Pyoluteorin production byPseudomonas fluorescens strain CHA0 is involved in the suppression ofPythium damping-off of cress but not of cucumber

Pseudomonas fluorescens strain CHA0 is an effective biocontrol agent of various soilborne pathogens. It controls damping-off or root rot caused byPythium ultimum on cucumber, wheat and cress. Strain CHA0 synthesizes several antibiotic metabolites such as hydrogen cyanide, 2,4-diacetylphloroglucinol, and pyoluteorin. The role of pyoluteorin in the suppression of damping-off was investigated. Two Tn5 mutants (CHA660 and CHA661) of strain CHA0 were isolated which had lost the capacity to produce pyoluteorin but still produced 2,4-diacteylphloroglucinol and HCN. These mutants still inhibitedP. ultimum on malt agar (which favours the production of 2,4-diacetylphloroglucinol) but had partially lost the ability to inhibit this pathogen on King's B agar (which favours the production of pyoluteorin). The two pyoluteorin-negative mutants showed a reduced capacity to suppress damping-off of cress caused byP. ultimum but were as effective in the protection of cucumber against this pathogen as the wild-type strain.These results indicate that, depending on the plant, pyoluteorin production plays a role in the suppression of damping-off by strain CHA0 without being a major mechanism in disease suppression. We suggest that the contribution of pyoluteorin to the biocontrol activity of strain CHA0 is determined by the quantity of this antibiotic produced in the rhizosphere, which might depend on the root exudates of the host plant.     

Biocontrol of Meloidogyne incognita on tomato using antagonistic fungi,plant‐growth‐promoting rhizobacteria and cattle manure

BACKGROUND: Biocontrol achieved by a single biocontrol agent is generally inconsistent under field conditions. The aim of the present study was to increase the competitiveness and efficacy of biocontrol agents by using them together with cattle manure. RESULTS: The effects of antagonistic fungi [Aspergillus niger v. Teigh., Paecilomyces lilacinus (Thom) Samson and Penicillium chrysogenum Thom] and plant‐growth‐promoting rhizobacteria (PGPR) [Azotobacter chroococcum Beijer., Bacillus subtilis (Ehrenberg) Cohn and Pseudomonas putida (Trev.) Mig.] were assessed with cattle manure on the growth of tomato and on the reproduction of Meloidogyne incognita (Kof. & White) Chitwood. Application of antagonistic fungi and PGPR alone and in combination with cattle manure resulted in a significant increase in the growth of nematode‐inoculated plants. The highest increase (79%) in the growth of nematode‐inoculated plants was observed when P. putida was used with cattle manure, followed by use of P. lilacinus plus cattle manure. Paecilomyces lilacinus resulted in a high reduction in galling and nematode multiplication, followed by P. putida, B. subtilis, A. niger, A. chroococcum and P. chrysogenum. The combined use of P. lilacinus with cattle manure resulted in a maximum reduction in galling and nematode multiplication. CONCLUSION: Application of P. lilacinus or P. putida with cattle manure was useful to achieve greater biocontrol of M. incognita on tomato. Copyright © 2009 Society of Chemical Industry     

Pseudomonas Fluorescens UP61 Isolated From Birdsfoot Trefoil Rhizosphere Produces Multiple Antibiotics and Exerts a Broad Spectrum of Biocontrol Activity

The efficient use of rhizospheric microorganisms to control plant pathogens has been reported worldwide in different plants. Pseudomonas fluorescens UP61 is a biocontrol strain isolated from the rhizosphere of Lotus corniculatus(birdsfoot trefoil) from Uruguayan soils. This strain exhibited in vitro antagonistic activity against a broad spectrum of fungal and bacterial phytopathogens. It was an effective biocontrol agent in different hosts, reducing the disease incidence caused by Sclerotium rolfsii in beans and Rhizoctonia solani in tomato. P. fluorescens UP61 produced three antibiotics possibly involved in its biocontrol activity: 2,4-diacetylphloroglucinol, pyrrolnitrin and pyoluteorin. Molecular techniques such as 16S rDNA RFLP, RAPD and rep-PCR, and partial sequence of the phlD gene, revealed the similarity of UP61 with other biocontrol strains isolated worldwide that are able to produce these antibiotics.     

