The application of salt-tolerant plant growth-promoting rhizobacteria (PGPR) represents a promising strategy to alleviate salt stress in crops. However, the mechanisms by which volatile organic compounds (VOCs) from actinomycetes mitigate salinity stress remain unclear. In this study, exposure to VOCs from the halotolerant actinomycete Glutamicibacter halophytocola KLBMP 5180 significantly promotes the growth of tomato seedlings under salt stress, as evidenced by increased fresh weight, lateral root number, and chlorophyll content. Physiological analyses show that VOCs exposure reduces oxidative damage and enhances antioxidant enzyme activity. Furthermore, elevated levels of total phenolics and flavonoids are detected, along with decreased sodium ion accumulation and an improved Na⁺/K⁺ ratio. Transcriptome analysis reveales that VOCs treatment upregulates genes associated with pathways involved in phenylpropanoid biosynthesis, glutathione metabolism, MAPK signaling, plant hormone signal transduction, and plant-pathogen interactions under saline conditions. VOCs also increase levels of endogenous auxin, jasmonic acid (JA), and 1-aminocyclopropane-1-carboxylic acid (ACC), while activating genes related to their respective signaling pathways. The salt tolerance enhancement mediated by VOCs is compromised by ethylene and JA inhibitors, suggesting that KLBMP 5180-derived VOCs improve tomato salt tolerance by stimulating ethylene synthesis and signal transduction. Additional VOCs profiling identifies several key bioactive compounds, including 3-methyl-1-butanol, phenylethyl alcohol, 2-aminopropanediamide, and 2-undecanone. These findings demonstrate that VOCs from G. halophytocola KLBMP 5180 enhance tomato growth under salt stress and advance our understanding of the role of actinomycetes in pronoting plant growth and mitigating salinity stress.

Root rot caused by Rhizopus stolonifer significantly diminished the yield and quality of sweet potatoes. Currently, the control of root rot predominantly relied on toxic chemical pesticides, with a notable lack of effective alternative strategies. Microbial volatile organic compounds (VOCs) are regarded as an environmentally friendly and safe method for controlling postharvest diseases. This study investigated the in vitro and in vivo effects of VOCs produced by the Nocardiopsis dassonvillei MI-S24 strain against R. stolonifer. The results demonstrated that VOCs exhibit strong antifungal activity against various pathogenic fungi. They significantly inhibited hyphal growth (91.85 %) and spore germination of R. stolonifer while inducing morphological changes. Furthermore, during storage at 28 degrees C for 20 days, VOCs fumigation effectively mitigated the incidence of postharvest sweet potato root rot. It enhanced the activity of CAT, POD, SOD, PPO, and PAL enzymes by 84.62 %, 38.19 %, 47.98 %, 84.52 %, and 19.50 % respectively in sweet potatoes, safeguarded soluble sugars and starch from degradation, and preserved overall quality. Analysis of volatile components coupled with antifungal validation revealed that 2-methyl-3-(methylthio) furan and 2-undecanol exhibited significant antifungal activity against R. stolonifer. Further research indicated that VOCs affected cell membrane integrity in R. stolonifer, stimulated reactive oxygen species (ROS) accumulation, reduced antioxidant enzyme activity, elevated malondialdehyde levels, induced oxidative damage, and disrupted ATP synthesis as well as TCA cycle processes by diminishing mitochondrial membrane potential alongside succinate dehydrogenase activity, ultimately leading to mitochondrial dysfunction. This study presents a novel biological strategy aimed at preventing and controlling soft rot disease in postharvest sweet potatoes.

