Mixtures of Plant-Growth-Promoting Rhizobacteria Enhance Biological Control of Multiple Plant Diseases and Plant-Growth Promotion in the Presence of Pathogens.

Several studies have shown that mixtures of plant-growth-promoting rhizobacteria (PGPR) could enhance biological control activity for multiple plant diseases through the mechanisms of induced systemic resistance or antagonism. Prior experiments showed that four individual PGPR strains-AP69 (Bacillus altitudinis), AP197 (B. velezensis), AP199 (B. velezensis), and AP298 (B. velezensis)-had broad-spectrum biocontrol activity via antagonism in growth chambers against two foliar bacterial pathogens (Xanthomonas axonopodis pv. vesicatoria and Pseudomonas syringae pv. tomato) and one of two tested soilborne fungal pathogens (Rhizoctonia solani and Pythium ultimum). Based on these findings, the overall hypothesis of this study was that a mixture of two individual PGPR strains would exhibit better overall biocontrol and plant-growth promotion than the individual PGPR strains. Two separate greenhouse experiments were conducted. In each experiment, two individual PGPR strains and their mixtures were tested for biological control of three different diseases and for plant-growth promotion in the presence of the pathogens. The results demonstrated that the two individual PGPR strains and their mixtures exhibited both biological control of multiple plant diseases and plant-growth promotion. Overall, the levels of disease suppression and growth promotion were greater with mixtures than with individual PGPR strains.

Paenibacillus nebraskensis sp. nov., isolated from the root surface of field-grown maize.

A Gram-positive-staining, aerobic, non-endospore-forming bacterial strain (JJ-59T), isolated from a field-grown maize plant in Dunbar, Nebraska in 2014 was studied by a polyphasic approach. Based on 16S rRNA gene sequence similarity comparisons, strain JJ-59T was shown to be a member of the genus Paenibacillus, most closely related to the type strains of Paenibacillus aceris (98.6 % 16S rRNA gene sequence similarity) and Paenibacillus chondroitinus (97.8 %). For all other type strains of species of the genus Paenibacillus lower 16S rRNA gene sequence similarities were obtained. DNA-DNA hybridization values of strain JJ-59T to the type strains of P. aceris and P. chondroitinus were 26 % (reciprocal, 59 %) and 52 % (reciprocal, 59 %), respectively. Chemotaxonomic characteristics such as the presence of meso-diaminopimelic acid in the peptidoglycan, the major quinone MK-7 and spermidine as the major polyamine were in agreement with the characteristics of the genus Paenibacillus. Strain JJ-59T shared with its next related species P. aceris the major lipids diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified aminophospholipid, but the presence/absence of certain lipids was clearly distinguishable. Major fatty acids of strain JJ-59T were anteiso-C15 : 0, iso-C15 : 0 and iso-C16 : 0, and the genomic G+C content is 47.2 mol%. Physiological and biochemical characteristics of strain JJ-59T were clearly different from the most closely related species of the genus Paenibacillus. Thus, strain JJ-59T represents a novel species of the genus Paenibacillus, for which the name Paenibacillus nebraskensis sp. nov. is proposed, with JJ-59T (=DSM 103623T=CIP 111179T=LMG 29764T) as the type strain.

Paenibacillus rhizoplanae sp. nov., isolated from the rhizosphere of Zea mays.

A Gram-stain-positive, aerobic, endospore-forming bacterial strain isolated from the rhizosphere of Zea mays was studied to determine its detailed taxonomic position. Based on 16S rRNA gene sequence similarity comparisons, strain JJ-64T was shown to be a member of the genus Paenibacillus, most closely related to the type strains of Paenibacillus silagei (99 %) and Paenibacillus borealis (97.5 %). 16S rRNA gene sequence similarity to all other Paenibacillus species was ≤97.5 %. DNA-DNA hybridization values to the type strains of P. silagei and P. borealis were 51 % (reciprocal 25 %) and 31 % (reciprocal 37 %), respectively. The presence of meso-diaminopimelic acid as the diagnostic diamino acid of the peptidoglycan, the major quinone MK-7 and the polyamine pattern with spermidine as the major component were well in line with the characteristics of the genus Paenibacillus. Furthermore, the polar lipid profile of strain JJ-64T with the predominant lipids diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylserine and two unidentified aminophospholipids reflected the close phylogenetic relatedness to P. silagei. Major fatty acids were iso- and anteiso-branched components. Physiological and biochemical characteristics allowed the further phenotypic differentiation of strain JJ-64T from the most closely related species. Thus, strain JJ-64T represents a novel species of the genus Paenibacillus, for which the name Paenibacillus rhizoplanae sp. nov. is proposed. The type strain is JJ-64T (=LMG 29875T=CCM 8725T).

