Colonization of maize and rice plants by strain Bacillus megaterium C4

Metagenomics approaches in unveiling the dynamics of Plant Growth-Promoting Microorganisms (PGPM) vis-à-vis Phytophthora sp. suppression in various crop ecological systems

Phytophthora species are destructive pathogens causing yield losses in different ecological systems, such as potato, black pepper, pepper, avocado, citrus, and tobacco. The diversity of plant growth-promoting microorganisms (PGPM) plays a crucial role in disease suppression. Knowledge of metagenomics approaches is essential for assessing the dynamics of PGPM and Phytophthora species across various ecosystems, facilitating effective management strategies for better crop protection. This review discusses the dynamic interplay between PGPM and Phytophthora sp. using metagenomics approaches that sheds light on the potential of PGPM strains tailored to specific crop ecosystems to bolster pathogen suppressiveness.

The Complex GH32 Enzyme Orchestra from Priestia megaterium Holds the Key to Better Discriminate Sucrose-6-phosphate Hydrolases from Other β-Fructofuranosidases in Bacteria

Inulin is widely used as a prebiotic and emerging as a priming compound to counteract plant diseases. We isolated inulin-degrading strains from the lettuce phyllosphere, identified as Bacillus subtilis and Priestia megaterium, species hosting well-known biocontrol organisms. To better understand their varying inulin degradation strategies, three intracellular β-fructofuranosidases from P. megaterium NBRC15308 were characterized after expression in Escherichia coli: a predicted sucrose-6-phosphate (Suc6P) hydrolase (SacAP1, supported by molecular docking), an exofructanase (SacAP2), and an invertase (SacAP3). Based on protein multiple sequence and structure alignments of bacterial glycoside hydrolase family 32 enzymes, we identified conserved residues predicted to be involved in binding phosphorylated (Suc6P hydrolases) or nonphosphorylated substrates (invertases and fructanases). Suc6P hydrolases feature positively charged residues near the structural catalytic pocket (histidine, arginine, or lysine), whereas other β-fructofuranosidases contain tryptophans. This correlates with our phylogenetic tree, grouping all predicted Suc6P hydrolases in a clan associated with genomic regions coding for transporters involved in substrate phosphorylation. These results will help to discriminate between Suc6P hydrolases and other β-fructofuranosidases in future studies and to better understand the interaction of B. subtilis and P. megaterium endophytes with sucrose and/or fructans, sugars naturally present in plants or exogenously applied in the context of defense priming.

Plants and endophytes interaction: a "secret wedlock" for sustainable biosynthesis of pharmaceutically important secondary metabolites

Many plants possess immense pharmacological properties because of the presence of various therapeutic bioactive secondary metabolites that are of great importance in many pharmaceutical industries. Therefore, to strike a balance between meeting industry demands and conserving natural habitats, medicinal plants are being cultivated on a large scale. However, to enhance the yield and simultaneously manage the various pest infestations, agrochemicals are being routinely used that have a detrimental impact on the whole ecosystem, ranging from biodiversity loss to water pollution, soil degradation, nutrient imbalance and enormous health hazards to both consumers and agricultural workers. To address the challenges, biological eco-friendly alternatives are being looked upon with high hopes where endophytes pitch in as key players due to their tight association with the host plants. The intricate interplay between plants and endophytic microorganisms has emerged as a captivating subject of scientific investigation, with profound implications for the sustainable biosynthesis of pharmaceutically important secondary metabolites. This review delves into the hidden world of the "secret wedlock" between plants and endophytes, elucidating their multifaceted interactions that underpin the synthesis of bioactive compounds with medicinal significance in their plant hosts. Here, we briefly review endophytic diversity association with medicinal plants and highlight the potential role of core endomicrobiome. We also propose that successful implementation of in situ microbiome manipulation through high-end techniques can pave the way towards a more sustainable and pharmaceutically enriched future.

Synergistic and non-synergistic impact of HAP-based nano fertilizer and PGPR for improved nutrient utilization and metabolite variation in hemp crops

Nanofertilizer prepared with urea-hydroxyapatite amalgamation along with PGPR promotes urea availability over longer period of plant growth and reduces wasteful urea expense in soil, curtailing environmental pollution.

Low-temperature straw biochar: Sustainable approach for sustaining higher survival of B. megaterium and managing phosphorus deficiency in the soil

Inoculation of phosphate-solubilizing bacteria (PSB) is a sustainable approach to increase the available P content in soils for crop production. This application, however, is constrained by the low survival rate of PSB in the field. Biochar, a carbon-rich biomaterial with a well-developed porous structure, has recently emerged as an appealing option to maintain the population size of inoculants in the soil. The efficacy of biochar as a PSB carrier is primarily determined by its physicochemical properties, which are dominated by the feedstocks and the pyrolysis temperatures. This study demonstrated a comprehensive assessment of the efficacy of straw-derived biochars prepared from different feedstocks (i.e., crop straws from cotton, peanut, maize, soybean, and wheat) and pyrolysis temperatures (i.e., 300 and 600 °C). We employed B. megaterium carrying green fluorescence protein and evaluated its survival rate and phosphate-solubilizing performance in various inoculated biochars that have distinct physicochemical properties. Our results showed that the pyrolysis temperature is more determinant of the beneficial effect of straw biochar than the feedstock species. Cotton straw biochar pyrolyzed at low temperature (i.e., 300 °C) sustained a survival rate of 6.17% for the B. megaterium and thereby entailed a significant increase in available P in soil by 30.05 mg kg^-1 soil, which were nearly 18-fold and 8-fold higher than that of the no carrier treatment respectively. The performance of biochar-assisted PSB was dominant-negatively affected by the increasing pH, ash content, surface area, and total pore volume of biochar, while larger H/C ratio, water holding capacity, pore size, and surface hydrophobicity were predominantly conducive to the colonization and survival of PSB. The results of this study were expected to provide valuable guidance for biochar preparation in practice to enhance the survival and activity of PSB and maximize the utility of PSB as sustainable phosphorus fertilizer with economic applicability.

Phytostimulating Potential of Endophytic Bacteria from Ethnomedicinal Plants of North-East Indian Himalayan Region

North-East Indian Himalayan Region has a humid subtropical climate having diverse ecosystems. The majority of the population of the region depends on agriculture for sustainable livelihood. However, it can produce only 1.5% of the country’s food grains, thereby importing from other parts of the country for consumption. To feed the increase in the population of the region, there is an urgent need to augment the agricultural and allied products to sustain the population and uplift the economic conditions. Plant beneficial endophytes isolated from ethnomedicinal plants of North-East India play an important role as a plant growth promoter by the production of phytohormones, solubilization and mobilization of mineral nutrients. It also indirectly promotes growth by protecting the plants from diseases through the production of antibiotics, enzymes and volatile compounds. The bacteria also have the potential to induce systemic resistance against various abiotic stresses. Since the region has various agro-climatic conditions, the plants are continuously affected by abiotic stress particularly, acidity, drought and waterlogging, there is a need to explore the indigenous endophytes that can mitigate the stress and enhance the sustainable development of agricultural products.

Improving Soil Resource Uptake by Plants Through Capitalizing on Synergies Between Root Architecture and Anatomy and Root-Associated Microorganisms

Root architectural and anatomical phenotypes are highly diverse. Specific root phenotypes can be associated with better plant growth under low nutrient and water availability. Therefore, root ideotypes have been proposed as breeding targets for more stress-resilient and resource-efficient crops. For example, root phenotypes that correspond to the Topsoil Foraging ideotype are associated with better plant growth under suboptimal phosphorus availability, and root phenotypes that correspond to the Steep, Cheap and Deep ideotype are linked to better performance under suboptimal availability of nitrogen and water. We propose that natural variation in root phenotypes translates into a diversity of different niches for microbial associations in the rhizosphere, rhizoplane and root cortex, and that microbial traits could have synergistic effects with the beneficial effect of specific root phenotypes. Oxygen and water content, carbon rhizodeposition, nutrient availability, and root surface area are all factors that are modified by root anatomy and architecture and determine the structure and function of the associated microbial communities. Recent research results indicate that root characteristics that may modify microbial communities associated with maize include aerenchyma, rooting angle, root hairs, and lateral root branching density. Therefore, the selection of root phenotypes linked to better plant growth under specific edaphic conditions should be accompanied by investigating and selecting microbial partners better adapted to each set of conditions created by the corresponding root phenotype. Microbial traits such as nitrogen transformation, phosphorus solubilization, and water retention could have synergistic effects when correctly matched with promising plant root ideotypes for improved nutrient and water capture. We propose that elucidation of the interactive effects of root phenotypes and microbial functions on plant nutrient and water uptake offers new opportunities to increase crop yields and agroecosystem sustainability.

