A novel Gram-stain-positive, rod-shaped bacterium with peritrichous flagella, designated C159T, was isolated from tomato rhizosphere soil. Growth of strain C159T occurred at 4-45 °C, pH 6.0-10.0 and in the presence of 0-5% NaCl. Based on 16S rRNA gene sequence analysis, strain C159T belongs to the genus Pseudoneobacillus and has a sequence similarity of 98.70% with Pseudoneobacillus rhizosphaerae JJ-79T. The whole genome of strain C159T was 4.65 Mb, with a DNA G+C content of 38.65 mol%. The average nucleotide identity between strain C159T and P. rhizosphaerae JJ-79T was 75.31%, and the digital DNA-DNA hybridization value was 19.6%, supporting its designation as a novel species of the genus Pseudoneobacillus. The major respiratory quinone of strain C159T was menaquinone 7; the major fatty acids were iso-C14:0, iso-C15:0 and anteiso-C15:0; and the major polar lipids are diphosphatidylglycerol, phosphatidylethanolamine and phosphatidylglycerol. Based on its phenotypic, chemotaxonomic, phylogenetic and genomic characteristics, strain C159T represents a novel species in the genus Pseudoneobacillus, for which the name Pseudoneobacillus rhizolycopersici sp. nov. is proposed. The type strain is C159T (=GDMCC 1.5552T=KCTC 43820T). In addition, strain C159T possesses the ability to produce indole-3-acetic acid (33.72 mg·l-1) and to mineralize insoluble organic phosphorus. In a pot experiment, this strain demonstrated a plant growth-promoting effect of 22.68-44.49% and a disease suppression efficacy of 89.29% against bacterial wilt of tomato caused by Ralstonia solanacearum. The above results confirm that strain C159T is a novel multifunctional rhizobacterium for the biocontrol of tomato bacterial wilt.

Root exudates serve a vital function in recruiting beneficial phosphate-solubilizing bacteria (PSB), thereby enhancing plant adaptation to phosphorus (P) deficiency. The C2H2-type zinc finger transcription factor STOP1 (sensitive to proton rhizotoxicity 1) regulates root organic acid (OA) exudation in plants. However, the impact of STOP1-regulated root OA exudation on rhizosphere microbial composition remains unexplored. This study revealed enhanced vegetation properties of soybean with higher P content in P-rich soils, while rhizosphere organic acid concentrations were elevated in P-poor soils. The soybean genotype YC03-3 in P-deficient soils specifically recruited three PSB in acid soils: Gammaproteobacteria_Incertae_Sedis, KF_JG30_C25, and Solirubrobacterales. These PSB abundances correlated positively with rhizosphere oxalate and citrate concentrations. Under P-sufficient conditions, GmSTOP1-3 overexpression in soybean plants increased oxalate and citrate exudation compared to YCO3-3 wild type (WT), leading to preferential colonization by the same three PSB species naturally present in P-deficient WT rhizosphere. The population dynamics of these PSB demonstrated strong positive correlations with the abundance of key genes involved in P cycling, particularly those governing acid/alkaline phosphatase activities and organic-P mineralization. Given the phosphate starvation-enhanced expression pattern of GmSTOP1-3, the findings indicate that specific PSB recruitment for organic-P remobilization in soybean rhizosphere depends on GmSTOP1-3-mediated oxalate and citrate exudation in P-deficient acid soils. This research establishes GmSTOP1-3 as a crucial regulator of rhizosphere microbiome assembly and P-acquisition efficiency in acid soils.