Bacillus zeae sp. nov., isolated from the rhizosphere of Zea mays.

A Gram-positive-staining, aerobic organism, isolated from the rhizosphere of Zea mays, was investigated in detail. Based on 16S rRNA gene sequence similarity comparisons, strain JJ-247T was grouped into the genus Bacillus, most closely related to Bacillus foraminis (98.4 %). The 16S rRNA gene sequence similarity to the sequences of the type strains of other species of the genus Bacillus was <97.4 %. The fatty acid profile with the major fatty acids, anteiso-C15 : 0, iso-C15 : 0, iso-C14 : 0 and iso-C16 : 0 supported the grouping of the strain to the genus Bacillus. The polar lipid profile contained the major components diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified aminophospholipid. The major quinone was menaquinone MK-7, and the major polyamine was spermidine. The genomic DNA G+C content of strain JJ-247T was 44.5 mol%. DNA-DNA hybridizations with the type strain B. foraminis LMG 23147T resulted in values below 70 %. In addition, physiological and biochemical test results allowed a clear phenotypic differentiation of strain JJ-247T from B. foraminis. As a consequence, JJ-247T represents a novel species of the genus Bacillus, for which we propose the name Bacillus zeae sp. nov., with JJ-247T (=CCM 8726T=LMG 29876T) as the type strain.

Selection and Assessment of Plant Growth-Promoting Rhizobacteria for Biological Control of Multiple Plant Diseases.

A study was designed to screen individual strains of plant growth-promoting rhizobacteria (PGPR) for broad-spectrum disease suppression in vitro and in planta. In a preliminary screen, 28 of 196 strains inhibited eight different tested pathogens in vitro. In a secondary screen, these 28 strains showed broad spectrum antagonistic activity to six different genera of pathogens, and 24 of the 28 strains produced five traits reported to be related to plant growth promotion, including nitrogen fixation, phosphate solubilization, indole-3-acetic acid production, siderophore production, and biofilm formation. In advanced screens, the 28 PGPR strains selected in vitro were tested in planta for biological control of multiple plant diseases including bacterial spot of tomato caused by Xanthomonas axonopodis pv. vesicatoria, bacterial speck of tomato caused by Pseudomonas syringae pv. tomato, damping-off of pepper caused by Rhizoctonia solani, and damping-off of cucumber caused by Pythium ultimum. In all, 5 of the 28 tested strains significantly reduced three of the four tested diseases, and another 19 strains showed biological control to two tested diseases. To understand the observed broad-spectrum biocontrol capacity, antiSMASH was used to predict secondary metabolite clusters of selected strains. Multiple gene clusters encoding for secondary metabolites, e.g., bacillibactin, bacilysin, and microcin, were detected in each strain. In conclusion, selected individual PGPR strains showed broad-spectrum biocontrol activity to multiple plant diseases.

Paenibacillus cucumis sp. nov., isolated from a cucumber plant.