Complete genome sequence of biocontrol strain Paenibacillus peoriae HJ-2 and further analysis of its biocontrol mechanism

Background

Paris polyphylla is a herb widely used in traditional Chinese medicine to treat various diseases. Stem rot diseases seriously affected the yield of P. polyphylla in subtropical areas of China. Therefore, cost-effective, chemical-free, eco-friendly strategies to control stem rot on P. polyphylla are valuable and urgently needed.

Results

In this paper, we reported the biocontrol efficiency of Paenibacillus peoriae HJ-2 and its complete genome sequence. Strain HJ-2 could serve as a potential biocontrol agent against stem rot on P. polyphylla in the greenhouse and field. The genome of HJ-2 consists of a single 6,001,192 bp chromosome with an average GC content of 45% and 5,237 predicted protein coding genes, 39 rRNAs and 108 tRNAs. The phylogenetic tree indicated that HJ-2 is most closely related to P. peoriae IBSD35. Functional analysis of genome revealed numerous genes/gene clusters involved in plant colonization, biofilm formation, plant growth promotion, antibiotic and resistance inducers synthesis. Moreover, metabolic pathways that potentially contribute to biocontrol mechanisms were identified.

Conclusions

This study revealed that P. peoriae HJ-2 could serve as a potential BCA against stem rot on P. polyphylla. Based on genome analysis, the genome of HJ-2 contains more than 70 genes and 12 putative gene clusters related to secondary metabolites, which have previously been described as being involved in chemotaxis motility, biofilm formation, growth promotion, antifungal activity and resistance inducers biosynthesis. Compared with other strains, variation in the genes/gene clusters may lead to different antimicrobial spectra and biocontrol efficacies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08330-0.

Spatial Distribution of the Pepper Blight (Phytophthora capsici) Suppressive Microbiome in the Rhizosphere

The properties of plant rhizosphere are dynamic and heterogeneous, serving as different habitat filters for or against certain microorganisms. Herein, we studied the spatial distribution of bacterial communities in the rhizosphere of pepper plants treated with a disease-suppressive or non-suppressive soil. The bacterial richness was significantly (p < 0.05) higher in plants treated with the disease-suppressive soil than in those treated with the non-suppressive soil. Bacterial richness and evenness greatly differed between root parts, with decrease from the upper taproot to the upper fibrous root, the lower taproot, and the lower fibrous root. As expected, the bacterial community in the rhizosphere differed between suppressive and non-suppressive soil. However, the spatial variation (36%) of the bacterial community in the rhizosphere was much greater than that explained by soils (10%). Taxa such as subgroups of Acidobacteria, Nitrosospira, and Nitrospira were known to be selectively enriched in the upper taproot. In vitro Bacillus antagonists against Phytophthora capsici were also preferentially colonized in the taproot, while the genera such as Clostridium, Rhizobium, Azotobacter, Hydrogenophaga, and Magnetospirillum were enriched in the lower taproot or fibrous root. In conclusion, the spatial distribution of bacterial taxa and antagonists in the rhizosphere of pepper sheds light on our understanding of microbial ecology in the rhizosphere.

Salt-Tolerant Compatible Microbial Inoculants Modulate Physio-Biochemical Responses Enhance Plant Growth, Zn Biofortification and Yield of Wheat Grown in Saline-Sodic Soil

A wide range of root-associated mutualistic microorganisms have been successfully applied and documented in the past for growth promotion, biofertilization, biofortification and biotic and abiotic stress amelioration in major crops. These microorganisms include nitrogen fixers, nutrient mobilizers, bio-remediators and bio-control agents. The present study aimed to demonstrate the impact of salt-tolerant compatible microbial inoculants on plant growth; Zn biofortification and yield of wheat (Triticum aestivum L.) crops grown in saline-sodic soil and insight of the mechanisms involved therein are being shared through this paper. Field experiments were conducted to evaluate the effects of Trichoderma harzianum UBSTH-501 and Bacillus amyloliquefaciens B-16 on wheat grown in saline-sodic soil at Research Farm, ICAR-Indian Institute of Seed Sciences, Kushmaur, India. The population of rhizosphere-associated microorganisms changed dramatically upon inoculation of the test microbes in the wheat rhizosphere. The co-inoculation induced a significant accumulation of proline and total soluble sugar in wheat at 30, 60, 90 and 120 days after sowing as compared to the uninoculated control. Upon quantitative estimation of organic solutes and antioxidant enzymes, these were found to have increased significantly in co-inoculated plants under salt-stressed conditions. The application of microbial inoculants enhanced the salt tolerance level significantly in wheat plants grown in saline-sodic soil. A significant increase in the uptake and translocation of potassium (K⁺) and calcium (Ca²⁺) was observed in wheat co-inoculated with the microbial inoculants, while a significant reduction in sodium (Na⁺) content was recorded in plants treated with both the bio-agents when compared with the respective uninoculated control plants. Results clearly indicated that significantly higher expression of TaHKT-1 and TaNHX1 in the roots enhances salt tolerance effectively by maintaining the Na⁺/K⁺ balance in the plant tissue. It was also observed that co-inoculation of the test inoculants increased the expression of ZIP transporters (2–3.5-folds) which ultimately led to increased biofortification of Zn in wheat grown in saline-sodic soil. Results suggested that co-inoculation of T. harzianum UBSTH-501 and B. amyloliquefaciens B-16 not only increased plant growth but also improved total grain yield along with a reduction in seedling mortality in the early stages of crop growth. In general, the present investigation demonstrated the feasibility of using salt-tolerant rhizosphere microbes for plant growth promotion and provides insights into plant-microbe interactions to ameliorate salt stress and increase Zn bio-fortification in wheat.

A Thermotolerant Marine Bacillus amyloliquefaciens S185 Producing Iturin A5 for Antifungal Activity against Fusarium oxysporum f. sp. cubense

Fusarium wilt of banana (also known as Panama disease), is a severe fungal disease caused by soil-borne Fusarium oxysporum f. sp. cubense (Foc). In recent years, biocontrol strategies using antifungal microorganisms from various niches and their related bioactive compounds have been used to prevent and control Panama disease. Here, a thermotolerant marine strain S185 was identified as Bacillus amyloliquefaciens, displaying strong antifungal activity against Foc. The strain S185 possesses multiple plant growth-promoting (PGP) and biocontrol utility properties, such as producing indole acetic acid (IAA) and ammonia, assimilating various carbon sources, tolerating pH of 4 to 9, temperature of 20 to 50 °C, and salt stress of 1 to 5%. Inoculation of S185 colonized the banana plants effectively and was mainly located in leaf and root tissues. To further investigate the antifungal components, compounds were extracted, fractionated, and purified. One compound, inhibiting Foc with minimum inhibitory concentrations (MICs) of 25 μg/disk, was identified as iturin A5 by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) and nuclear magnetic resonance (NMR). The isolated iturin, A5, resulted in severe morphological changes during spore germination and hyphae growth of Foc. These results specify that B. amyloliquefaciens S185 plays a key role in preventing the Foc pathogen by producing the antifungal compound iturin A5, and possesses potential as a cost-effective and sustainable biocontrol strain for Panama disease in the future. This is the first report of isolation of the antifungal compound iturin A5 from thermotolerant marine B. amyloliquefaciens S185.