Phosphorus (P) availability in acidic red soils is primarily driven by phosphate-solubilizing bacteria (PSB), particularly those harboring the phoD and pqqC genes. However, the differential effect of soil pH on the community structure, interaction networks, and functional contributions of these two key PSB taxa remains unclear. In this study, we analyzed soil chemical properties as well as PSB abundance and community across typical acidic red soil regions. Our findings revealed that soil pH was the sole edaphic factor significantly correlated with the absolute abundance of both phoD and pqqC genes, showing a positive correlation with phoD but a negative correlation with pqqC. Redundancy analysis showed that the phoD-harboring community was influenced by a broader set of soil chemical properties, while the pqqC-harboring community was more exclusively and strongly shaped by pH and specific P fractions. Co-occurrence network analysis further demonstrated that non-acidic conditions (pH >= 5.5) promoted more complex and stable networks for the phoD-harboring community, whereas acidic conditions (pH < 5.5) intensified connectivity within the pqqC-harboring community. Random forest analysis revealed that pqqC-harboring bacteria explained a substantially greater proportion of available P (AP) variation (an incremental contribution of 14.72%) than did phoD-harboring bacteria (an incremental contribution of 3.50%). Further, core pqqC-harboring species belonging to the phylum Actinomycetota, particularly the genus Trebonia, were strongly positively correlated with AP content. In conclusion, our study suggests an association between pqqC-harboring bacteria (particularly Actinomycetota) and phosphorus availability in acidic red soils, offering a basis for targeted screening of PSB to improve soil fertility.

Mango stem-end rot is a globally significant postharvest disease that severely threatens the mango industry, primarily caused by Botryosphaeria dothidea. However, information on biocontrol agents targeting this pathogen in mango remains limited. In this study, we isolated and identified a strain of Bacillus velezensis MGL-B1 from mango leaf tissues for the first time, which exhibited broad-spectrum antifungal activity. Both in vitro and in vivo assays demonstrated that MGL-B1 effectively inhibited the growth of B. dothidea, with an in vivo biocontrol efficacy reaching 83.72 ± 5.10%, comparable to that of the commonly used chemical fungicide thiabendazole. Further mechanistic analysis revealed that MGL-B1 acts by directly disrupting the integrity of the pathogen's mycelial cell membrane. In addition, its released volatile organic compounds (VOCs) also displayed significant antifungal activity, with components such as 2-nonanone, 2-nonanol, and phenylethyl alcohol being confirmed to exert antifungal effects in in vitro fumigation assays. qPCR analysis showed that MGL-B1 treatment significantly upregulated the transcriptional levels of genes involved in plant-pathogen interaction, phenylpropanoid biosynthesis, and antioxidant defense pathways in mango fruits, with upregulation folds of 16.32, 37.19, and 75.93, respectively; meanwhile, the expression of browning-related genes such as polyphenol oxidase (PPO) was markedly suppressed. Whole-genome sequencing further revealed 14 biosynthetic gene clusters for antimicrobial compounds, including five unknown gene clusters. Collectively, B. velezensis MGL-B1 represents a promising biocandidate strain with multiple antifungal mechanisms and excellent control efficacy, providing a valuable resource for green and sustainable management of mango diseases.

Verticillium dahliae is a destructive soil-borne fungus with a broad host range, and its persistence in soil complicates control. Current measures, mainly resistant cultivars and chemicals, are limited and environmentally risky, promoting biocontrol as a green alternative. Here, we investigated the biocontrol mechanisms of Bacillus velezensis L33a against V. dahliae JR2 in tomato. In vitro assays on PDA plates at 26°C for 9 d showed that L33a inhibited JR2 by 58.6%, caused hyphal malformation and disruption, and its volatile organic compounds suppressed pathogen growth. In pot experiments, tomato roots dipped in JR2 suspension (1 ×10⁶ CFU/mL) for 30 min at 7 d after transplanting and grown for 21 d achieved 60.9% control efficacy. Physiological assays indicated reduced peroxidase and catalase activities, while qPCR revealed that L33a alone upregulated JA signaling (SlJAZ1, SlMYC2, SlPI II) and antioxidant (SlCAT, SlAPX) genes, with further enhancement upon JR2 co-treatment. To track their interactions, we generated GFP-labeled JR2 and RFP-labeled L33a; dual fluorescence labeling showed that L33a endophytically colonized Arabidopsis thaliana roots and competed with JR2 for the same niche, correlating with reduced pathogen colonization. Integrated metabolomic and transcriptomic analysis further revealed that L33a treatment altered pathways related to ABC transporters, amino acid metabolism, cell wall integrity, and energy metabolism in JR2, with tyrosine metabolism significantly enriched at both levels. Collectively, these findings suggest that L33a is a promising biocontrol strain for green management of tomato Verticillium wilt.