A Gram-positive-staining, aerobic, endospore-forming bacterial strain, isolated from the stem of a cucumber plant, was studied in detail for its taxonomic position. Based on 16S rRNA gene sequence similarity comparisons, strain AP-115T was grouped into the genus Paenibacillus, most closely related to Paenibacillus amylolyticus (98.8 %), Paenibacillus tundrae and Paenibacillus barcinonensis (both 98.4 %). The 16S rRNA gene sequence similarity to other species of the genus Paenibacillus was ≤98.4 %. Chemotaxonomic characterization supported allocation of the strain to the genus Paenibacillus. The quinone system contained exclusively menaquinone MK-7, and in the polar lipid profile diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and phosphatidylserine were predominating. The major component in the polyamine pattern was spermidine, and the diagnostic diamino acid of the peptidoglycan was meso-diaminopimelic acid. The major fatty acids were iso- and anteiso-branched fatty acids. The results of physiological and biochemical tests allowed phenotypic differentiation of strain AP-115T from closely related species. Thus, AP-115T represents a novel species of the genus Paenibacillus, for which the name Paenibacillus cucumis sp. nov. is proposed, with AP-115T (=LMG 29222T=CCM 8653T) as the type strain.

Bacillus cucumis sp. nov. isolated from the rhizosphere of cucumber (Cucumis sativus).

A facultative anaerobic, Gram-positive staining, endospore-forming bacterium, isolated from the rhizosphere of cucumber (Cucumis sativus), was taxonomically investigated. Based on 16S rRNA gene sequence similarity comparisons, strain AP-6T clustered together with other species of the genus Bacillus and showed highest similarities with Bacillus drentensis LMG 21831T (99.1 %), Bacillus vireti LMG 21834T (98.7 %) and Bacillus soli LMG 21838T (98.5 %). The 16S rRNA gene sequence similarity to the sequences of the type strains of other species of the genus Bacillus was 98.5 % or less. Chemotaxonomic features supported the grouping of the strain in the genus Bacillus; for example, the major fatty acids were anteiso-C15 : 0, iso-C15 : 0 and C16 : 0, the polar lipid profile contained the major components diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified glycolipid, the major quinone was menaquinone MK-7 and the major compound in the polyamine pattern was spermidine. Additionally, DNA-DNA hybridization with B. drentensis LMG 21831T, B. vireti LMG 21834T and B. soli LMG 21838T resulted in relatedness values that were clearly below 70 %. Physiological and biochemical test results were also different from those of the most closely related species. As a consequence, AP-6T represents a novel species of the genus Bacillus, for which the name Bacillus cucumis sp. nov. is proposed, with AP-6T ( = CIP 110974T = CCM 8651T) as the type strain.

Isoptericola cucumis sp. nov., isolated from the root tissue of cucumber (Cucumis sativus).

A Gram-stain-positive, aerobic organism, showing an irregular cell morphology, was isolated from the root tissue of cucumber (Cucumis sativus) and investigated in detail for its taxonomic position. On the basis of the 16S rRNA gene sequence analysis, strain AP-38T was shown to be most closely related to Isoptericola variabilis (99.1 %) and Isoptericola nanjingensis (98.9 %). The 16S rRNA gene sequence similarity to all other species of the genus Isoptericola was ≤98.5 %. DNA-DNA relatedness to Isoptericola variablis DSM 10177T and Isoptericola nanjingensis DSM 24300T was 31(reciprocal 41 %) and 34 (reciprocal 34 %), respectively. The diagnostic diamino acid of the peptidoglycan was l-lysine. The quinone system contained predominantly menaquinones MK-9(H4) and MK-9(H2). In the polar lipid profile, major compounds were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol and two phosphatidylinositol mannosides. The polyamine pattern contained the major components spermidine and spermine and significant amounts of tyramine. In the fatty acid profile, anteiso-C15 : 0 and iso-C15 : 0 were present in major amounts. These data support the allocation of the strain to the genus Isoptericola. The results of physiological and biochemical characterization additionally provide phenotypic differentiation of strain AP-38T from I. variabilis and I. nanjingensis. AP-38T represents a novel species of the genus Isoptericola, for which we propose the name Isoptericola cucumis sp. nov., with AP-38T (= LMG 29223T=CCM 8653T) as the type strain.

Mitochondrial genome sequences reveal evolutionary relationships of the Phytophthora 1c clade species.