The "beauty in the beast"-the multiple uses of Priestia megaterium in biotechnology

Abstract

Over 30 years, the Gram-positive bacterium Priestia megaterium (previously known as Bacillus megaterium) was systematically developed for biotechnological applications ranging from the production of small molecules like vitamin B₁₂, over polymers like polyhydroxybutyrate (PHB) up to the in vivo and in vitro synthesis of multiple proteins and finally whole-cell applications. Here we describe the use of the natural vitamin B₁₂ (cobalamin) producer P. megaterium for the elucidation of the biosynthetic pathway and the subsequent systematic knowledge-based development for production purposes. The formation of PHB, a natural product of P. megaterium and potential petro-plastic substitute, is covered and discussed. Further important biotechnological characteristics of P. megaterium for recombinant protein production including high protein secretion capacity and simple cultivation on value-added carbon sources are outlined. This includes the advanced system with almost 30 commercially available expression vectors for the intracellular and extracellular production of recombinant proteins at the g/L scale. We also revealed a novel P. megaterium transcription-translation system as a complementary and versatile biotechnological tool kit. As an impressive biotechnology application, the formation of various cytochrome P450 is also critically highlighted. Finally, whole cellular applications in plant protection are completing the overall picture of P. megaterium as a versatile giant cell factory.

Key points

• The use of Priestia megaterium for the biosynthesis of small molecules and recombinant proteins through to whole-cell applications is reviewed.

• P. megaterium can act as a promising alternative host in biotechnological production processes.

Bacillus subtilis MBI600 Promotes Growth of Tomato Plants and Induces Systemic Resistance Contributing to the Control of Soilborne Pathogens

Bacillus subtilis MBI600 (Bs MBI600) is a recently commercialized plant-growth-promoting rhizobacterium (PGPR). In this study, we investigated the effects of Bs MBI600 on the growth of tomato and its biocontrol efficacy against three main soilborne tomato pathogens (Rhizoctonia solani, Pythium ultimum, and Fusarium oxysporum f.sp. radicis-lycopersici-Forl). Furthermore, the root colonization ability of the Bs MBI600 strain on tomato roots was analyzed in vivo with a yellow fluorescence protein (yfp)-labeled strain, revealing strong colonization ability, which was affected by the root growth substrate. The application of Bs MBI600 on tomato plants resulted in significant increases in shoot and root lengths. Transcriptional activation of two auxin-related genes (SiPin6 and SiLax4) was observed. Single applications of Bs MBI600 on inoculated tomato plants with pathogens revealed satisfactory control efficacy compared to chemical treatment. Transcriptomic analysis of defense-related genes used as markers of the salicylic acid (SA) signaling pathway (PR-1A and GLUA) or jasmonic acid/ethylene (JA/ET) signaling pathway (CHI3, LOXD, and PAL) showed increased transcription patterns in tomato plants treated with Bs MBI600 or Forl. These results indicate the biochemical and molecular mechanisms that are activated after the application of Bs MBI600 on tomato plants and suggest that induction of systemic resistance (ISR) occurred.

Restructuring the Cellular Responses: Connecting Microbial Intervention With Ecological Fitness and Adaptiveness to the Maize (Zea mays L.) Grown in Saline-Sodic Soil

Salt stress hampers plant growth and development. It is now becoming one of the most important threats to agricultural productivity. Rhizosphere microorganisms play key roles in modulating cellular responses and enable plant tolerant to salt stress, but the detailed mechanisms of how this occurs need in-depth investigation. The present study elucidated that the microbe-mediated restructuring of the cellular responses leads to ecological fitness and adaptiveness to the maize (Zea mays L.) grown in saline-sodic soil. In the present study, effects of seed biopriming with B. safensis MF-01, B. altitudinis MF-15, and B. velezensis MF-08 singly and in consortium on different growth parameters were recorded. Soil biochemical and enzymatic analyses were performed. The activity and gene expression of High-Affinity K+ Transporter (ZmHKT-1), Sodium/Hydrogen exchanger 1 (zmNHX1), and antioxidant enzymes (ZmAPX1.2, ZmBADH-1, ZmCAT, ZmMPK5, ZmMPK7, and ZmCPK11) were studied. The expression of genes related to lateral root development (ZmHO-1, ZmGSL-1, and ZmGSL-3) and root architecture were also carried out. Seeds bioprimed with consortium of all three strains have been shown to confer increased seed germination (23.34-26.31%) and vigor indices (vigor index I: 38.71-53.68% and vigor index II: 74.11-82.43%) as compared to untreated control plant grown in saline-sodic soil at 30 days of sowing. Results indicated that plants treated with consortium of three strains induced early production of adventitious roots (tips: 4889.29, forks: 7951.57, and crossings: 2296.45) in maize compared to plants primed with single strains and untreated control (tips: 2019.25, forks: 3021.45, and crossings: 388.36), which was further confirmed by assessing the transcript level of ZmHO-1 (7.20 folds), ZmGSL-1 (4.50 folds), and ZmGSL-3 (12.00 folds) genes using the qPCR approach. The uptake and translocation of Na+, K+, and Ca2+ significantly varied in the plants treated with bioagents alone or in consortium. qRT-PCR analysis also revealed that the ZmHKT-1 and zmNHX1 expression levels varied significantly in the maize root upon inoculation and showed a 6- to 11-fold increase in the plants bioprimed with all the three strains in combination. Further, the activity and gene expression levels of antioxidant enzymes were significantly higher in the leaves of maize subjected seed biopriming with bioagents individually or in combination (3.50- to 12.00-fold). Our research indicated that ZmHKT-1 and zmNHX1 expression could effectively enhance salt tolerance by maintaining an optimal Na+/K+ balance and increasing the antioxidant activity that keeps reactive oxygen species at a low accumulation level. Interestingly, up-regulation of ZmHKT-1, NHX1, ZmHO-1, ZmGSL-1, and ZmGSL-3 and genes encoding antioxidants regulates the cellular responses that could effectively enhance the adaptiveness and ultimately leads to better plant growth and grain production in the maize crop grown in saline-sodic soil.

From the roots to the stem: unveiling pear root colonization and infection pathways by Erwinia amylovora

Fire blight caused by Erwinia amylovora affects pome fruit worldwide, generating serious economic losses. Despite the abundant literature on E. amylovora infection mechanisms of aerial plant organs, root infection routes remain virtually unexplored. Assessing these infection pathways is necessary for a full understanding of the pathogen's ecology. Using the pathosystem Pyrus communis-E. amylovora and different experimental approaches including a green fluorescent protein transformant (GFP1) and epifluorescence microscopy (EFM) and laser confocal scanning microscopy (LCSM), we demonstrated the pathogen's ability to infect, colonize and invade pear roots and cause characteristic fire blight symptoms both in the aerial part and in the root system. Plant infections after soil irrigation with E. amylovora-contaminated water were favored by root damage, which agreed with EFM and LCSM observations. E. amylovora GFP1 cells formed aggregates/biofilms on root surfaces and invaded the cortex through wounds and sites of lateral root emergence. Sugars, sugar-alcohols and amino acids typically secreted by roots, favored the in vitro biofilm development by E. amylovora. Migration of E. amylovora cells to aerial tissues mainly occurred after xylem penetration. Overall, our findings revealed, for the first time, common root infection patterns between E. amylovora and well-known soil borne plant pathogens and endophytes.

Colonization on Cotton Plants with a GFP Labeled Strain of Bacillus axarquiensis

Verticillium dahliae was one of the most important diseases caused Verticillium wilt of cotton. In our previous study, Bacillus axarquiensis TUBP1 was screened and found to be an antagonistic strain against V. dahliae with 43% biocontrol effect in the cotton field. In order to uncover the functional mechanism of B. axarquiensis against Verticillium wilt in cotton, the colonization of B. axarquiensis labeled with a green fluorescent protein (GFP) was investigated in cotton plants and the rhizosphere soil. Firstly, a plasmid (pHT-315) containing gfp gene was successfully transformed into wild B. axarquiensis TUBP1 and labeled a green fluorescence by electroporation, which didn't change the bioactivity in vitro. In gnotobiotic conditions, cotton seeds were then inoculated with the gfp-labeled strain and grown in green house. Observation with a confocal laser scanning microscope and a scanning electron microscope showed that GFP-labeled B. axarquiensis TUBP1 infected cotton roots and widely distributed in epidermis, cortical parenchyma, intercellular spaces, the xylem vessels, and pith cells as well as root hair cells through cracks formed at the lateral root junctions, followed by a slow migration from roots to stems and leaves. Quantitative fluorescence and flow cytometry (FACS) approaches showed a gradual decrease in the number of TUBP1-315gfp with increasing inoculation time. However, TUBP1-315gfp levels were detectable till 45 days after planting. In contrast, no fluorescence signal was detected in the non-inoculated groups. Therefore, GFP-labeled B. axarquiensis TUBP1 exhibited colonization in different parts of cotton plants from the rhizosphere soil.