Inhibitory effect of ACC deaminase‐producing bacteria on crown gall formation in tomato plants infected by Agrobacterium tumefaciens or A. vitis

This study showed that various rhizosphere bacteria producing the enzyme 1‐aminocyclopropane‐1‐carboxylate (ACC) deaminase (ACCD), which can degrade ACC, the immediate precursor of ethylene in plants, and thereby lower plant ethylene levels, can act as promising biocontrol agents of pathogenic strains of Agrobacterium tumefaciens and A. vitis. Soaking the roots of tomato (Solanum lycopersicum) seedlings in a suspension of the ACCD‐producing Pseudomonas putida UW4, Burkholderia phytofirmans PsJN or Azospirillum brasilense Cd1843 transformed by plasmid pRKTACC carrying the ACCD‐encoding gene acdS from UW4, significantly reduced the development of tumours on tomato plants injected 4–5 days later with pathogenic Agrobacterium strains via wounds on the plant stem. The fresh mass of tumours formed by plants pretreated with ACCD‐producing strains was typically four‐ to fivefold less than that of tumours formed on control plants inoculated only with a pathogenic Agrobacterium strain. Simultaneously, the level of ethylene evolution per amount of tumour mass on plants pretreated with ACCD‐producing bacteria decreased four to eight times compared with that from tumours formed on control plants or plants pretreated with bacteria deficient in ACCD production. Moreover, transgenic tomato plants expressing a bacterial ACCD were found to be highly resistant to crown gall formation relative to the parental, non‐transformed tomato plants. The results support the hypothesis that ethylene is a crucial factor in Agrobacterium tumour formation, and that ACCD‐produced rhizosphere bacteria may protect plants infected by pathogenic Agrobacteria from crown gall disease.     

Characterization,phylogenetic analyses and pathogenicity of Bipolaris setariae associated with brown stripe disease on sugarcane in Yunnan,China

Brown stripe disease is a severe foliar fungal disease of sugarcane worldwide and is widespread in all sugarcane planting areas in China. Brown stripe is a major disease that seriously affects the output and quality of the sugarcane industry in Yunnan Province, China's second-largest sugar base, while the pathogen of this disease remains not yet fully understood. To address this, we isolated and identified the fungi associated with 68 leaf samples showing typical symptoms of brown stripe from 22 sugarcane varieties in different areas of Yunnan Province. A total of 113 isolates were obtained, which were morphologically similar. Of these, 64 representative isolates were sequenced for the internal transcribed spacer region (ITS), GAPDH and EF-1α loci. All representative isolates grouped with the type strain of Bipolaris setariae in the phylogenetic trees inferred with individual and concatenated sequences of ITS, GAPDH and EF1-α. Pathogenicity test results showed that B. setariae strains were able to induce typical symptoms of brown stripe. The results obtained in this study clarify that only B. setariae is associated with sugarcane brown stripe in Yunnan, China. It is recorded here for the first time as a pathogen causing sugarcane brown stripe in Yunnan, and it is able to infect many major cultivars and new varieties, posing a new threat to the sugar industry in Yunnan Province. In addition, these results provide the scientific basis for the future breeding of disease-resistant varieties and effective prevention and control of sugarcane brown stripe disease.     

A stable Leifsonia xyli subsp. xyli GFP‐tagged strain reveals a new colonization niche in sugarcane tissues

Ratoon stunting disease (RSD), caused by the bacterium Leifsonia xyli subsp. xyli (Lxx), is one of the most economically important diseases of sugarcane worldwide. Because knowledge on the interaction of Lxx with its host at the microscopic level is limited, the development of tools to monitor Lxx during the colonization process could shed new light on the processes that control disease development. In this investigation, a transformation protocol was optimized and a mutant Lxx strain engineered that stably expressed the gfp gene in sugarcane tissues. In vitro, the growth of the mutant did not differ from that of the wild type. Also, plants inoculated with both strains showed comparable growth and development when analysed 180 days after inoculation (dai). Fluorescence microscopy of roots, stalks, meristems and leaf tissues of Lxx‐GFP‐inoculated plants was performed at 180 dai. In the leaves, Lxx‐tagged cells were observed within the xylem vessels as has been described before but, in addition, they were found in a new niche within the host tissues, in the mesophyll and in the bundle sheath cells surrounding the vascular system. This finding indicates that Lxx is able to move from the xylem to the parenchyma of the leaf cells. This first report of an Lxx mutant expressing a heterologous gene revealed that colonization of sugarcane by this pathogen is not limited to the xylem vessels as commonly reported.     