Phytophthora infestans is one of the most destructive plant pathogens of potato and tomato globally. The pathogen is closely related to four other Phytophthora species in the 1c clade including P. phaseoli, P. ipomoeae, P. mirabilis and P. andina that are important pathogens of other wild and domesticated hosts. P. andina is an interspecific hybrid between P. infestans and an unknown Phytophthora species. We have sequenced mitochondrial genomes of the sister species of P. infestans and examined the evolutionary relationships within the clade. Phylogenetic analysis indicates that the P. phaseoli mitochondrial lineage is basal within the clade. P. mirabilis and P. ipomoeae are sister lineages and share a common ancestor with the Ic mitochondrial lineage of P. andina. These lineages in turn are sister to the P. infestans and P. andina Ia mitochondrial lineages. The P. andina Ic lineage diverged much earlier than the P. andina Ia mitochondrial lineage and P. infestans. The presence of two mitochondrial lineages in P. andina supports the hybrid nature of this species. The ancestral state of the P. andina Ic lineage in the tree and its occurrence only in the Andean regions of Ecuador, Colombia and Peru suggests that the origin of this species hybrid in nature may occur there.

Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans.

Phytophthora infestans is the most destructive pathogen of potato and a model organism for the oomycetes, a distinct lineage of fungus-like eukaryotes that are related to organisms such as brown algae and diatoms. As the agent of the Irish potato famine in the mid-nineteenth century, P. infestans has had a tremendous effect on human history, resulting in famine and population displacement. To this day, it affects world agriculture by causing the most destructive disease of potato, the fourth largest food crop and a critical alternative to the major cereal crops for feeding the world's population. Current annual worldwide potato crop losses due to late blight are conservatively estimated at $6.7 billion. Management of this devastating pathogen is challenged by its remarkable speed of adaptation to control strategies such as genetically resistant cultivars. Here we report the sequence of the P. infestans genome, which at approximately 240 megabases (Mb) is by far the largest and most complex genome sequenced so far in the chromalveolates. Its expansion results from a proliferation of repetitive DNA accounting for approximately 74% of the genome. Comparison with two other Phytophthora genomes showed rapid turnover and extensive expansion of specific families of secreted disease effector proteins, including many genes that are induced during infection or are predicted to have activities that alter host physiology. These fast-evolving effector genes are localized to highly dynamic and expanded regions of the P. infestans genome. This probably plays a crucial part in the rapid adaptability of the pathogen to host plants and underpins its evolutionary potential.

Recent Genotypes of Phytophthora infestans in the Eastern United States Reveal Clonal Populations and Reappearance of Mefenoxam Sensitivity.

Isolates of Phytophthora infestans (n = 178) were collected in 2002 to 2009 from the eastern United States, Midwestern United States, and eastern Canada. Multilocus genotypes were defined using allozyme genotyping, and DNA fingerprinting with the RG-57 probe. Several previously described and three new mulitilocus genotypes were detected. The US-8 genotype was found commonly on commercial potato crops but not on tomato. US-20 was found on tomato in North Carolina from 2002 through 2007 and in Florida in 2005. US-21 was found on tomato in North Carolina in 2005 and Florida in 2006 and 2007. US-22 was detected on tomato in 2007 in Tennessee and New York and became widespread in 2009. US-22 was found in 12 states on tomato and potato and was spread on tomato transplants. This genotype accounted for about 60% of all the isolates genotyped. The US-23 genotype was found in Maryland, Virginia, Pennsylvania, and Delaware on both tomato and potato in 2009. The US-24 genotype was found only in North Dakota in 2009. A1 and A2 mating types were found in close proximity on potato and tomato crops in Pennsylvania and Virginia; therefore, the possibility of sexual reproduction should be monitored. Whereas most individuals of US-8 and US-20 were resistant to mefenoxam, US-21 appeared to be intermediately sensitive, and isolates of US-22, US-23, and US-24 were largely sensitive to mefenoxam. On the basis of sequence analysis of the ras gene, these latter three genotypes appear to have been derived from a common ancestor. Further field and laboratory studies are underway using simple sequence repeat genotyping to monitor current changes in the population structure of P. infestans causing late blight in North America.