Gray mold disease caused by Botrytis cinerea severely compromises postharvest strawberry quality. As an ecofriendly alternative to chemical fungicides, biological control agents based on microbial volatile organic compounds (VOCs) have emerged as promising candidates. This study developed a VOC-producing fumigant derived from wheat grain-cultured Streptomyces setonii WY228 to control gray mold in postharvest strawberries. Key parameters influencing the fumigant's antifungal activity, including inoculation volume, culture temperature, incubation time, moisture content and dosage of wheat grains, were systematically optimized to establish a preparation strategy. Storage at 4 degrees C with vacuum maintained 70 % B. cinerea inhibition with sustained viability of fumigant for 90 d. Application of the fumigant at 26.7 g L-1 suppressed gray mold in postharvest strawberries. The firmness, ascorbic acid content, titratable acidity, and superoxide dismutase activity of fumigated strawberries were 42 %, 54 %, 24 %, and 160 % higher than those of the positive control group, respectively. Four main compounds in VOCs were identified as methyl 2-furoate, 2-methylbutyric acid, 4-methylpentanoic acid, and cyclopentanone, and exhibited antifungal activity and gray mold suppression in strawberries. The wheat grain solid fumigant prepared by S. setonii WY228 demonstrates potential as a biological control agent for gray mold suppression in postharvest strawberries.

Botrytis cinerea causes gray mold disease on postharvest fruit, resulting in serious economic losses. Fumigation with natural volatile organic compounds can replace artificially synthesized fungicides to control postharvest fungal decay of agricultural products. The antifungal effects and potential mechanisms of microbial volatile compound 2-methylbutyric acid (2-MBA) against B. cinerea were investigated in this study. Results showed that 2-MBA inhibited mycelial growth and spores germination, altered mycelial morphology and reduced metabolic vitality of B. cinerea. Moreover, 2-MBA induced oxidative damage by stimulating ROS accumulation, increasing malondialdehyde levels, and decreasing the ratio of reduced/oxidized glutathione. Further studies indicated that 2-MBA reduced mitochondrial membrane potential, interfered TCA cycle by decreasing succinate dehydrogenase activity and ATP content, ultimately leading to decreased mitochondrial activity. In addition, key genes involved in botrydial biosynthesis and mycelial growth were down-regulated. 2-MBA controlled postharvest gray mold by inducing resistance to B. cinerea in cherry tomatoes. Taken together, 2-MBA is an effective and promising fumigant to control gray mold caused by B. cinerea.

Plant-associated streptomycetes play important roles in plant growth and development. However, knowledge of volatile-mediated crosstalk between Streptomyces spp. and plants remains limited. In this study, we investigated the impact of volatiles from nine endophytic Streptomyces strains on the growth and development of plants. One versatile strain, Streptomyces setonii WY228, was found to significantly promote the growth of Arabidopsis thaliana and tomato seedlings, confer salt tolerance, and induce early flowering and increased fruit yield following volatile treatment. Analysis of plant growth-promoting traits revealed that S. setonii WY228 could produce indole-3-acetic acid, siderophores, ACC deaminase, fix nitrogen, and solubilize inorganic phosphate. These capabilities were further confirmed through genome sequencing and analysis. Volatilome analysis indicated that the volatile organic compounds emitted from ISP-2 medium predominantly comprised sesquiterpenes and 2-ethyl-5-methylpyrazine. Further investigations showed that 2-ethyl-5-methylpyrazine and sesquiterpenoid volatiles were the primary regulators promoting growth, as confirmed by experiments using the terpene synthesis inhibitor phosphomycin, pure compounds, and comparisons of volatile components. Transcriptome analysis, combined with mutant and inhibitor studies, demonstrated that WY228 volatiles promoted root growth by activating Arabidopsis auxin signaling and polar transport, and enhanced root hair development through ethylene signaling activation. Additionally, it was confirmed that volatiles can stimulate plant abscisic acid signaling and activate the MYB75 transcription factor, thereby promoting anthocyanin synthesis and enhancing plant salt stress tolerance. Our findings suggest that aerial signaling-mediated plant growth promotion and abiotic stress tolerance represent potentially overlooked mechanisms of Streptomyces-plant interactions. This study also provides an exciting strategy for the regulation of plant growth and the improvement of horticultural crop yields within sustainable agricultural practices.