Citrus bacterial canker (CBC) is a globally important citrus disease caused by Xanthomonas citri subsp. citri (Xcc). Increasing evidence shows that the plant microbiome is crucial for host growth performance and health maintenance, among which the keystone taxa stand out due to their indispensable roles in microbiome homeostasis. However, how keystone taxa in the phyllosphere microbiome contribute to disease resistance remains unclear. In this study, we characterized the phyllosphere bacterial community of pomelo across an annual cycle and found that amino acids in leaves were the main drivers of the bacterial community structure. Meanwhile, five OTUs were identified as keystone taxa. A total of 587 phyllosphere bacterial strains were isolated, among which six strains belonging to Methylobacterium, Sphingomonas, Massilia, and Paenibacillus were identified as the corresponding keystone strains. We further constructed a consortium with these six strains to test its role in controlling CBC in planta. Network analysis reveals that consortium inoculation increased the phyllosphere bacterial community stability, whereas Xcc inoculation decreased it. However, dual inoculation of the consortium and Xcc restored community stability compared to the control. Interestingly, the inoculated keystone strains, if not all, still appeared as keystone taxa in the microbiomes of the control, consortium inoculation, and dual inoculation treatments, but not in the Xcc-inoculated treatment. Moreover, the consortium inoculation significantly increased the defense-related enzyme activities such as PPO, POD, and PAL in leaves, suggesting a triggered plant immune response. In vitro assays indicated that these keystone strains showed either antagonistic activity against Xcc or siderophore-producing activity. Finally, the consortium inoculation significantly reduced the disease index by 78% in planta. Taken together, these results suggest that the keystone taxa of the phyllosphere microbiome can confer disease resistance to the host via multiple mechanisms, especially by maintaining phyllosphere microbiome homeostasis.

Phosphorus (P) limitation is a major constraint of agricultural production due to the fixation of P by metal ions, especially in tropical and subtropical soils. Cover cropping in association with arbuscular mycorrhizal (AM) fungi can alleviate this stress in subtropical orchard soils. Siderophore-producing phosphate solubilizing bacteria (SPPSBs) hold potentials in effectively solubilizing sparingly soluble inorganic phosphate (SS-Pi) via secreting both ferric iron chelators and organic acids; however, the interactive relation between SP-PSBs and AM fungi in mobilizing SS-Pi has been largely underexplored. In this study, we inoculated clover (Trifolium repens) with Rhizophagus irregularis in compartmented rhizoboxes, characterized the PSB community, and isolated SP-PSB strains to assay their capacity to solubilize SS-Pi in subtropical orchard soils. Results demonstrate that AM inoculation enhanced the activities of gluconic dehydrogenase and pyrroloquinoline quinone synthase, facilitating the solubilization of SS-Pi and the increased plant biomass. Moreover, AM inoculation significantly reshaped both gcd- and pqqC-haboring communities, maintained the PSB community homeostasis via recruiting the keystone taxa. The capacity of isolated SP-PSB strains to solubilize SS-Pi was significantly positively related to their siderophore production. Bradyrhizobium elkanii (ASV37 and ASV322) enriched by AM inoculation showed high potentials in producing siderophores and solubilizing SS-Pi (e.g. Fe-bound P), representing a promising bacterial resource in increasing SS-Pi availability. Together, these data suggest a synergistic effect of SP-PSBs and AM fungi in promoting plant P nutrition via mobilizing SS-Pi, especially Fe-bound P, in the subtropical soils.