Genetic structure of Phytophthora infestans populations in China indicates multiple migration events.

One hundred isolates of Phytophthora infestans collected from 10 provinces in China between 1998 and 2004 were analyzed for mating type, metalaxyl resistance, mitochondrial DNA (mtDNA) haplotype, allozyme genotype, and restriction fragment length polymorphism (RFLP) with the RG-57 probe. In addition, herbarium samples collected in China, Russia, Australia, and other Asian countries were also typed for mtDNA haplotype. The Ia haplotype was found during the first outbreaks of the disease in China (1938 and 1940), Japan (1901, 1930, and 1931), India (1913), Peninsular Malaysia (1950), Nepal (1954), The Philippines (1910), Australia (1917), Russia (1917), and Latvia (1935). In contrast, the Ib haplotype was found after 1950 in China on both potato and tomato (1952, 1954, 1956, and 1982) and in India (1968 and 1974). Another migration of a genotype found in Siberia called SIB-1 (Glucose-6-phosphate isomerase [Gpi] 100/100, Peptidase [Pep] 100/100, IIa mtDNA haplotype) was identified using RFLP fingerprints among 72% of the isolates and was widely distributed in the north and south of China and has also been reported in Japan. A new genotype named CN-11 (Gpi 100/111, Pep 100/100, IIb mtDNA haplotype), found only in the south of China, and two additional genotypes (Gpi 100/100, Pep 100/100, Ia mtDNA haplotype) named CN-9 and CN-10 were identified. There were more diverse genotypes among isolates from Yunnan province than elsewhere. The SIB-1 (IIa) genotype is identical to those from Siberia, suggesting later migration of this genotype from either Russia or Japan into China. The widespread predominance of SIB-1 suggests that this genotype has enhanced fitness compared with other genotypes found. Movement of the pathogen into China via infected seed from several sources most likely accounts for the distribution of pathogen genotypes observed. MtDNA haplotype evidence and RFLP data suggest multiple migrations of the pathogen into China after the initial introduction of the Ia haplotype in the 1930s.

Phylogenetic relationships of Phytophthora andina, a new species from the highlands of Ecuador that is closely related to the Irish potato famine pathogen Phytophthora infestans.

Phylogenetic relationships of Phytophthora infestans sensu lato in the Andean highlands of South America were examined. Three clonal lineages (US-1, EC-1, EC-3) and one heterogeneous lineage (EC-2) were found in association with different host species in genus Solanum. The EC-2 lineage includes two mitochondrial (mtDNA) haplotypes, Ia and Ic. Isolates of P. infestans sensu lato EC-2 fit the morphological description of P. infestans but are different from any genotypes of P. infestans described to date. All isolates of P. infestans sensu lato from Ecuador were amplified by a P. infestans specific primer (PINF), and restriction fragment length patterns were identical in isolates amplified with ITS primers 4 and 5. The EC-1 clonal lineage of P. infestans sensu lato from S. andreanum, S. columbianum, S. paucijugum, S. phureja, S. regularifolium, S. tuberosum and S. tuquerense was confirmed to be P. infestans based on sequences of the cytochrome oxidase I (cox I) gene and intron 1 of ras gene. The EC-2 isolates with the Ic haplotype formed a distinct branch in the same clade with P. infestans and P. mirabilis, P. phaseoli and P. ipomoeae for both cox I and ras intron 1 phylogenies and were identified as the newly described species P. andina. Ras intron 1 sequence data suggests that P. andina might have arisen via hybridization between P. infestans and P. mirabilis.

Construction of probe of the plant growth-promoting bacteria Bacillus subtilis useful for fluorescence in situ hybridization.