Mitochondrial impacts of insecticidal formate esters in insecticide‐resistant and insecticide‐susceptible Drosophila melanogaster

BACKGROUND: Previous research on insecticidal formate esters in flies and mosquitoes has documented toxicity profiles, metabolism characteristics and neurological impacts. The research presented here investigated mitochondrial impacts of insecticidal formate esters and their hydrolyzed metabolite formic acid in the model dipteran insect Drosophila melanogaster Meig. These studies compared two Drosophila strains: an insecticide‐susceptible strain (Canton‐S) and a strain resistant by cytochrome P450 overexpression (Hikone‐R). RESULTS: In initial studies investigating inhibition of mitochondrial cytochrome c oxidase, two proven insecticidal materials (hydramethylnon and sodium cyanide) caused significant inhibition. However, for insecticidal formate esters and formic acid, no significant inhibition was identified in either fly strain. Mitochondrial impacts of formate esters were then investigated further by tracking toxicant‐induced cytochrome c release from mitochondria into the cytoplasm, a biomarker of apoptosis and neurological dysfunction. Formic acid and three positive control treatments (rotenone, antimycin A and sodium cyanide) induced cytochrome c release, verifying that formic acid is capable of causing mitochondrial disruption. However, when comparing formate ester hydrolysis and cytochrome c release between Drosophila strains, formic acid liberation was only weakly correlated with cytochrome c release in the susceptible Canton‐S strain (r² = 0.70). The resistant Hikone‐R strain showed no correlation (r² < 0.0001) between formate ester hydrolysis and cytochrome c release. CONCLUSION: The findings of this study provide confirmation of mitochondrial impacts by insecticidal formate esters and suggest links between mitochondrial disruption, respiratory inhibition, apoptosis and formate‐ester‐induced neurotoxicity. Copyright © 2009 Society of Chemical Industry     

Isolation and characterisation of a bacterial strain degrading the herbicide sulcotrione from an agricultural soil

BACKGROUND: The dissipation kinetics of the herbicide sulcotrione sprayed 4 times on a French soil was studied using a laboratory microcosm approach. An advanced cultivation‐based method was then used to isolate the bacteria responsible for biotransformation of sulcotrione. Chromatographic methods were employed as complementary tools to define its metabolic pathway. RESULTS: Soil microflora was able quickly to biotransform the herbicide (DT₅₀ ≈ 8 days). 2‐Chloro‐4‐mesylbenzoic acid, one of its main metabolites, was clearly detected. However, no accelerated biodegradation process was observed. Eight pure sulcotrione‐resistant strains were isolated, but only one (1OP) was capable of degrading this herbicide with a relatively high efficiency and to use it as a sole source of carbon and energy. In parallel, another sulcotrione‐resistant strain (1TRANS) was shown to be incapable of degrading the herbicide. Amplified ribosomal restriction analysis (ARDRA) and repetitive extragenic palendromic PCR genomic (REP‐PCR) fingerprinting of strains 1OP and 1TRANS gave indistinguishable profiles. CONCLUSION: Sequencing and aligning analysis of 16S rDNA genes of each pure strain revealed identical sequences and a close phylogenetic relationship (99% sequence identity) to Pseudomonas putida. Such physiological and genetic properties of 1OP to metabolise sulcotrione were probably governed by mobile genetic elements in the genome of the bacteria. Copyright © 2011 Society of Chemical Industry     

Evaluating the importance of the tan spot ToxA–Tsn1 interaction in Australian wheat varieties