Screening and Application of Bacillus Strains Isolated from Nonrhizospheric Rice Soil for the Biocontrol of Rice Blast

Rice blast, caused by Magnaporthe oryzae, is one of the most destructive rice diseases worldwide. The aim of this study was to screen bacterial isolates to efficiently prevent the occurrence of rice blast. A total of 232 bacterial isolates were extracted from nonrhizospheric rice soil and were screened for antifungal activity against M. oryzae using a leaf segment assay. Strains S170 and S9 showed significant antagonistic activity against M. oryzae in vitro and in leaf disk assays, and controlled M. oryzae infection under greenhouse conditions. The results showed that strains S170 and S9 could effectively control rice leaf blast and panicle neck blast after five spray treatments in field. This suggested that the bacterial strains S170 and S9 were valuable and promising for the biocontrol of rice disease caused by M. oryzae. Based on 16S rDNA, and gyrA and gyrB gene sequence analyses, S170 and S9 were identified as Bacillus amyloliquefaciens and B. pumilus, respectively. The research also demonstrated that B. amyloliquefaciens S170 and B. pumilus S9 could colonize rice plants to prevent pathogenic infection and evidently suppressed plant disease caused by 11 other plant pathogenic fungi. This is the first study to demonstrate that B. amyloliquefaciens and B. pumilus isolated from nonrhizospheric rice soil are capable of recolonizing internal rice stem tissues.

Diversity of nitrogen-fixing rhizobacteria associated with sugarcane: a comprehensive study of plant-microbe interactions for growth enhancement in Saccharum spp

BACKGROUND

Nitrogen is an essential element for sugarcane growth and development and is generally applied in the form of urea often much more than at recommended rates, causing serious soil degradation, particularly soil acidification, as well as groundwater and air pollution. In spite of the importance of nitrogen for plant growth, fewer reports are available to understand the application and biological role of N2 fixing bacteria to improve N2 nutrition in the sugarcane plant.

RESULTS

In this study, a total of 350 different bacterial strains were isolated from rhizospheric soil samples of the sugarcane plants. Out of these, 22 isolates were selected based on plant growth promotion traits, biocontrol, and nitrogenase activity. The presence and activity of the nifH gene and the ability of nitrogen-fixation proved that all 22 selected strains have the ability to fix nitrogen. These strains were used to perform 16S rRNA and rpoB genes for their identification. The resulted amplicons were sequenced and phylogenetic analysis was constructed. Among the screened strains for nitrogen fixation, CY5 (Bacillus megaterium) and CA1 (Bacillus mycoides) were the most prominent. These two strains were examined for functional diversity using Biolog phenotyping, which confirmed the consumption of diverse carbon and nitrogen sources and tolerance to low pH and osmotic stress. The inoculated bacterial strains colonized the sugarcane rhizosphere successfully and were mostly located in root and leaf. The expression of the nifH gene in both sugarcane varieties (GT11 and GXB9) inoculated with CY5 and CA1 was confirmed. The gene expression studies showed enhanced expression of genes of various enzymes such as catalase, phenylalanine-ammonia-lyase, superoxide dismutase, chitinase and glucanase in bacterial-inoculated sugarcane plants.

CONCLUSION

The results showed that a substantial number of Bacillus isolates have N-fixation and biocontrol property against two sugarcane pathogens Sporisorium scitamineum and Ceratocystis paradoxa. The increased activity of genes controlling free radical metabolism may at least in part accounts for the increased tolerance to pathogens. Nitrogen-fixation was confirmed in sugarcane inoculated with B. megaterium and B. mycoides strains using N-balance and 15N2 isotope dilution in different plant parts of sugarcane. This is the first report of Bacillus mycoides as a nitrogen-fixing rhizobacterium in sugarcane.

Effect of endophytic Bacillus megaterium colonization on structure strengthening, microbial community, chemical composition and stabilization properties of Hybrid Pennisetum

BACKGROUND

This study was conducted to analyze the effects of endophytic Bacillus megaterium (BM 18-2) colonization on structure strengthening, microbial community, chemical composition and stabilization properties of Hybrid Pennisetum.

RESULTS

The BM 18-2 had successfully colonized in the interior tissues in both leaf and stem of Hybrid Pennisetum. During ensiling, the levels of pH, acetic acid (AA), butyric acid (BA), propionic acid (PA), and the population of yeast and aerobic bacteria were significantly (P > 0.05) lower, while lactic acid bacteria (LAB) and lactic acid (LA) were significantly (P < 0.001) higher with the steps forward of ensiling in with BM 18-2 as compared to without BM 18-2 colonized of Hybrid Pennisetum. During the different ensiling days, at days 3, 6, 15, and 30, the genus Brevundimonas, Klebsiella, Lactococcus, Weissella, Enterobacter, Serratia, etc. population were significantly decreased, while genus Pediococcus acidilactici and Lactobacillus plantarum were significantly influenced in treated groups as compared to control. The genus Lactobacillus and Pediococcus were positively correlated with treatment groups.

CONCLUSIONS

It is concluded that the endophytic bacteria strain BM 18-2 significantly promoted growth characteristics and biomass yield before ensiling and after ensiling inoculated with or without Lactobacillus plantarum could improve the distinct changes of the undesirable microbial diversity, chemical composition, and stabilization properties in with BM 18-2 as compared to without BM 18-2 colonized Hybrid Pennisetum. © 2019 Society of Chemical Industry.

Diversity of cultivable endophytic bacteria in mulberry and their potential for antimicrobial and plant growth-promoting activities

Endophytic bacteria-based biocontrol is regarded as a potential plant disease management strategy. Present study analyzed the diversity of mulberry endophytic bacteria basing on a culture-dependent approach and further evaluated their antimicrobial and plant growth-promoting (PGP) activities. A total of 608 cultivable endophytic bacteria, belonging to 4 phyla and 36 genera, were isolated from four mulberry cultivars having different resistance to sclerotiniosis in three seasons. Taxonomic compositional analysis results showed that Proteobacteria, Firmicutes, and Actinobacteria were the three dominant bacterial phyla in all communities, with the representative genera Pantoea, Bacillus, Pseudomonas, Curtobacterium, and Sphingomonas. Diversity analysis results indicated that the diversity of winter community was higher than that of spring or autumn, and higher diversities were detected in the resistant cultivar communities compared with the susceptible cultivar. Antagonism assays results showed that 33 isolates exhibited strong and stable activity against three phytopathogens which are Sclerotinia sclerotiorum, Botrytis cinerea, and Colletotrichum gloeosporioide. Eight endophytic bacteria were selected out from 33 antagonists based on the evaluation of antagonistic and PGP activities. Furthermore, pot experiment results revealed that all the 8 tested endophytes stimulated the growth of mulberry seedlings at different levels, and Bacillus sp. CW16-5 exhibited the highest promotion capacity, which the shoot length and the root fresh weight were increased by 83.37% and 217.70%, respectively. Altogether, present study revealed that mulberry harbors a large amount of diverse cultivable endophytic bacteria and they also serve as novel sources of beneficial bacteria and bioactive metabolites.