A novel Gram-stain-negative, aerobic, non-spore-forming, non-motile, golden-yellow pigmented, rod-shaped bacterium with starch-degrading ability, designated RRHN12T, was isolated from the rind of Hami melon with bacterial fruit blotch collected from Hainan Province, PR China. The novel strain exhibits the ability to produce siderophores and indole acetic acid, as well as to inhibit the hyphae growth and sporangia germination of Peronophythora litchii. The 16S rRNA gene sequence analysis showed that strain RRHN12T belonged to the genus Chryseobacterium with the highest similarities to Chryseobacterium aquifrigidense CW9T (99.1%) and Chryseobacterium gleum ATCC 35910T (99.0%). Phylogenomic analysis based on 92 core genes clearly showed that strain RRHN12T was most closely related to C. gleum ATCC 35910T. Genome-derived average nucleotide identity and digital DNA-DNA hybridization values between the novel strain and the closely related type strains were 87.14-92.68% and 32.49-49.40%, respectively, which are below the threshold values for species delineation. The major fatty acids were identified as iso-C15 : 0, iso-C17 : 0 3-OH, summed feature 3 (C16 : 1  ω7c and/or C16 : 1  ω6c) and summed feature 9 (10-methyl C16 : 0 and/or iso-C17 : 1  ω9c). Menaquinone-6 (MK-6) was the predominant respiratory quinone. The major polar lipids contained phosphatidylethanolamine and several unidentified aminolipids. The genomic DNA G+C content of strain RRHN12T was 37.5 mol%. Based on genomic, phenotypic and chemotaxonomic analyses, it should be considered as a novel species of the genus Chryseobacterium, for which the name Chryseobacterium amylolyticum sp. nov. was proposed, with RRHN12T (=GDMCC 1.4744T=JCM 37545T) as the type strain.

Cover crops are key components in agroecological systems, which promote soil fertility and are, in turn, promoted by soil microbes. Arbuscular mycorrhizal (AM) fungi and phosphorus-mobilizing bacteria (PMBs) can cooperate to enhance plant phosphorus (P) nutrition. However, it is unclear what kinds of PMBs are suitable for mobilizing P together with AM fungi. In a pot culture of bahiagrass, the widely used cover crop in subtropical orchards, we distinguished PMBs enriched by AM fungus, isolated PMB strains, and characterized their P-mobilizing ability. Results indicate that AM inoculation enriched Streptomyces, Bradyrhizobium, and Ralstonia in terms of both organic P (phoD) and inorganic P (pqqC) mobilizing bacterial communities, and depleting Ca-P, Al-P, and labile organic P in the rhizosphere. Among these genera, only Streptomyces PMBs were isolated at the highest frequency but displayed relatively low P-mobilizing abilities. Subsequently, we constructed two kinds of PMB consortia, one composed of PMB strains (Proteobacteria and Firmicutes) with high-P mobilizing abilities in vitro and the other one mainly composed of Streptomyces PMB strains (Actinobacteria) with low-P mobilizing abilities. Compared with the former, Streptomyces consortium inoculation enriched bacteria belonging to Actinobacteria and Bacteroidetes, and more efficiently promoted mycorrhizal cover crop plant P content, growth, and soil alkaline phosphatase activity by 7.2%-10.4%, 9.6%-10.9%, and 26.7%-31.0%. Our results suggested that a consortium of Streptomyces-dominated AM fungi-enriched PMBs was more suitable for mycorrhizal cover crops, providing new insights for the choice of PMBs in association with AM fungi to promote the plant growth of cover crops.

ABSTRACT Arbuscular mycorrhizal (AM) fungi can form symbiotic associations with plants and play a significant role in enhancing plant tolerance to acidic stress, wherein arbuscules serve as key structures in this process. However, the response patterns of arbuscule development and function under low pH conditions remain poorly understood. Previous studies have shown that abscisic acid (ABA) can regulate arbuscule development, but whether ABA regulates arbuscule development and function under low pH conditions is unknown. In this study, the model plant tomato (Solanum lycopersicum) was used as the host plant, inoculated with AM fungi to investigate the regulatory effects of low pH and exogenous ABA on arbuscule development and function. The results showed that (1) as time progressed, the mycorrhizal colonization increased, and arbuscules gradually developed from the main trunk to mature and senescent stages; however, low pH values inhibited arbuscule development. (2) High concentrations of ABA inhibited root growth and mycorrhizal colonization, whereas low concentrations promoted mycorrhizal colonization, with 10−7 M identified as the optimal concentration for maximizing mycorrhizal colonization. (3) Both low pH and ABA‐deficient mutants significantly inhibited mycorrhizal colonization, alkaline phosphatase activity, and the expression of genes related to arbuscular development. However, exogenous ABA did not significantly affect the expression of genes associated with arbuscular function. Low concentrations of ABA could restore the inhibition of arbuscule development and function caused by low pH and ABA‐deficient mutants. Additionally, low pH significantly inhibited the ABA content in mycorrhizae, while exogenous ABA treatment significantly increased the ABA content in mycorrhizae. Our research results indicate that low dosage of ABA enhances arbuscule formation and function, and recovers the inhibitory effect of low pH on this process. Low pH may regulate arbuscule development and function by modulating ABA in roots, and ABA may regulate mycorrhizal development by affecting lipid synthesis and transport.