The necrotrophic fungal pathogen Pyrenophora tritici‐repentis (Ptr) causes the major wheat disease tan spot, and produces multiple necrotrophic effectors that contribute to virulence. The proteinaceous effector ToxA induces necrosis in wheat genotypes possessing the Tsn1 gene, although the importance of the ToxA–Tsn1 interaction itself in varietal disease development has not been well studied. Here, 40 Australian spring wheat varieties were assessed for ToxA sensitivity and disease response to a race 1 wildtype Ptr isolate and ToxA‐deleted strain at both seedling and tillering growth stages. ToxA sensitivity was generally associated with disease susceptibility, but did not always predict spreading necrotic symptoms. Whilst the majority of Tsn1 varieties exhibited lower disease scores following toxa mutant infection, several exhibited no distinct differences between wildtype and toxa symptoms. This implies that ToxA is not the major determinant in tan spot disease development in some host backgrounds and indicates the presence of additional effectors. Unexpectedly, several tsn1 varieties exhibited a reduction in disease severity following toxa mutant inoculation, which may suggest an indirect role for ToxA in pathogen fitness. Additionally, increased chlorosis was observed following toxa mutant infection in three varieties, and further work is required to determine whether this is likely to be due to ToxA epistasis of ToxC symptoms. Taken together, these observations demonstrate that Ptr interacts with the host in a complex and intricate manner, leading to a variety of disease reactions that are dependent or independent of the ToxA–Tsn1 interaction.     

Aerial Contamination of Sugarcane in Guadeloupe by Two Strains of Xanthomonas albilineans

Two sugarcane plots were set up in Guadeloupe with disease-free tissue cultured plants in a banana growing location distant from sugarcane fields. Thirteen weeks after planting sugarcane in the field, a Xanthomonas albilineans strain belonging to serotype 3 (strain XaS3) was detected in water sampled at sunrise on the leaves in the first plot. This strain randomly invaded the sugarcane canopy. Seven weeks later, a new strain belonging to serotype 1 (strain XaS1) appeared on leaves and populations of strain XaS1 progressively increased on the leaf surface, whereas populations of strain XaS3 progressively decreased. Leaf scald symptoms were first noted 26 weeks after sugarcane planting. However, only strain XaS1 was isolated from leaves and a few sugarcane stalks showing symptoms. Both strains also colonized the second field plot, which was studied at the end of the experiment to avoid human interference of aerial contamination of sugarcane. After inoculation of three sugarcane cultivars by the decapitation technique, strain XaS1 was as virulent or more virulent than five other strains of X. albilineans isolated from diseased sugarcane plants in Guadeloupe. Although strain XaS3 colonized a few stalks, it failed to produce any symptoms and was the least virulent strain. Leaf surface colonization by X. albilineans was reproduced in a greenhouse trial by spraying the pathogen on sugarcane foliage. After 8 weeks, the pathogen was isolated from disinfected leaf blades. Although the leaf scald pathogen is thought to be mainly transmitted by infected cuttings, aerial transmission of X. albilineans is also known to occur. These results indicate the importance of sugarcane phyllosphere colonization by virulent strains in the epidemiological cycle of leaf scald disease in Guadeloupe.     

Inoculation of Malus genotypes with a set of Erwinia amylovora strains indicates a gene‐for‐gene relationship between the effector gene eop1 and both Malus floribunda 821 and Malus ‘Evereste’

The Gram‐negative bacterium Erwinia amylovora, causal agent of fire blight disease in pome fruit trees, encodes a type three secretion system (T3SS) that translocates effector proteins into plant cells that collectively function to suppress host defences and enable pathogenesis. Until now, there has only been limited knowledge about the interaction of effector proteins and host resistance presented in several wild Malus species. This study tested disease responses in several Malus wild species with a set of effector deletion mutant strains and several highly virulent E. amylovora strains, which are assumed to influence the host resistance response of fire blight‐resistant Malus species. The findings confirm earlier studies that deletion of the T3SS abolished virulence of the pathogen. Furthermore, a new gene‐for‐gene relationship was established between the effector protein Eop1 and the fire blight resistant ornamental apple cultivar Evereste and the wild species Malus floribunda 821. The results presented here provide new insights into the host–pathogen interactions between Malus sp. and E. amylovora.     