Root colonization and growth promotion of soybean, wheat and Chinese cabbage by Bacillus cereus YL6

Although phosphate-solubilizing bacteria (PSBs) are used in agricultural production, comprehensive research on PSB that colonize the rhizosphere of different plants and promote plant growth is lacking. This study was conducted to examine the growth-promoting effects and colonizing capacity of strain YL6, a PSB. YL6 not only increased the biomass of soybean and wheat in pot experiments but also increased the yield and growth of Chinese cabbage under field conditions. The observed growth promotion was related to the capacity of YL6 to dissolve inorganic and organic phosphorus and to produce indole-3-acetic (IAA) and gibberellin (GA). After applying YL6 to soybean, wheat and Chinese cabbage, the rhizosphere soil available phosphorus (available P) content increased by 120.16%, 62.47% and 7.21%, respectively, and the plant total phosphorus content increased by 198.60%, 6.20% and 78.89%, respectively, compared with plants not treated with YL6. To examine plant colonization, YL6 labeled with green fluorescent protein (YL6-GFP) was inoculated into the plant rhizosphere and found to first colonize the root surface and hairs and then to penetrate into the intercellular spaces and vessels. Collectively, these results demonstrate that YL6 promotes the growth of three different crops and colonizes them in a similar manner. The findings therefore provide a solid foundation for probing the mechanisms by which PSB affect plant growth.

Enhanced removal of nitrate in the maize rhizosphere by plant growth-promoting Bacillus megaterium NCT-2, and its colonization pattern in response to nitrate

High soil nitrate concentrations can lead to the secondary salinization of soils. Bacillus megaterium NCT-2 is a wild-type strain isolated from secondary salinized soil and is very effective in reducing nitrate. Laboratory and greenhouse experiments were carried out to investigate its nitrate reduction capabilities, colonization pattern, and plant growth promotion responses to nitrate content in the soil. B. megaterium NCT-2 was marked with a green fluorescent protein (gfp) gene and was left to successfully colonize maize roots and the rhizosphere. Inoculation with gfp-tagged NCT-2 significantly promoted nitrate removal from the soil and improved plant growth. Confocal microscopy results revealed that NCT-2 is an endophyte that can colonize the meristematic and elongation zones of the root tip, and the middle segment of the root. Soil nitrate concentration had no significant effect on NCT-2 distribution. The gfp-tagged NCT-2 populations in the roots and rhizosphere soil first increased, but then decreased, and at the end of the experiment, colonization levels in the rhizosphere soil stabilized at ∼5 × 10⁴ CFU g^-1 soil. However, the levels in the roots increased again to 1-3 × 10⁴ CFU g^-1 root in the different treatments. The NCT-2 population in the roots was significantly affected by nitrate content. A nitrate-nitrogen concentration of 72 mg kg^-1 was the optimum concentration for NCT-2 colonization of maize roots. This study will improve the agricultural application of NCT-2 as a biofertilizer for nitrate removal and plant growth promotion.

Bacillus velezensis CC09: A Potential 'Vaccine' for Controlling Wheat Diseases

Biocontrol bacteria that can act like a "vaccine", stimulating plant resistance to pathogenic diseases, are still not fully elucidated. In this study, an endophytic bacterium, Bacillus velezensis CC09, labeled with green fluorescent protein, was tested for its colonization, migration, and expression of genes encoding iturin A synthetase within wheat tissues and organs as well as for protective effects against wheat take-all and spot blotch diseases. The results showed that strain CC09 not only formed biofilm on the root surface but was also widely distributed in almost every tissue, including the epidermis, cortex, and xylem vessels, and even migrated to stems and leaves, resulting in 66.67% disease-control efficacy (DCE) of take-all and 21.64% DCE of spot blotch. Moreover, the gene cluster encoding iturin A synthase under the control of the p_itu promoter is expressed in B. velezensis CC09 in wheat tissues, which indicates that iturin A might contribute to the in-vivo antifungal activity and leads to the disease control. All these data suggested that strain CC09 can act like a 'vaccine' in the control of wheat diseases, with a single treatment inoculated on roots through multiple mechanisms.

Growth stage and tissue specific colonization of endophytic bacteria having plant growth promoting traits in hybrid and composite maize (Zea mays L.)

Maize, a crop cultivated worldwide, was investigated for plant tissue and crop stage specific colonization of endophytic bacteria. Such bacterial interactions have high potential to enhance maize grain yield by means of biological nitrogen fixation and/or plant growth promoting activities. In this study endophytic bacteria were isolated from a hybrid PEEHM-5 and composite PC-4 maize varieties using root, stem and leaf tissues of plants at vegetative, flowering and maturity stages of growth. PEEHM-5 harbored higher endophytic bacterial population than PC-4 at all growth stages, with highest in roots and at flowering stage. Morphologically 188 different endophytic isolates (82 from PEEHM-5, 106 from PC-4) were screened for plant growth promoting attributes viz. P, K, Zn solubilization, production of hormones, siderophore, ACC deaminase, HCN, biological nitrogen fixation and biocontrol of two maize fungal pathogens. Thirty one potential PGP isolates on RFLP analysis of their amplified 16S rRNA gene, were clustered in 13 phylogenetic groups. On sequencing and blasting of amplified 16S rRNA gene of representative isolates from each group identified PC-4 endophytic bacterial isolates as Bacillus aryabhattai, Pantoea cypripedii, Bacillus licheniformis, Klebsiella sp., Pantoea dispersa, Klebsiella variicola, Pantoea sp., Agrobacterium larrymoorei and PEEHM-5 endophytic bacterial isolates as Bacillus sp., Bacillus amyloliquefaciens, Lactococcus lactis, Bacillus cereus and Staphylococcus hominis. In planta evaluation of potential isolates at variable chemical fertilizer input indicated their potential in compensating nearly 25% of the fertilizer input as observed on their improvement of shoot and root parameters. Lactococcus lactis inoculation influenced maximum followed by Pantoea and Klebsiella isolates.

Bacterial Endophyte Colonization and Distribution within Plants

The plant endosphere contains a diverse group of microbial communities. There is general consensus that these microbial communities make significant contributions to plant health. Both recently adopted genomic approaches and classical microbiology techniques continue to develop the science of plant-microbe interactions. Endophytes are microbial symbionts residing within the plant for the majority of their life cycle without any detrimental impact to the host plant. The use of these natural symbionts offers an opportunity to maximize crop productivity while reducing the environmental impacts of agriculture. Endophytes promote plant growth through nitrogen fixation, phytohormone production, nutrient acquisition, and by conferring tolerance to abiotic and biotic stresses. Colonization by endophytes is crucial for providing these benefits to the host plant. Endophytic colonization refers to the entry, growth and multiplication of endophyte populations within the host plant. Lately, plant microbiome research has gained considerable attention but the mechanism allowing plants to recruit endophytes is largely unknown. This review summarizes currently available knowledge about endophytic colonization by bacteria in various plant species, and specifically discusses the colonization of maize plants by Populus endophytes.

Genetic Diversity of Nitrogen-Fixing and Plant Growth Promoting Pseudomonas Species Isolated from Sugarcane Rhizosphere