Pseudomonas chlororaphis subsp. aureofaciens SPS-41, a beneficial bacterium isolated from rhizosphere of sweet potato, has been explored as a biological control agent for the management of soil-borne diseases. In this study, its plant growth-promoting and antifungal properties against two major tomato fungal pathogens (Fusarium oxysporum and Botrytis cinerea) were evaluated using in vitro and in vivo experiments. Strain SPS-41 produced indole-3-acetic acid (IAA), siderophore, biofilm, protease, amylase, chitinase, and could solubilize phosphate. It also inhibited fungal mycelial growth, with an inhibition rate of more than 80% in dual culture assays. Volatile organic compounds (VOCs) produced by SPS-41 also exhibited strong antifungal activity. In addition, SPS-41 promoted the growth of tomato seedlings in greenhouse conditions, and the severity of wilt disease and grey mold decreased significantly with rhizospheric inoculation of cell suspension and VOC fumigation comparing to untreated controls. The composition of VOCs emitted by SPS-41 was analyzed, and 57 volatiles were identified by SPME/GC-MS. Of these, 2-tridecanone, 2-undecanone, 2-nonanone, 2-acetylthiazole, 9-decenyl acetate, 2-hexadecanol, α-farnesene, and nonanal were the major volatiles. Nonanal, 2-nonanone and 2-acetylthiazole were confirmed to inhibit the growth of all three pathogens. The complete genome of strain SPS-41 was sequenced and annotated. We identified a series of gene clusters responsible for the synthesis of active secondary metabolites with antimicrobial and biocontrol functions. Moreover, several genes related to plant growth promotion, such as nitrogen fixation and indole-3-acetic acid and siderophore biosynthesis were also identified. This research indicates that strain SPS-41 is a plant growth promoting bacterium with the potential for development as a new biocontrol agent for tomato fungal pathogens.

Microbial exopolysaccharides (EPSs) can promote plants growth and protect them against various abiotic stresses, but the role of actinobacteria-produced EPSs in plant growth promoting is still less known. Here, we aim to explore the effect of EPSs from an endophyte Glutamicibacter halophytocota KLBMP 5180 on tomato seeds germination and seedlings growth under salt stress. Our study revealed that 2.0 g/L EPSs resulted in increased seed germination rate by 23.5 % and 11.0 %, respectively, under 0 and 200 mM NaCl stress conditions. Further pot experiment demonstrated that EPSs significantly promoted seedlings growth under salt stress, with increased height, root length and fibrous roots number. Plant physiological traits revealed that EPSs increased chlorophyll content, enhanced the activity of antioxidant enzymes, soluble sugar, and K+ concentration in seedlings; malondialdehyde and Na+ contents were reduced. Additionally, auxin, abscisic acid, jasmonic acid, and salicylic acid were accumulated significantly in seedlings after EPSs treatment. Furthermore, we identified 1233 differentially expressed genes, and they were significantly enriched in phytohormone signal transmission, phenylpropanoid biosynthesis, and protein processing in endogenous reticulum pathways, etc. Our results suggest that KLBMP 5180-produced EPSs effectively ameliorated NaCl stress in tomato plants by triggering complex regulation mechanism, and showed application potentiality in agriculture.

Botrytis cinerea is a necrotrophic fungal pathogen that causes gray mold disease in more than 1400 plant species. The biocontrol effect of microbial volatile compounds on postharvest diseases has become a research hotspot. This study revealed the biocontrol effects of volatiles released by Bacillus tequilensis XK29 on B. cinerea in vitro and in vivo. The results showed that the strain XK29 volatiles inhibited the mycelial growth of B. cinerea. The inhibition effect of strain XK29 volatiles against B. cinerea was increased by optimizing the wheat seeds weight, suspension concentration and inoculation volume of strain XK29, and the pre-culture time. The volatiles of strain XK29 affected the mycelial development, the formation of conidia, and the metabolic activity of B. cinerea. Selected individual monomers and artificial mixtures showed strong antifungal activity against B. cinerea. The volatiles of strain XK29 reduced the lesion diameter and disease severity of cherry tomatoes after inoculation with B. cinerea. These results indicate that the fumigation with B. tequilensis XK29 volatiles is a promising approach to protect cherry tomatoes from gray mold disease. This is also the first study to prove the antifungal effect of B. tequilensis on B. cinerea and its biocontrol effect on postharvest cherry tomatoes.

Black spot disease caused by Ceratocystis fimbriata has caused huge economic losses to worldwide sweet potato production. At present, the control of C. fimbriata mainly depends on toxic fungicides, and there is a lack of effective alternative strategies.