Microbial responses to environmental gradients are often evaluated at community level, where bacterial communities are treated as a metaorganism. Considering that tolerance to physicochemical factors is an attribute of microbial lineages and functional groups, it seems necessary to figure out how well each taxon’s adaptation and sensitivity to different environments, especially for the prediction model of microbial response to climate change. Here, we reported the response of the soil predator myxobacteria, a group of bacteria cooperating and communicating to form multicellular structures and performing complex behaviors to prey on other microorganisms, across different forest soil by a comprehensive meta-analysis. The results showed that myxobacteria frequently occurred in forest soils. Soil properties explained a larger fraction of community variation (11.10%) than climatic factors (10.20%) and geographic variables (4.91%). The myxobacterial community showed a lower rate of species turnover than that of the whole bacterial community, and the proportions of both general and core myxobacteria in forest soil were higher than those of bacteria. Furthermore, the niche breadth of myxobacteria was wider than that of other micro-predators and common bacterial guilds inhabiting forest soil, providing statistical evidence for their potentially extensive environmental adaptation. This study provided robust statistical evidence for wider ranges of environmental breadths of myxobacteria and indicated the need to discriminate different taxa in predicting microbial responses to environmental gradients.

Denitrifying bacteria with flocculation capacity were dual-function microorganisms that can simultaneously remove nitrogen (N) and reduce suspended particles in wastewater, providing a sustainable bioremediation strategy. In this study, a novel denitrifying bacterium capable of producing bioflocculants, Thauera sp. JM12B12, was isolated and investigated. The results confirmed that this strain could completely remove NO3−-N and NO2−-N under microaerobic conditions with a low C/N ratio of 5, using lactate as the optimal carbon source. Notably, no other harmful inorganic N species were produced during denitrification, and total N removal efficiency consistently exceeded 93.0%. Optimal denitrification conditions include a pH range of 7–9, salinity of 0–1.5%, temperature of 25–40 °C, and static incubation. Remarkably, this strain synthesized extracellular bioflocculants during NO3−-N removal, achieving 91.4% flocculation efficiency with cell-free supernatant. Genome analyses revealed a complete denitrification pathway (possessing napA, two nirS, norB, nosZ) and 80 bioflocculant-related genes (polysaccharide production and protein secretion), highlighting its dual capacity for N and suspended particle removal. PCR also confirmed key denitrification genes. Therefore, JM12B12 could be a multifunctional microbial agent for N removal and flocculation, offering a sustainable solution for low C/N wastewater treatment, particularly valuable in recirculating aquaculture systems.

Two aerobic, Gram-stain-negative, gliding motility and rod-shaped bacterial strains, designated as B11T and D4T, were isolated from rhizosphere soil of Litchi (Litchi chinensis Sonn.) in Guangzhou, Guangdong Province, P.R. China. Phylogenetic analysis based on the 16S rRNA gene sequences showed that strains B11T and D4T belonged to the genus Flavobacterium and shared the highest similarity to Flavobacterium ginsenosidimutans THG 01T (98.2%) and Flavobacterium daemonensis JCM 19455T (98.0%), respectively. The DNA G+C contents of strains B11T and D4T were 33.9 and 34.1 mol%, respectively. Both the genome-derived average nucleotide identity and digital DNA-DNA hybridization values between the two strains and their closely related type species Flavobacterium sharifuzzamanii KCTC 62405T and Flavobacterium denitrificans DSM 15936T were below 39.3 and 90.5%, respectively. They all took iso-C15:0 and summed feature 3 (C16:1 ω6c and/or C16:1  ω7c) as the major fatty acids. The polar lipids of strain B11T contained phosphatidylethanolamine (PE), two unidentified aminophospholipids (APL1-2) and four unidentified lipids (L1-4), while strain D4T took phosphatidylethanolamine (PE), three aminophospholipids (APL1-3) and four unidentified lipids (L1-4). Menaquinone-6 was their predominant quinone. Based on the phenotypic, chemotaxonomic, phylogenetic and genomic analyses, strains B11T and D4T should be considered as two novel species of the genus Flavobacterium, for which the names Flavobacterium movens (type strain B11T=GDMCC 1.4103T=JCM 36423T) and Flavobacterium mesophilum (type strain D4T=GDMCC 1.4105T=JCM 36424T) are proposed.