The study was designed to isolate and characterize Pseudomonas spp. from sugarcane rhizosphere, and to evaluate their plant- growth- promoting (PGP) traits and nitrogenase activity. A biological nitrogen-fixing microbe has great potential to replace chemical fertilizers and be used as a targeted biofertilizer in a plant. A total of 100 isolates from sugarcane rhizosphere, belonging to different species, were isolated; from these, 30 isolates were selected on the basis of preliminary screening, for in vitro antagonistic activities against sugarcane pathogens and for various PGP traits, as well as nitrogenase activity. The production of IAA varied from 312.07 to 13.12 μg mL^-1 in tryptophan supplemented medium, with higher production in AN15 and lower in CN20 strain. The estimation of ACC deaminase activity, strains CY4 and BA2 produced maximum and minimum activity of 77.0 and 15.13 μmoL mg^-1 h^-1. For nitrogenase activity among the studied strains, CoA6 fixed higher and AY1 fixed lower in amounts (108.30 and 6.16 μmoL C₂H₂ h^-1 mL^-1). All the strains were identified on the basis of 16S rRNA gene sequencing, and the phylogenetic diversity of the strains was analyzed. The results identified all strains as being similar to Pseudomonas spp. Polymerase chain reaction (PCR) amplification of nifH and antibiotic genes was suggestive that the amplified strains had the capability to fix nitrogen and possessed biocontrol activities. Genotypic comparisons of the strains were determined by BOX, ERIC, and REP PCR profile analysis. Out of all the screened isolates, CY4 (Pseudomonas koreensis) and CN11 (Pseudomonas entomophila) showed the most prominent PGP traits, as well as nitrogenase activity. Therefore, only these two strains were selected for further studies; Biolog profiling; colonization through green fluorescent protein (GFP)-tagged bacteria; and nifH gene expression using quantitative real-time polymerase chain reaction (qRT-PCR) analysis. The Biolog phenotypic profiling, which comprised utilization of C and N sources, and tolerance to osmolytes and pH, revealed the metabolic versatility of the selected strains. The colonization ability of the selected strains was evaluated by genetically tagging them with a constitutively expressing GFP-pPROBE-pTet^r-OT plasmid. qRT-PCR results showed that both strains had the ability to express the nifH gene at 90 and 120 days, as compared to a control, in both sugarcane varieties GT11 and GXB9. Therefore, our isolated strains, P. koreensis and P. entomophila may be used as inoculums or in biofertilizer production for enhancing growth and nutrients, as well as for improving nitrogen levels, in sugarcane and other crops. The present study, to the best of our knowledge, is the first report on the diversity of Pseudomonas spp. associated with sugarcane in Guangxi, China.

Colonization and Maize Growth Promotion Induced by Phosphate Solubilizing Bacterial Isolates

Phosphorus (P) limits the production of maize, one of the major food crops in China. Phosphate-solubilizing bacteria (PSB) have the capacity to solubilize phosphate complexes into plant absorbable and utilizable forms by the process of acidification, chelation, and exchange reactions. In this study, six bacteria, including one Paenibacillus sp. B1 strain, four Pseudomonas sp. strains (B10, B14, SX1, and SX2) and one Sphingobium sp. SX14 strain, were those isolated from the maize rhizosphere and identified based on their 16S rRNA sequences. All strains could solubilize inorganic P (Ca₃(PO₄)₂, FePO₄ and AlPO₄), and only B1 and B10 organic P (lecithin). All strains, except of SX1, produced IAA, and SX14 and B1 showed the highest level. B1 incited the highest increase in root length and the second increase in shoot and total dry weight, shoot length, and total P and nitrogen (N), along with increased root length. In addition, by confocal laser scanning microscopy (CLSM), we found that green fluorescent protein (GFP)-labeled B1 mainly colonized root surfaces and in epidermal and cortical tissue. Importantly, B1 can survive through forming spores under adverse conditions and prolong quality guarantee period of bio-fertilizer. Therefore, it can act as a good substitute for bio-fertilizer to promote agricultural sustainability.

Colonization of Wheat, Maize and Cucumber by Paenibacillus polymyxa WLY78

Paenibacillus polymyxa WLY78 is a nitrogen fixer and it can be potentially applied to biofertilizer in agriculture. In this study, P. polymyxa WLY78 is labelled with gfp gene. The GFP-labelled P. polymyxa WLY78 is used to inoculate wheat, maize and cucumber seedlings grown in the gnotobiotic system and in soil, respectively. Observation by confocal laser scanning microscope reveals that the GFP-labeled bacterial cells are mainly located on the root surface and epidermis of wheat, and only a few cells are present within cortical cells. In maize and cucumber seedlings, bacterial cells were colonized in epidermal and cortical cells, intercellular spaces and vascular system of root, stem and leaf tissue interiors besides on root surfaces. Higher densities of the bacterial cells in roots, stems and leaves indicated that P. polymyxa WLY78 cells could migrate from roots to stems and leaves of maize and cucumber. This study will provide insight into interaction between P. polymyxa WLY78 and host cells.

Effects of Selected Diazotrophs on Maize Growth

Laboratory, greenhouse, and field experiments were conducted at the University of KwaZulu-Natal, Pietermaritzburg, South Africa in the 2010/2011 and 2011∖2012 seasons to study the effects of eight strains of diazotrophic bacteria on the growth and yield of maize. Maize seeds were treated with Bacillus megaterium (V16), Pseudomonas sp. (StB5, A3, A6, and A61), Burkholderia ambifaria (V9), Enterobacter cloacae (L1) and Pantoea ananatis (LB5), aiming to stimulate plant growth, and maintain or increase yields while reducing the need for N fertilization. All the diazotrophic bacteria increased germination of maize seed, and Pseudomonas sp. (StB5) and B. megaterium (V16) significantly increased shoot length. Pseudomonas sp. (StB5), B. megaterium (V16), E. cloacae (L1), B. ambifaria (V9), and Pseudomonas sp. (A3) very significantly increased root length and seed vigor index. Under greenhouse conditions, plants treated with diazotrophic bacteria developed more leaf chlorophyll and greater dry weight, albeit not significantly (n.s.). In a field trial in 2010/2011, application of the best five diazotrophic bacteria, with or without 33% N-fertilizer, had no significant effect on germination, grain yield, dry weight, plant height and leaf chlorophyll. In the 2011/2012 growing season, at 60 days after planting (DAP), all the diazotrophic bacteria increased plant dry weights to equal that of the fertilized control (33%N-fertilizer) (n.s.). After inoculation with the diazotrophs alone increased plant heights (n.s.), and chlorophyll contents (n.s.). With the addition of 33%N-fertilizer at planting, the diazotrophs still caused increases of chlorophyll content relative to the control with 33%N (n.s.). It may be concluded that the tested diazotrophs alone may be beneficial for use on maize growth.

Suppression of Magnaporthe oryzae and interaction between Bacillus subtilis and rice plants in the control of rice blast

Magnaporthe oryzae, the causative pathogen of rice blast, has caused extensive losses to rice cultivation worldwide. Strains of the bacterium Bacillus subtilis have been used as biocontrol agents against rice blast. However, little has been reported about the interaction between B. subtilis and the rice plant and its mechanism of action. Here, the colonization process and induced disease resistance by B. subtilis SYX04 and SYX20 in rice plants was examined. Strains of B. subtilis labeled with green fluorescent protein reached population of more than 5 × 10⁶ CFU/g after 20 days on mature rice leaves and were detected after 3 days on newly grown leaves. Results showed that SYX04 and SYX20 not only inhibited spore germination, germ tube length, and appressorial formation but also caused a series of alterations in the structures of hyphae and conidia. The cell walls and membrane structures of the fungus showed ultrastructural abnormalities, which became severely degraded as observed through scanning electron microscopy and transmission electron microscopy. The mixture of both B. subtilis and M. oryzae resulted in enhanced activity of peroxidase, and polyphenol oxidase while there was significantly more superoxide dismutase activity in plants that had been sprayed with B.subtilis alone. The present study suggests that colonized SYX04 and SYX20 strains protected rice plants and exhibited antifungal activity and induced systemic resistance, thus indicating their potential biological control agents.

Metagenomic Signatures of Bacterial Adaptation to Life in the Phyllosphere of a Salt-Secreting Desert Tree

The leaves of Tamarix aphylla, a globally distributed, salt-secreting desert tree, are dotted with alkaline droplets of high salinity. To successfully inhabit these organic carbon-rich droplets, bacteria need to be adapted to multiple stress factors, including high salinity, high alkalinity, high UV radiation, and periodic desiccation. To identify genes that are important for survival in this harsh habitat, microbial community DNA was extracted from the leaf surfaces of 10 Tamarix aphylla trees along a 350-km longitudinal gradient. Shotgun metagenomic sequencing, contig assembly, and binning yielded 17 genome bins, six of which were >80% complete. These genomic bins, representing three phyla (Proteobacteria,Bacteroidetes, and Firmicutes), were closely related to halophilic and alkaliphilic taxa isolated from aquatic and soil environments. Comparison of these genomic bins to the genomes of their closest relatives revealed functional traits characteristic of bacterial populations inhabiting the Tamarix phyllosphere, independent of their taxonomic affiliation. These functions, most notably light-sensing genes, are postulated to represent important adaptations toward colonization of this habitat.