绿针假单胞菌(Pseudomonas chlororaphis)是目前研究较多的生防菌种之一.19世纪初被Miguela首次分离,将其鉴定为假单胞菌(Pseudomonas),并将机会性病原菌绿脓杆菌作为其模式菌株,而后Peix于2007年重新将其分类为绿针假单胞菌(P.chlororaphis).目前该菌种已报道有4个亚种,均可产生有颜色的吩嗪类化合物抗生素.该种的菌株多分离自植物根际,对植物抵抗病原菌、线虫等的侵染起到保护作用.本文结合我们分离获得的1株绿针假单胞菌的功能研究、基因组分析及相关文献资料,系统描述了绿针假单胞菌的分类学特征、基因组特点、代谢产物功能及其应用开发前景等,以期为绿针假单胞菌在活性代谢产物发掘、基因功能研究及其农业应用等方面提供借鉴.

Arctium lappa L. is one of the medicinal and food homologous plants in China, which is rich in nutrients and medicinal ingredients. The use of plant growth-promoting (PGP) endophytic bacteria is an alternative to reducing chemical fertilizers in agricultural production. The aim of this study was to analyze the diversity of endophytic bacteria in different cultivars of A. lappa L. collected from two geographical locations in China and evaluate PGP traits of the isolates and their potential PGP ability in greenhouse condition. Endophytic bacterial community was investigated by culture-dependent and culture-independent methods. Isolates were screened and investigated for multiple PGP traits, and representative strains were inoculated host seedlings to evaluate the growth promoting effect. A total of 348 endophytic bacteria were obtained and they were distributed into 4 phyla and 30 genera. In addition, high throughput sequencing revealed more abundant bacterial community, including 17 bacterial phyla and 207 genera. A high proportion of PGP traits were detected, including production of indole acetic acid, siderophore, ammonia and phosphate solubilization. Four representative strains with multiple PGP traits of the most dominant genera (Bacillus, Pantoea, Microbacterium and Pseudomonas) were further selected for host inoculation and growth promoting evaluation, and they significantly increase seedlings length, root length and fresh weight. This study demonstrated that A. lappa L. harbors abundant endophytic bacteria, and some endophytic bacteria showed good potential for the development of microbial fertilizer in the future.

Pseudomonas chlororaphis subsp. aureofaciens SPS-41 is a plant growth-promoting rhizobacetrium (PGPR) with biocontrol potential, which was isolated from the rhizosphere of sweet potato in Xuzhou, Jiangsu Province, China. Our previous study demonstrated that volatile organic compounds (VOCs) produced by SPS-41 inhibited black spot disease fungi Ceratocystis fimbriata in postharvest sweet potatoes and a variety of other plant pathogens, and the VOCs also displayed strong nematocidal activity. In order to further explore the application potential of this strain, here, we report the complete genome sequence of strain SPS-41. The genome consists of one chromosome (6,757,898 bp) with a G+C content 63.10%, which contains 5,951 coding genes, 67 tRNA genes, 16 rRNA genes and 85 other ncRNA genes. No plasmid was detected. The information of the genome will provide resources for studying the biocontrol mechanism of this strain.

[This corrects the article DOI: 10.1371/journal.pone.0240187.].

In this study, a novel halotolerant phenol-degrading yeast strain, SDP-1, was isolated from a coastal soil in Jiangsu, China, and identified as Candida tropicalis by morphology and rRNA internal transcribed space region sequence analysis. Strain SDP-1 can efficiently remove phenol at wide ranges of pH (3.0-9.0), temperature (20-40 °C), and NaCl (0-5%, w/v), as well as the tolerance of Mn2+, Zn2+ and Cr3+ in aquatic phase. It also utilized multiple phenol derivatives and aromatic hydrocarbons as sole carbon source and energy for growth. Free cells of SDP-1 were able to degrade the maximum phenol concentration of 1800 mg/L within 56 h under the optimum culture conditions of 10% inoculum volume, pH 8.0, 35 °C and 200 rpm agitation speed. Meanwhile, SDP-1 was immobilized on sodium alginate, and the capability of efficiently phenol degradation of free cells and immobilized SDP-1 were evaluated. Shortened degradation time and long-term utilization and recycling for immobilized SDP-1 was achieved compared to free cells. The 1200 mg/L of phenol under 5% NaCl stress could be completely degraded within 40 h by immobilized cells. In actual industrial coking wastewater, immobilized cells were able to completely remove 383 mg/L phenol within 20 h, and the corresponding chemical oxygen demand (COD) value was decreased by 50.38%. Besides, in phenol-contained salinity soil (3% NaCl), 100% of phenol (500 and 1000 mg/kg) removal efficiency was achieved by immobilized SDP-1 within 12 and 26 days, respectively. Our study suggested that versatile yeast Candida tropicalis SDP-1 could be potentially used for enhanced treatment of phenol-contaminated wastewater and soil under hypersaline or no-salt environmental conditions.