Rhizobacteria‐mediated resistance against the blackeye cowpea mosaic strain of bean common mosaic virus in cowpea (Vigna unguiculata)

BACKGROUND: The present study investigated the effect of seven Bacillus‐species plant‐growth‐promoting rhizobacteria (PGPR) seed treatments on the induction of disease resistance in cowpea against mosaic disease caused by the blackeye cowpea mosaic strain of bean common mosaic virus (BCMV). RESULTS: Initially, although all PGPR strains recorded significant enhancement of seed germination and seedling vigour, GBO3 and T4 strains were very promising. In general, all strains gave reduced BCMV incidence compared with the non‐bacterised control, both under screen‐house and under field conditions. Cowpea seeds treated with Bacillus pumilus (T4) and Bacillus subtilis (GBO3) strains offered protection of 42 and 41% against BCMV under screen‐house conditions. Under field conditions, strain GBO3 offered 34% protection against BCMV. The protection offered by PGPR strains against BCMV was evaluated by indirect enzyme‐linked immunosorbent assay (ELISA), with lowest immunoreactive values recorded in cowpea seeds treated with strains GBO3 and T4 in comparison with the non‐bacterised control. In addition, it was observed that strain combination worked better in inducing resistance than individual strains. Cowpea seeds treated with a combination of strains GBO3 + T4 registered the highest protection against BCMV. CONCLUSION: PGPR strains were effective in protecting cowpea plants against BCMV under both screen‐house and field conditions by inducing resistance against the virus. Thus, it is proposed that PGPR strains, particularly GBO3, could be potential inducers against BCMV and growth enhancers in cowpea. Copyright © 2009 Society of Chemical Industry     

Antagonistic rhizoplane bacteria induce diverse morphological alterations in Peronosporomycete hyphae during <Emphasis Type="Italic">in vitro</Emphasis> interaction

A total of 150 bacteria were isolated from rhizoplanes of the host and non-host plants of a phytopathogenic Peronosporomycete Aphanomyces cochlioides. Upon screening, 5% of the isolates were evaluated as antagonists as they inhibited radial growth of A. cochlioides AC-5 hyphae in a dual culture assay. In addition, those antagonistic bacteria also induced characteristic morphological alterations in the A. cochlioides AC-5 hyphae that grew towards bacterial colonies. Hyphal morphological alterations observed in AC-5 and other tested strains of Peronosporomycetes included excessive branching, curly growth, unusually longer and pointed tip formation and swelling; all of these were comparable to the alterations induced by known antimicrobial compounds. Among the antagonistic bacteria, Pseudomonas sp. strain EC-S101 induced a unique branching pattern (tree-like) in AC-5 hyphae by continuous apical bifurcation of successive hyphae, where increases in number of branches and hyphal area were linearly correlated with time up to 10 h. Our observations suggested that the pathogen might have lost its ability of normal branch production; however maintained the capability of self-branching. Soluble extracts from the culture fluids of Pseudomonas sp. strain EC-S101 and Stenotrophomonas maltophilia EC-S105 induced similar excessive branching and curly growth in A. cochlioides hyphae as the respective bacterium. These results revealed that bacterial metabolites appeared to be responsible for induction of morphological alterations. Interestingly, the antagonistic bacteria that induced hyphal morphological alterations, also efficiently suppressed in vivo damping-off disease caused by AC-5. We suggest that antagonistic rhizoplane bacteria have the capability to induce diverse morphological alterations in Peronosporomycetes hyphae during in vitro interactions. Hyphal morphological alterations associated with growth inhibition and the induction of characteristic morphological changes indicate antagonistic activity against the Peronosporomycete.     

Molecular identification and prevalence of Acidovorax avenae subsp. avenae causing red stripe of sugarcane in China

Red stripe caused by the bacterium Acidovorax avenae subsp. avenae (Aaa) is a disease of sugarcane that is distributed worldwide. In this study, 108 sugarcane leaf samples were collected in 2013–2016 from nine sugarcane‐growing regions in China. Aaa was detected by PCR with specific and novel primers from the 16S–23S rDNA internal transcribed spacer region in 81 of 84 (96%) leaves with red stripe symptoms and in 20 of 24 (83%) leaves without symptoms. Furthermore, Aaa was detected in all nine sampling locations representing six sugarcane‐producing provinces in China. The 101 amplified fragments were cloned and sequenced. The size of the nucleotide sequences varied from 436 to 454 bp and the sequence identity ranged from 89.2% to 100%, suggesting a significant genetic variation among Aaa strains from China. Five major restriction fragment length polymorphism (RFLP) profiles were obtained by in silico and polyacrylamide gel electrophoresis analyses of the PCR products digested with HindIII and EcoRI. The causal agent of sugarcane red stripe was also successfully isolated from a diseased plant and its pathogenicity confirmed by inoculation of healthy sugarcane plantlets and reproduction of disease symptoms. The data showed that Aaa is currently widespread in China, suggesting that control methods should be implemented to limit the impact of red stripe on sugarcane production.     