IMPORTANCE

Plant leaves are an extensive and diverse microbial habitat, forming the main interface between solar energy and the terrestrial biosphere. There are hundreds of thousands of plant species in the world, exhibiting a wide range of morphologies, leaf surface chemistries, and ecological ranges. In order to understand the core adaptations of microorganisms to this habitat, it is important to diversify the type of leaves that are studied. This study provides an analysis of the genomic content of the most abundant bacterial inhabitants of the globally distributed, salt-secreting desert tree Tamarix aphylla Draft genomes of these bacteria were assembled, using the culture-independent technique of assembly and binning of metagenomic data. Analysis of the genomes reveals traits that are important for survival in this habitat, most notably, light-sensing and light utilization genes.

Quantitative Analysis of the Migration and Accumulation of Bacillus subtilis in Asparagus officinalis

Bacillus subtilis B96-II is a broad-spectrum biological control strain. It effectively suppresses soil-borne fungal diseases in vegetables. A green fluorescence protein (GFP) was expressed in B96-II to detect migration of B96-II into the root and stem of asparagus. The GFP-tagged B96-II (B96-II-GFP) strain exhibited bright green fluorescence under a fluorescence microscope. GFP was stable and had no apparent effects on the growth of the strain. Asparagus plants were planted in the soil inoculated with B96-II-GFP. Our results showed that B96-II-GFP was detected in both the root and stem 15, 30, and 45 days after the asparagus seedlings were planted. B96-II-GFP was also detected in leaves but at a lower concentration. The highest concentration was detected in 15 days, and the number of bacteria decreased subsequently irrespective of duration of growth or sampling period. The highest concentration of B96-II-GFP was present in the root base suggesting that the root base served as the hub of bacterial migration from the soil to the stem.

Biocontrol of verticillium wilt and colonization of cotton plants by an endophytic bacterial isolate

AIMS

To explore biocontrol potential of 39 DAEB isolates (doubly antagonistic towards both Verticillium dahliae Kleb and Fusarium oxysporum) against verticillium wilt of cotton and to elucidate colonization and category characteristics of an endophytic bacterium with significant biocontrol activity.

METHODS AND RESULTS

Thirty-nine antagonistic endophytic bacteria strains were tested for their ability to control verticillium wilt in cotton plants caused by a defoliating pathotype of V. dahliae 107 in cotton under controlled conditions. The biocontrol trial revealed that an endophytic bacterium, designated HA02, showed a significant biocontrol activity to V. dahliae 107. After cotton seedlings were inoculated with a gfp gene-tagged HA02 (HA02-gfp), HA02-gfp populations were higher in the root than in the stem; in addition, the HA02-gfp was distributed in the maturation zone of cotton root. Furthermore, HA02-gfp also colonized seedlings of maize, rape and soybean after the bacteria inoculation. Phylogenetic trees based on 16S rDNA sequences combined with morphological, physiological and identification showed that the bacterium belongs to the Enterobacter genus.

CONCLUSIONS

Our results showed that only 1 of 39 DAEB isolates demonstrated more efficient biocontrol potential towards V. dahliae 107 in greenhouse and field trials. HA02-gfp mainly colonized cotton in roots. In addition, we quantitatively observed HA02 colonization in other hosts. HA02 belongs to the Enterobacter genus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study on biocontrol to defoliating pathotype of V. dahliae Kleb by endophytic bacteria. The HA02 showed effective biocontrol to V. dahliae 107 in greenhouse and field trials. Furthermore, we assessed the quantitative and qualitative colonization of HA02 in cotton seedlings. Our study provides basic information to further explore managing strategies to control this critical disease.

Isolation, characterization and colonization of 1-aminocyclopropane-1-carboxylate deaminase-producing bacteria XG32 and DP24

Two 1-aminocyclopropane-1-carboxylate deaminase-producing bacterial strains (DP24 and XG32) were isolated from surface-sterilized tomato roots and rizhospere soil. The strains were identified as Pseudomonas fluorescens biovar. IV (XG2) and Erwinia herbicola (DP24) by physiological and biochemical tests, and 16S rRNA gene analysis. Both strains showed positive plant growth-promoting activity when inoculated into cucumber (Cucumis sativus), tomato (Lycopersicon esculentum), pepper (Capsicum annuum) and rapeseed (Brassica napus L.). Colonization ability and behavior of these two strains were determined by treating mutant strains with rifampicin and fluorescence in situ hybridization (FISH) assay with rRNA targeted probes, respectively. Both strains were endophytic colonizers of pepper plants. The behavior of the two strains was not identical. Strain XG32 only colonized the root and reached the max level of 27.7 × 10(7) c.f.u./g (fresh weight), after 12 days postinoculation, while strain DP24 was able to colonize the roots, stems and leaves. The max level was reached at 40.87 × 10(7) c.f.u./g (fresh weight) in the roots, 17 × 10(7) c.f.u./g in the stems after 7 days postinoculation and 44.84 × 10(7) c.f.u./g in the leaves after 12 days postinoculation.

Construction of EGFP-labeling system for visualizing the infection process of Xanthomonas axonopodis pv. citri in planta

Xanthomonas axonopodis pv. citri (Xac) is the causal agent of citrus bacterial canker, an economically important disease to world citrus industry. To monitor the infection process of Xac in different citrus plants, the enhanced green florescent protein (EGFP) visualizing system was constructed to visualize the propagation and localization in planta. First, the wild-type Xac was isolated from the diseased leaves of susceptible 'Bingtang' sweet orange, and then the isolated Xac was labeled with EGFP by triparental mating. After PCR identification, the growth kinetics and pathogenicity of the transformants were analyzed in comparison with the wild-type Xac. The EGFP-labeled bacteria were inoculated by spraying on the surface and infiltration in the mesophyll of 'Bingtang' sweet orange leaves. The bacterial cell multiplication and diffusion processes were observed directly under confocal laser scanning microscope at different intervals after inoculation. The results indicated that the EGFP-labeled Xac releasing clear green fluorescence light under fluorescent microscope showed the infection process and had the same pathogenicity as the wild type to citrus. Consequently, the labeled Xac demonstrated the ability as an efficient tool to monitor the pathogen infection.

Identification of the cellular location of internalized Escherichia coli O157:H7 in mung bean, Vigna radiata, by immunocytochemical techniques

Escherichia coli O157:H7 has been associated with numerous outbreaks involving fresh produce. Previous studies have shown that bacteria can be internalized within plant tissue and that this can be a source of protection from antimicrobial chemicals and environmental conditions. However, the types of tissue and cellular locations the bacteria occupy in the plant following internalization have not been addressed. In this study, immunocytochemical techniques were used to localize internalized E. coli O157:H7 expressing green fluorescent protein in germinated mung bean (Vigna radiata) hypocotyl tissue following contamination of intact seeds. An average of 13 bacteria per mm(3) were localized within the sampled tissue. The bacteria were found to be associated with every major tissue and corresponding cell type (cortex, phloem, xylem, epidermis, and pith). The cortical cells located on the outside of the vascular bundles contained the majority of the internalized bacteria (61%). In addition, the bacteria were localized primarily to the spaces between the cells (apoplast) and not within the cells. Growth experiments were also performed and demonstrated that mung bean plants could support the replication of bacteria to high levels (10(7) CFU per plant) following seed contamination and that these levels could be sustained over a 12-day period. Therefore, E. coli O157:H7 can be internalized in many different plant tissue types after a brief seed contamination event, and the bacteria are able to grow and persist within the plant.