Pseudomonas chlororaphis subsp. aureofaciens SPS-41 is a plant growth-promoting rhizobacterium with biocontrol potential that was isolated from the rhizosphere of sweet potato in Xuzhou, Jiangsu Province, China. Our previous study demonstrated that volatile organic compounds (VOCs) produced by SPS-41 inhibited black spot disease fungi Ceratocystis fimbriata in postharvest sweet potatoes and a variety of other plant pathogens, and the VOCs also displayed strong nematocidal activity. In order to further explore the application potential of this strain, we here report the complete genome sequence of strain SPS-41. The genome consists of one chromosome (6,757,898 bp) with a G+C content 63.10%, which contains 5,951 coding genes, 67 transfer RNA genes, 16 ribosome RNA genes, and 85 other non-coding RNA genes. No plasmid was detected. The information of the genome will provide resources for studying the biocontrol mechanism of this strain.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Black spot disease caused by Ceratocystis fimbriata is destructive to the production, transportation, and storage of sweet potato. The antifungal effects of Bacillus tequilensis XK29 against C. fimbriata through volatile organic compounds (VOCs) were evaluated in this study. The activated carbon assay proved that XK29 could exert antibiotic effects through volatiles. By optimizing the wheat seed weight, inoculation method, concentration, volume, and time, the antifungal activity of XK29 was significantly improved. XK29 fumigation inhibited spore formation and germination and changed the cell morphology of C. fimbriata. During the storage of sweet potato tuber roots, XK29 effectively controlled black spot disease and reduced the weight loss and malondialdehyde content. Metabolomic analysis revealed that 21 volatile compounds were released from XK29. Isovaleric acid, isobutyric acid, and 2-methylbutanoic acid effectively inhibited the growth of C. fimbriata. These results indicate that B. tequilensis XK29 has a good potential to be developed as a microbial fumigation agent.

Ceratocystis fimbriata is the pathogen of black rot disease, which widely exists in sweet potato producing areas all over the world. The antifungal activity of volatile organic compounds (VOCs) released by Pseudomonas chlororaphis subsp. aureofaciens SPS-41 against C. fimbriata was reported in our previous study. In this study, we attempted to reveal the underlying antifungal mechanism of SPS-41 volatiles. Our results showed that the VOCs released by SPS-41 caused the morphological change of hyphae, destroyed the integrity of cell membrane, reduced the content of ergosterol, and induced massive accumulation of reactive oxygen species in C. fimbriata cells. Furthermore, SPS-41 fumigation decreased the mitochondrial membrane potential, acetyl-CoA and pyruvate content of C. fimbriata cells, as well as the mitochondrial dehydrogenases activity. In addition, the VOCs generated by SPS-41 reduced the intracellular ATP content and increased the extracellular ATP content of C. fimbriata. In summary, SPS-41 fumigation exerted its antifungal activity by inducing oxidative stress and mitochondrial dysfunction in C. fimbriata.