Phyllosphere microbiome plays important roles in crop adaptation to the changing environments. Perennial woody crops undergo annual cycles with the changing weather parameters and the biological factors, which might shape the phyllosphere microbial community. In this study, we aimed to investigate the dynamics of phyllosphere microbiome of pomelo (Citrus maxima (Burm.) Merr.), an economically important horticultural crops worldwide, and to compare the respective contribution of the weather parameters and the biotic factors to the microbial community assembly, with special focus on the amino acids in leaves. Hi-Seq analysis revealed that both bacterial and fungal communities showed annual cycle dynamics, and the bacterial community in summer was much different from those in other seasons probably due to high temperature and precipitation. However, contribution of the biotic factors (e.g., leaf traits) (12%-29%) to microbial community assembly was higher than that of the weather parameters (4%-15%). Redundancy analysis indicated that the leaf amino acids significantly affected bacterial community while sugars significantly affected fungal community, highlighting the differential patterns of bacterial and fungal community as affected by the biotic factors. Finally, structure equation model showed that the weather parameters influenced microbial community colonizing pomelo leaves both in a direct way and in an indirect way via leaf traits (mainly amino acids). These results demonstrate the primary role of weather parameters and the key role of leaf amino acids in shaping phyllosphere microbiome.

Bacteriolytic myxobacteria are versatile micropredators and are proposed as potential biocontrol agents against diverse bacterial and fungal pathogens. Isolation of new myxobacteria species and exploration of effective predatory products are necessary for successful biocontrol of pathogens. In this study, a myxobacterium strain CY-1 was isolated from a soil sample of a pig farm using the Escherichia coli baiting method. Based on the morphological observation, physiological test, 16S rRNA gene sequence, and genomic data, strain CY-1 was identified as a novel species of the myxobacterial genus Archangium, for which the name Archangium lipolyticum sp. nov. was proposed. Subsequent predation tests indicated that the strain efficiently lysed drug-resistant pathogens, with a higher predatory activity against E. coli 64 than Staphylococcus aureus GDMCC 1.771 (MRSA). The lysis of extracellular proteins against ester-bond-containing substrates (tributyrin, tween 80, egg-yolk, and autoclaved drug-resistant pathogens) inspired the mining of secreted predatory products with lipolytic activity. Furthermore, a lipase ArEstA was identified from the genome of CY-1, and the heterologously expressed and purified enzyme showed bacteriolytic activity against Gram-negative bacteria E. coli 64 but not against Gram-positive MRSA, possibly due to different accessibility of enzyme to lipid substrates in different preys. Our research not only provided a novel myxobacterium species and a candidate enzyme for the development of new biocontrol agents but also reported an experimental basis for further study on different mechanisms of secreted predatory products in myxobacterial killing and degrading of Gram-negative and Gram-positive preys.

Myxobacteria are special bacteria with wide adaptability, which are rich sources of structurally diverse natural products with intriguing biological properties. Here, a gram-negative myxobacterium strain s54d21T was isolated from the sediment of a wetland park in China using the Escherichia coli baiting method. Based on 16S rRNA gene sequence and genomic data, the strain was demonstrated to be a novel species of a rare genus Hyalangium, designated Hyalangium ruber sp. nov (type strain s54d21T = GDMCC 1.1945T = JCM 39263T). The subsequent chemical investigation of the strain s54d21T led to the isolation of three rare 3,5,6-trisubstituted 2(1H)-pyrazinones, namely, hyalanones A–C (1–3), together with a known macrolactin A (4). Those new structures and their absolute configurations were unambiguously assigned by extensive analyses of spectroscopic data and density functional theory (DFT) calculations. In biological assays, compound 4 exhibited moderate cytotoxic activities against human cell lines RKO, A549, and NCM460 with IC50 values ranging from 27.21 to 32.14 μM.

