of phenotypic and phylogenetic data, strain 7MK25T is proposed to represent a novel species of a novel genus with name Terrirubrum flagellatum gen. nov., sp. nov., within a novel family Terrirubraceae fam. nov., with 7MK25T (=KCTC 62738T=GDMCC 1.1452T) as its type strain. Strain 6Y81T represents a novel species in the genus Lichenibacterium, for which the name Lichenibacterium dinghuense sp. nov. (type strain 6Y81T=KACC 21 727T=GDMCC 1.2176T) is proposed. Rhodoblastaceae fam. nov. with Rhodoblastus as the type genus is also proposed to solve the non-monophylectic problem of the family Roseiarcaceae.