Strains of Bacillus subtilis are plant growth-promoting bacteria (PGPB) of many crops and are used as inoculants. PGPB colonization is an important trait for success of a PGPB on plants. A specific probe, based on the 16 s rRNA of Bacillus subtilis, was designed and evaluated to distinguishing, by fluorescence in situ hybridization (FISH), between this species and the closely related Bacillus amyloliquefaciens. The selected target for the probe was between nucleotides 465 and 483 of the gene, where three different nucleotides can be identified. The designed probe successfully hybridized with several strains of Bacillus subtilis, but failed to hybridize not only with B. amyloliquefaciens, but also with other strains such as Bacillus altitudinis, Bacillus cereus, Bacillus gibsonii, Bacillus megaterium, Bacillus pumilus; and with the external phylogenetic strains Azospirillum brasilense Cd, Micrococcus sp. and Paenibacillus sp. The results showed the specificity of this molecular probe for B. subtilis.

Different signaling pathways of induced resistance by rhizobacteria in Arabidopsis thaliana against two pathovars of Pseudomonas syringae.

• The mechanisms by which plant growth-promoting rhizobacteria (PGPR) mediate induced systemic resistance are currently being intensively investigated from the viewpoint of signal transduction pathways within plants. • Here, we determined whether our well-characterized PGPR strains, which have demonstrated induced resistance on various plants, also elicit induced resistance in Arabidopsis thaliana. Nine different PGPR strains were evaluated for their capacity to cause induced resistance on Arabidopsis against two pathovars of Pseudomonas syringae. Six strains significantly reduced severity of P. syringae pv. tomato, whereas seven strains reduced severity of P. syringae pv. maculicola. • From the initial screenings, four strains (90-166, SE34, 89B61 and T4) were selected because of their consistent induced resistance capacity. Elicitation of induced resistance with these strains depended on how disease severity was measured. Three strains (90-166, 89B61 and T4) induced resistance in NahG plants (SA-deficient), indicating a salicylic acid-independent pathway, which agrees with the previously reported pathway for induced resistance by PGPR. However, differences from the reported pathway were noted with strain 89B61, which did not require jasmonic acid or ethylene signaling pathways for induced resistance, and with strain T4, which induced resistance in npr1 plants. • These results indicate that strains 89B61 and T4 induce resistance via a new pathway or possibly a variation of the previously reported pathway. This information will broaden our understanding of ways in which microorganisms can signal physiological changes in plants.

Symptoms of Fern Distortion Syndrome resulting from inoculation with opportunistic endophytic fluorescent Pseudomonas spp.

BACKGROUND: Fern Distortion Syndrome (FDS) is a serious disease of Leatherleaf fern (Rumohra adiantiformis). The main symptom of FDS is distortion of fronds, making them unmarketable. Additional symptoms include stunting, irregular sporulation, decreased rhizome diameter, and internal discoloration of rhizomes. We previously reported an association of symptoms with increased endophytic rhizome populations of fluorescent pseudomonads (FPs). The aim of the current study was to determine if FPs from ferns in Costa Rica with typical FDS symptoms would recreate symptoms of FDS. METHODOLOGY AND FINDINGS: Greenhouse tests were conducted over a 29-month period. Micro-propagated ferns derived from tissue culture were first grown one year to produce rhizomes. Then, using an 8×9 randomized complete block experimental design, 8 replicate rhizomes were inoculated by dipping into 9 different treatments before planting. Treatments included water without bacteria (control), and four different groups of FPs, each at a two concentrations. The four groups of FPs included one group from healthy ferns without symptoms (another control treatment), two groups isolated from inside rhizomes of symptomatic ferns, and one group isolated from inside roots of symptomatic ferns. Symptoms were assessed 12 and 17 months later, and populations of FPs inside newly formed rhizomes were determined after 17 months. Results showed that inoculation with mixtures of FPs from ferns with FDS symptoms, but not from healthy ferns, recreated the primary symptom of frond deformities and also the secondary symptoms of irregular sporulation, decreased rhizome diameter, and internal discoloration of rhizomes. CONCLUSIONS: These results suggest a model of causation of FDS in which symptoms result from latent infections by multiple species of opportunistic endophytic bacteria containing virulence genes that are expressed when populations inside the plant reach a minimum level.

DNA barcoding of oomycetes with cytochrome c oxidase subunit I and internal transcribed spacer.