Black spot disease, which is caused by the pathogenic fungal Ceratocystis fimbriata, seriously affects the production of sweet potato and its quality during postharvest storage. In this study, the preliminary identification of the rhizosphere actinomycete strain SPS-33, and its antifungal activity of volatiles in vitro and in vivo was investigated. Based on morphological identification and phylogenetic analysis of the 16S rRNA gene sequence, strain SPS-33 was identified as Streptomyces lavendulae. Volatile organic compounds (VOCs) emitted by SPS-33 inhibited mycelial growth and sporulation of C. fimbriata in vitro and also induced a series of observable hyphae morphological changes. In an in vivo pathogenicity assay, exposure to SPS-33 significantly decreased the lesion diameter and water loss rate in sweet potato tuberous roots (TRs) inoculated with C. fimbriata. It increased the antioxidant enzymes’ activities of peroxidase, catalase, and superoxide dismutase as well as decreased malondialdehyde and increased total soluble sugar. In the VOC profile of SPS-33 detected by a headspace solid-phase micro extraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS), heptadecane, tetradecane, and 3-methyl-1-butanol were the most abundant compounds. 2-Methyl-1-butanol, 3-methyl-1-butanol, pyridine, and phenylethyl alcohol showed strong antifungal effects against C. fimbriata. These findings suggest that VOCs from S. lavendulae SPS-33 have the potential for pathogen C. fimbriata control in sweet potato postharvest storage by fumigant action.

Plant parasitic nematodes (PPNs) cause huge agricultural and silvicultural losses every year. Most studies that investigated various mechanisms for killing nematodes used the model organism Caenorhabditis elegans. Pseudomonas chlororaphis is an important biocontrol bacterium with great application potential in agriculture. Pseudomonas chlororaphis strains have the potential to control PPNs. In our previous study, we obtained one P. chlororaphis-like bacterium, namely, SPS-41, from the rhizosphere of sweet potato. This strain exhibited wide-spectrum of antifungal activity. In the present study, the nematicidal activity of the P. chlororaphis SPS-41 strain against C. elegans was examined. This strain was identified as Pseudomonas chlororaphis subsp. aureofaciens via 16S rRNA phylogenetic analysis. Results showed that this strain could kill nematodes in a short time on both fast- and slow-media. Both the liquid culture and volatile organic compounds (VOCs) produced by this strain exhibited strong nematicidal activity. VOCs fumigation by the SPS-41 strain triggered the immune response of C. elegans. A total of 23 VOCs from this strain were analyzed via solid-phase microextraction gas chromatography–mass spectrometry. Seven major VOCs were picked to test its nematicidal activity. Four of these VOCs, namely, 2-methyl-1-butanol, octanoic acid ethyl ester, ethyl acetate, and isobutyl acetate, could kill nematodes. Octanoic acid ethyl ester displayed the strongest nematocidal activity. The results suggested that the SPS-41 strain and its VOCs can be used as a new strategy for controlling nematodes.

Halophytes play an important role in the bioremediation of saline soils. Increased evidence has revealed that plant growth-promoting rhizobacteria (PGPR) have colonized the halophytic rhizosphere, and they have evolved the capacity to reduce salt stress damage to the host. However, the mechanism by which halophytes attract and recruit beneficial PGPR has rarely been reported. This study reports the interaction between the halophyte Limonium sinense and its rhizosphere PGPR strain Bacillus flexus KLBMP 4941, as well as the mechanism by which KLBMP 4941 promotes host plant growth under salt stress. After salt stress treatment, we collected the root exudates (REs) of L. sinense and found that the REs could promote the growth and chemotaxis of the bacterium KLBMP 4941. In addition, the components of the REs under salt stress were analyzed, and some organic acids (2-methylbutyric acid, stearic acid, palmitic acid, palmitoleic acid, and oleic acid) were detected as the major components. Further assessment showed that each of these components had positive effects on the growth, motility, chemotaxis, and root colonization of strain KLBMP 4941. Further pot experiments revealed the potential PGP mechanisms induced by strain KLBMP 4941 on the host plant under salt stress. Inoculation with KLBMP 4941 promoted the accumulation of chlorophyll to enhance photosynthesis, increased osmotic regulator contents, enhanced flavonoid and antioxidant enzymes, and regulated Na+/K+ homeostasis to help the host ameliorate salinity stress damage. Our findings indicate that the halophyte L. sinense can attract and recruit beneficial rhizosphere bacteria by REs under salt stress, and the recruited B. flexus KLBMP 4941 elicited PGP effects under salinity stress through complex plant physiological regulatory mechanisms. This study provides a foundation for the enhancement of the rhizosphere colonization ability of the PGP strain KLBMP 4941, which shows potential applications in phytoremediation of saline soils.