小麦全蚀病菌拮抗细菌的筛选及生防效果

为筛选小麦全蚀病菌Gaeumannomyces tritici的拮抗细菌,从健康小麦根际土壤中分离和筛选拮抗细菌,采用对峙培养法和室内盆栽试验测定拮抗细菌对小麦全蚀病菌的抑制活性及生防效果,通过形态学特征观察、生理生化特性测定和保守序列分析确定其分类地位。结果表明,从小麦根际土壤中分离并筛选获得2株具有较好拮抗效果的菌株XJ-3和XJ-4,对小麦全蚀病菌菌丝生长的抑制率分别为64.31%和65.25%。基于菌株XJ-3和XJ-4的形态学特征、生理生化特性和gyrB基因系统进化分析结果,将其均鉴定为贝莱斯芽胞杆菌Bacillus velezensis。菌株XJ-3和XJ-4的发酵液中均含有一类具抗真菌活性、耐热性的非蛋白类次生代谢产物;经菌株XJ-3和XJ-4发酵液处理小麦幼苗后,株高较对照分别显著增加了6.48%和13.12%,干重较对照分别增加了1.30%和14.29%,其中菌株XJ-4的促生效果相对较好;对小麦全蚀病的盆栽防效分别达到51.29%和52.46%。表明筛选得到的贝莱斯芽胞杆菌XJ-3和XJ-4可用于小麦全蚀病的生物防治,具有开发为生防制剂的潜力。     

The effect of post‐harvest storage conditions on the development of black dot (Colletotrichum coccodes) on potato in crops grown for different durations

The effects of post‐harvest curing and storage temperature on severity of black dot, caused by Colletotrichum coccodes, were investigated for potato crops grown for different crop durations (days from 50% emergence to harvest) in soils that posed a low, medium and high risk of disease. In field trials over four growing seasons (2005–8), black dot severity at harvest increased with increasing crop duration, within the range 103–146 days from 50% emergence to harvest (P < 0.05). In field trials over three growing seasons (2006–8), black dot severity on tubers at harvest increased significantly with increasing soil inoculum in each year, within the range 43–4787 pg C. coccodes DNA/g soil (P < 0.05). Storage trials were conducted to measure the influence of accumulated post‐harvest temperature on black dot. In 2005, no difference in black dot severity was observed on tubers stored for 20 weeks at 2.5 and 3.5 °C. In 2006 (but not 2007), increasing the duration of curing after harvest from 4 to 14 days increased black dot severity on tubers from 8.9 to 11.2% (P < 0.01) in long duration crops (>131 days after 50% emergence) grown under high (>1000 pg C. coccodes DNA/g soil) soil inoculum. The number of days of curing did not affect disease severity for shorter duration crops grown at high soil inoculum, or on crops grown at medium or low (100–1000 and <100 pg C. coccodes DNA/g soil, respectively) soil inoculum concentrations. Soil inoculum and crop duration together provided a reasonable prediction of black dot severity at harvest and after a 20‐week storage period.     