Oomycete species occupy many different environments and many ecological niches. The genera Phytophthora and Pythium for example, contain many plant pathogens which cause enormous damage to a wide range of plant species. Proper identification to the species level is a critical first step in any investigation of oomycetes, whether it is research driven or compelled by the need for rapid and accurate diagnostics during a pathogen outbreak. The use of DNA for oomycete species identification is well established, but DNA barcoding with cytochrome c oxidase subunit I (COI) is a relatively new approach that has yet to be assessed over a significant sample of oomycete genera. In this study we have sequenced COI, from 1205 isolates representing 23 genera. A comparison to internal transcribed spacer (ITS) sequences from the same isolates showed that COI identification is a practical option; complementary because it uses the mitochondrial genome instead of nuclear DNA. In some cases COI was more discriminative than ITS at the species level. This is in contrast to the large ribosomal subunit, which showed poor species resolution when sequenced from a subset of the isolates used in this study. The results described in this paper indicate that COI sequencing and the dataset generated are a valuable addition to the currently available oomycete taxonomy resources, and that both COI, the default DNA barcode supported by GenBank, and ITS, the de facto barcode accepted by the oomycete and mycology community, are acceptable and complementary DNA barcodes to be used for identification of oomycetes.

Bacterial volatiles induce systemic resistance in Arabidopsis.

Plant growth-promoting rhizobacteria, in association with plant roots, can trigger induced systemic resistance (ISR). Considering that low-molecular weight volatile hormone analogues such as methyl jasmonate and methyl salicylate can trigger defense responses in plants, we examined whether volatile organic compounds (VOCs) associated with rhizobacteria can initiate ISR. In Arabidopsis seedlings exposed to bacterial volatile blends from Bacillus subtilis GB03 and Bacillus amyloliquefaciens IN937a, disease severity by the bacterial pathogen Erwinia carotovora subsp. carotovora was significantly reduced compared with seedlings not exposed to bacterial volatiles before pathogen inoculation. Exposure to VOCs from rhizobacteria for as little as 4 d was sufficient to activate ISR in Arabidopsis seedlings. Chemical analysis of the bacterial volatile emissions revealed the release of a series of low-molecular weight hydrocarbons including the growth promoting VOC (2R,3R)-(-)-butanediol. Exogenous application of racemic mixture of (RR) and (SS) isomers of 2,3-butanediol was found to trigger ISR and transgenic lines of B. subtilis that emitted reduced levels of 2,3-butanediol and acetoin conferred reduced Arabidopsis protection to pathogen infection compared with seedlings exposed to VOCs from wild-type bacterial lines. Using transgenic and mutant lines of Arabidopsis, we provide evidence that the signaling pathway activated by volatiles from GB03 is dependent on ethylene, albeit independent of the salicylic acid or jasmonic acid signaling pathways. This study provides new insight into the role of bacteria VOCs as initiators of defense responses in plants.

Bacterial volatiles promote growth in Arabidopsis.

Several chemical changes in soil are associated with plant growth-promoting rhizobacteria (PGPR). Some bacterial strains directly regulate plant physiology by mimicking synthesis of plant hormones, whereas others increase mineral and nitrogen availability in the soil as a way to augment growth. Identification of bacterial chemical messengers that trigger growth promotion has been limited in part by the understanding of how plants respond to external stimuli. With an increasing appreciation of how volatile organic compounds signal plants and serve in plant defense, investigations into the role of volatile components in plant-bacterial systems now can follow. Here, we present chemical and plant-growth data showing that some PGPR release a blend of volatile components that promote growth of Arabidopsis thaliana. In particular, the volatile components 2,3-butanediol and acetoin were released exclusively from two bacterial strains that trigger the greatest level of growth promotion. Furthermore, pharmacological applications of 2,3-butanediol enhanced plant growth whereas bacterial mutants blocked in 2,3-butanediol and acetoin synthesis were devoid in this growth-promotion capacity. The demonstration that PGPR strains release different volatile blends and that plant growth is stimulated by differences in these volatile blends establishes an additional function for volatile organic compounds as signaling molecules mediating plant-microbe interactions.