The inoculants of arbuscular mycorrhizal fungi (AMF) propagated by the in vitro culture system is important in scientific research; however, the long-term storage reduces the spore germination rate. The propagules of AMF consist of three components, including spores, hyphae and colonized root fragments. It is well known that cold storage can improve the germination rate of AMF spores, with limited investigations on the germination of other propagules. In this study, AMF inoculants were stored at 25°C or at 4°C (cold storage) to investigate the effect of cold storage on the propagule viability of the AMF Rhizophagus irregularis DAOM197198. The germination rate of propagules (spores, hyphae, root fragments) and their colonization ability were determined at 3 and 6 months after storage. The results showed that the spore germination rate remained unchanged after storage for 0 and 1 month at 25°C, but decreased rapidly after storage for 3 months. Furthermore, we investigated the hyphal germination rate for the first time. The germination rates of spores, hyphae and root fragments were significantly higher under cold storage compared to those at 25°C. Additionally, we classified the germ tubes of hypha into two types: long-type (L-type) and short type (S-type). The germination rate and the proportion of L-type germ tubes of hyphae significantly increased with cold storage time, which was conducive to colonization. The results of mycorrhizal colonization confirmed that cold storage significantly increased the colonization of hypha compared with 25°C treatment. Cold storage may break the dormancy of AMF propagules and activate related enzymes to promote the germination and colonization of propagules, which needs further investigation.

A novel plant- beneficial bacterium strain, designated as JGH33T, which inhibited Peronophythora litchii sporangia germination, was isolated on Reasoner's 2A medium from a litchi rhizosphere soil sample collected in Gaozhou City, Guangdong Province, PR China. Cells of strain JGH33T were Gram- stain- positive, aerobic, non- motile, bent rods. The strain grew optimally at 30-37 degrees C and pH 6.0-8.0. Sequence similarity analysis based on 16S rRNA genes indicated that strain JGH33T exhibited highest sequence similarity to Sinomonas albida LC13T (99.2 %). The genomic DNA G+C content of the isolate was 69.1 mol%. The genome of JGH33T was 4.7 Mbp in size with the average nucleotide identity value of 83.45 % to the most related reference strains, which is lower than the species delineation threshold of 95 %. The digital DNA-DNA hybridization of the isolate resulted in a relatedness value of 24.9 % with its closest neighbour. The predominant respiratory quinone of JGH33T was MK- 9(H2). The major fatty acids were C15: 0 anteiso (43.4 %), C16: 0 iso (19.1 %) and C17: 0 anteiso (19.3 %), and the featured component was C18:3 omega 6c (1.01 %). The polar lipid composition of strain JGH33T included diphosphatidylglycerol, phosphatidylglycerol, dimannosylglyceride, phosphatidylinositol and glycolipids. On the basis of polyphasic taxonomy analyses data, strain JGH33T represents a novel species of the genus Sinomonas, for which the name Sinomonas terricola sp. nov. is proposed, with JGH33T (=JCM 35868T=GDMCC 1.3730T) as the type strain.

Citrus is the typical mycorrhizal fruit tree species establishing symbiosis with arbuscular mycorrhizal (AM) fungi. However, arbuscule development and senescence in colonized citrus roots, especially in response to drought stress, remain unclear, which is mainly due to the difficulty in clearing and staining lignified roots with the conventional method. Here, we improved the observation of colonized roots of citrus plants with the sectioning method, which enabled the clear observation of AM fungal structures. Furthermore, we investigated the effects of one week of drought stress on arbuscule development and senescence with the sectioning method. Microscopy observations indicated that drought stress significantly decreased mycorrhizal colonization (F % and M %) although it did not affect plant growth performance. Fluorescence probes (WGA 488 and/or Nile red) revealed that drought stress inhibited arbuscule development by increasing the percentage of arbuscules at the early stage and decreasing the percentages of arbuscules at the midterm and mature stages. Meanwhile, drought stress accelerated arbuscule senescence, which was characterized by the increased accumulation of neutral lipids. Overall, the sectioning method developed in this study enables the in-depth investigation of arbuscule status, and drought stress can inhibit arbuscule development but accelerate arbuscule senescence in the colonized roots of citrus plants. This study paves the way to elaborately dissecting the arbuscule dynamics in the roots of fruit tree species in response to diverse abiotic stresses.