Efficacy of olive mill waste water and its derivatives in the suppression of crown gall disease of bitter almond

Olive mill waste water (OMW) and some of its indigenous bacterial strains were tested in vitro and in planta for their efficacy against crown gall disease caused by Agrobacterium tumefaciens. OMW and polyphenols displayed a high level of antibacterial activity, however the volatile fraction was less efficient as only a bacteriostatic effect was observed. In pot experiments, the percentage of bitter almond rootstock showing symptoms of crown gall was significantly reduced with the dosage rate of OMW 1% as compared to the control (highly natural infected soil treated with water). Five indigenous bacterial strains isolated from OMW exhibited an antagonistic effect against the bacterium. Based on the gene 16S rRNA sequence analysis, one isolate showed 99.2% similarity to known sequences of Bacillus subtilis, one isolate demonstrated high percentage similarities (99.3%) to the genera Bacillus pumilis, and two isolates were associated with Stenotrophomonas maltophilia and Pseudomonas putida 100% and 99.6% similarities respectively. Among these bacteria, the strain B1 proved efficient against the soil borne pathogen in vitro and pot experiments. Our study in controlled conditions suggested that the addition of OMW to soil exerts significant disease suppressiveness against A. tumefaciens. Thabet Yangui and Ali Rhouma contributed equally to this work and are regarded as joint first authors.     

Biocontrol of <Emphasis Type="Italic">Fusarium</Emphasis> wilt of <Emphasis Type="Italic">Capsicum annuum</Emphasis> by rhizospheric bacteria isolated from turmeric endowed with plant growth promotion and disease suppression potential

In the present study, 129 rhizospheric bacteria isolated from Curcuma longa were screened for their antagonistic potential against six fungal phytopathogens. Among them, 32 isolates that showed significant antagonistic potential were screened for their in vitro plant growth promoting (PGP) traits. The identification of potential isolates was confirmed by 16S rRNA gene sequencing and results revealed Bacillus as the dominant genus followed by Staphylococcus, Pseudomonas, Sphingomonas and Achromobacter. Based on the antagonistic activity and PGP traits; two strains (BPSRB4 and BPSRB14), identified as Bacillus amyloliquefaciens, were further tested for their in vivo PGP and disease suppression potential on Capsicum annuum seedlings under greenhouse conditions. The results demonstrated that BPSRB4 and BPSR14 strains suppress fungal pathogen infection and promote plant growth. Further, the BPSRB4 strain was positive for the production of the phytohormone indole acetic acid (IAA) detected by thin layer chromatography (TLC). In addition, nitrogen fixation and plant growth promotion activity were also confirmed by amplification and sequencing of nitrogen fixation gene (nifH) and ACC (1-aminocyclopropane-1-carboxylate) deaminase (acdS) gene from strains BPSRB4 and BPSRB14. The present study demonstrated that the B. amyloliquefaciens strains BPSRB4 and BPSR14 possess antagonistic activity and PGP potential which could be explored for the development of biofertilizers and biocontrol agents for the growth of chilli seedlings.     

Induction of resistance against pine wilt disease caused by Bursaphelenchus xylophilus using selected pine endophytic bacteria

Pine wilt disease (PWD) is the most destructive disease threatening pine worldwide. The disease is mainly caused by the pine wood nematode, Bursaphelenchus xylophilus, which is vectored by pine sawyer longhorn-beetles, Monochamus spp. This study aimed to select resistance-inducing pine endophytic bacteria for management of PWD. To set up a defence-related genes expression pattern for screening, four chemical inducers (salicylic acid, γ-aminobutyric acid (GABA), β-aminobutyric acid and α-aminobutyric acid) were tested in vitro on pine calli and in vivo on pine seedlings. Treatment with GABA had the greatest reduction in PWD severity on pine seedlings. The pattern of defence-related gene expression in calli treated with GABA was used to select potential resistance-inducing bacterial strains. In addition, 92 bacterial strains were isolated from pine tree needles and stems and were tested for expression of defence-related genes in pine calli in vitro. Among the tested strains, 13 showed a similar pattern to GABA treatment in at least four tested defence-related genes and were selected for the seedling assay. From the seedling assay, three bacterial strains (16YSM-E48, 16YSM-P180 and 16YSM-P39) showed significant reduction in PWD severity compared to the untreated control. Moreover, among the selected strains, cell-free culture supernatant of strain 16YSM-P180 significantly reduced PWD severity in inoculated pine seedlings. The selected strains were identified based on the 16S rRNA sequence as Pseudomonas putida 16YSM-E48, Curtobacterium pusillum 16YSM-P180 and Stenotrophomonas rhizophila 16YSM-P39. These selected strains are suggested as potential alternatives for management of PWD by induction of systemic resistance.