Biocontrol of Rhizoctonia solani damping-off disease in cucumber with Bacillus pumilus SQR-N43

Biological control is an efficient and environmentally friendly way to prevent damping-off disease. Micrographs were used to investigate the ability of Bacillus pumilus (B. pumilus) SQR-N43 to control Rhizoctonia solani (R. solani) Q1 in cucumbers. The root colonization ability of B. pumilus SQR-N43 was analyzed in vivo with a green fluorescent protein (GFP) tag. A pot experiment was performed to assess the in vivo disease-control efficiency of B. pumilus SQR-N43 and its bio-organic fertilizer. Results indicate that B. pumilus SQR-N43 induced hyphal deformation, enlargement of cytoplasmic vacuoles and cytoplasmic leakage in R. solani Q1 mycelia. A biofilm on the root surface was formed when the roots were inoculated with 10(7)-10(8)cells g(-1) of soil of GFP-tagged B. pumilus SQR-N43. In the pot experiment, the biocontrol reduced the concentration of R. solani. In contrast to applications of only B. pumilus SQR-N43 (N treatment), which produced control efficiencies of 23%, control efficiencies of 68% were obtained with applications of a fermented organic fertilizer inoculated with B. pumilus SQR-N43 (BIO treatment). After twenty days of incubation, significant differences in the number of CFUs and the percentage of spores of B. pumilus SQR-N43 were recorded between the N treatment (2.20×10(7)CFU g(-1) of soil and 79%, respectively) and the BIO treatment (1.67×10(8)CFU g(-1) of soil and 52%, respectively). The results indicate that B. pumilus SQR-N43 is a potent antagonist against R. solani Q1. The BIO treatment was more effective than the N treatment because it stabilized the population and increased the active form of the antagonist.

Colonization of Morus alba L. by the plant-growth-promoting and antagonistic bacterium Burkholderia cepacia strain Lu10-1

Background

Anthracnose, caused by Colletotrichum dematium, is a serious threat to the production and quality of mulberry leaves in susceptible varieties. Control of the disease has been a major problem in mulberry cultivation. Some strains of Burkholderia cepacia were reported to be useful antagonists of plant pests and could increase the yields of several crop plants. Although B. cepacia Lu10-1 is an endophytic bacterium obtained from mulberry leaves, it has not been deployed to control C. dematium infection in mulberry nor its colonization patterns in mulberry have been studied using GFP reporter or other reporters. The present study sought to evaluate the antifungal and plant-growth-promoting properties of strain Lu10-1, to clarify its specific localization within a mulberry plant, and to better understand its potential as a biocontrol and growth-promoting agent.

Results

Lu10-1 inhibited conidial germination and mycelial growth of C. dematium in vitro; when applied on leaves or to the soil, Lu10-1 also inhibited the development of anthracnose in a greenhouse, but the effectiveness varied with the length of the interval between the strain treatment and inoculation with the pathogen. Strain Lu10-1 could survive in both sterile and non-sterile soils for more than 60 days. The strain produced auxins, contributed to P solubilization and nitrogenase activity, and significantly promoted the growth of mulberry seedlings. The bacteria infected mulberry seedlings through cracks formed at junctions of lateral roots with the main root and in the zone of differentiation and elongation, and the cells were able to multiply and spread, mainly to the intercellular spaces of different tissues. The growth in all the tissues was around 1-5 × 10⁵ CFU per gram of fresh plant tissue.

Conclusions

Burkholderia cepacia strain Lu10-1 is an endophyte that can multiply and spread in mulberry seedlings rapidly and efficiently. The strain is antagonistic to C. dematium and acts as an efficient plant-growth-promoting agent on mulberry seedlings and is therefore a promising candidate as a biocontrol and growth-promoting agent.

Bacillus megaterium strain XTBG34 promotes plant growth by producing 2-pentylfuran

Several chemical changes in soil are associated with plant growth-promoting rhizobacteria. An endosporeforming bacterium, strain XTBG34, was isolated from a Xishuangbanna Tropical Botanical Garden soil sample and identified as Bacillus megaterium. The strain's volatiles had remarkable plant growth promotion activity in Arabidopsis thaliana plants; after 15 days treatment, the fresh weight of plants inoculated with XTBG34 was almost 2-fold compared with those inoculated with DH5alpha. Head space volatile compounds produced by XTBG34, trapped with headspace solid phase microextraction and identified by gas chromatography-mass spectrometry, included aldehydes, alkanes, ketones and aroma components. Of the 11 compounds assayed for plant growth promotion activity in divided Petri plates, only 2-pentylfuran increased plant growth. We have therefore identified a new plant growth promotion volatile of B. megaterium XTBG34, which deserves further study in the mechanisms of interaction between plant growth-promoting rhizobacteria and plants.

Monitoring the colonization of sugarcane and rice plants by the endophytic diazotrophic bacterium Gluconacetobacter diazotrophicus marked with gfp and gusA reporter genes

AIMS

To evaluate the colonization process of sugarcane plantlets and hydroponically grown rice seedlings by Gluconacetobacter diazotrophicus strain PAL5 marked with the gusA and gfp reporter genes.

METHODS AND RESULTS

Sugarcane plantlets inoculated in vitro with PAL5 carrying the gfp::gusA plasmid pHRGFPGUS did not present green fluorescence, but beta-glucuronidase (GUS)-stained bacteria could be observed inside sugarcane roots. To complement this existing inoculation methodology for micropropagated sugarcane with a more rapid colonization assay, we employed hydroponically grown gnotobiotic rice seedlings to study PAL5-plant interaction. PAL5 could be isolated from the root surface (10(8) CFU g(-1)) and from surface-disinfected root and stem tissues (10(4) CFU g(-1)) of inoculated plants, suggesting that PAL5 colonized the internal plant tissues. Light microscopy confirmed the presence of bacteria inside the root tissue. After inoculation of rice plantlets with PAL5 marked with the gfp plasmid pHRGFPTC, bright green fluorescent bacteria could be seen colonizing the rice root surface, mainly at the sites of lateral root emergence, at root caps and on root hairs.

CONCLUSION

The plasmids pHRGFPGUS and pHRGFPTC are valid tools to mark PAL5 and monitor the colonization of micropropagated sugarcane and hydroponic rice seedlings.

SIGNIFICANCE AND IMPACT OF THE STUDY

These tools are of use to: (i) study PAL5 mutants affected in bacteria-plant interactions, (ii) monitor plant colonization in real time and (iii) distinguish PAL5 from other bacteria during the study of mixed inoculants.

Systemic colonization of potato plants by a soilborne, green fluorescent protein-tagged strain of Dickeya sp. biovar 3

ABSTRACT Colonization of potato plants by soilborne, green fluorescent protein (GFP)-tagged Dickeya sp. IPO2254 was investigated by selective plating, epifluorescence stereo microscopy (ESM), and confocal laser scanning microscopy (CLSM). Replicated experiments were carried out in a greenhouse using plants with an intact root system and plants from which ca. 30% of the lateral roots was removed. One day after soil inoculation, adherence of the pathogen on the roots and the internal colonization of the plants were detected using ESM and CLSM of plant parts embedded in an agar medium. Fifteen days post-soil inoculation, Dickeya sp. was found on average inside 42% of the roots, 13% of the stems, and 13% of the stolons in plants with undamaged roots. At the same time-point, in plants with damaged roots, Dickeya sp. was found inside 50% of the roots, 25% of the stems, and 25% of the stolons. Thirty days postinoculation, some plants showed true blackleg symptoms. In roots, Dickeya sp. was detected in parenchyma cells of the cortex, both inter- and intracellularly. In stems, bacteria were found in xylem vessels and in protoxylem cells. Microscopical observations were confirmed by dilution spread-plating the plant extracts onto agar medium directly after harvest. The implications of infection from soilborne inoculum are discussed.

The Gram-positive side of plant-microbe interactions

Plant growth and development are significantly influenced by the presence and activity of microorganisms. To date, the best-studied plant-interacting microbes are Gram-negative bacteria, but many representatives of both the high and low G+C Gram-positives have excellent biocontrol, plant growth-promoting and bioremediation activities. Moreover, actinorhizal symbioses largely contribute to the global biological nitrogen fixation and many Gram-positive bacteria promote other types of symbioses in tripartite interactions. Finally, several prominent and devastating phytopathogens are Gram-positive. We summarize the present knowledge of the beneficial and detrimental interactions of Gram-positive bacteria with plants to underline the importance of this particular group of bacteria.