Plant beneficial endophytic bacteria: Mechanisms, diversity, host range and genetic determinants

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.

https://www.mdpi.com/2076-2607/5/4/77/pdf

Bacterial Endophyte Colonization and Distribution within Plants

Bacterial endophytes are ubiquitous to virtually all terrestrial plants. With the increasing appreciation of studies that unravel the mutualistic interactions between plant and microbes, we increasingly value the beneficial functions of endophytes that improve plant growth and development. However, still little is known on the source of established endophytes as well as on how plants select specific microbial communities to establish associations. Here, we used cultivation-dependent and -independent approaches to assess the endophytic bacterrial community of surface-sterilized rice seeds, encompassing two consecutive rice generations. We isolated members of nine bacterial genera. In particular, organisms affiliated with Stenotrophomonas maltophilia and Ochrobactrum spp. were isolated from both seed generations. PCR-based denaturing gradient gel electrophoresis (PCR-DGGE) of seed-extracted DNA revealed that approximately 45% of the bacterial community from the first seed generation was found in the second generation as well. In addition, we set up a greenhouse experiment to investigate abiotic and biotic factors influencing the endophytic bacterial community structure. PCR-DGGE profiles performed with DNA extracted from different plant parts showed that soil type is a major effector of the bacterial endophytes. Rice plants cultivated in neutral-pH soil favoured the growth of seed-borne Pseudomonas oryzihabitans and Rhizobium radiobacter, whereas Enterobacter-like and Dyella ginsengisoli were dominant in plants cultivated in low-pH soil. The seed-borne Stenotrophomonas maltophilia was the only conspicuous bacterial endophyte found in plants cultivated in both soils. Several members of the endophytic community originating from seeds were observed in the rhizosphere and surrounding soils. Their impact on the soil community is further discussed.

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Dynamics of Seed-Borne Rice Endophytes on Early Plant Growth Stages

Screening of Brazilian cacti rhizobacteria for plant growth promotion under drought

Endophytic microbes are ubiquitous in most plant species. Endophytic microbes enter plants mainly through wounds, naturally occurring as a result of plant growth or through root hairs and at epidermal conjunctions. Besides gaining entrance to plants through natural openings or wounds, endophytic microbes appear to actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase. Diverse community structure of endophytes can be analyzed using culture-dependent and culture-independent method. Endophytic bacteria belong to different phyla such as Acidobacteria, Actinobacteria, Ascomycota, Bacteroidetes, Basidiomycota, Deinococcus-Thermus, and Firmicutes. Endophytic archaea (Euryarchaeota) were reported using only culture-independent method. Endophytic microbes were most predominant and studied and belonged to three major phyla Actinobacteria, Proteobacteria, and Firmicutes. Among reported genera Achromobacter, Bacillus, Burkholderia, Enterobacter, Herbaspirillum, Pantoea, Pseudomonas, Rhizobium, and Streptomyces were dominant in most host plants. Along with common endophytic microbial genera, there were many niche-specific microbial genera that have been reported from different host plants. Application of associative microbes for sustainable agriculture holds immense potential. Endophytic microbes are known to enhance growth and yield of plants by fixing atmospheric nitrogen and solubilization of phosphorus, potassium, and zinc; production of phytohormones (cytokinins, auxins, and gibberellins), ammonia, hydrogen cyanide, and siderophores; and possession of antagonistic activity as well as reducing the level of stress ethylene in host plants. Endophytes seem to contribute to plant fitness and development, displaying beneficial traits that can be exploited in agricultural biotechnology. The interactions between endophytes and plants can promote plant health and play a significant role in low-input sustainable agriculture for both food and nonfood crops. This chapter summarizes part of the work being done on endophytic microbes, including their isolation, identification, diversity, distribution, and applications for sustainable agriculture.

Endophytic Microbes in Crops: Diversity and Beneficial Impact for Sustainable Agriculture

Twenty endophytic bacteria were isolated from the meristematic tissues of three varieties of strawberry cultivated in vitro, and further identified, by FAME profile, into the genera Bacillus and Sphingopyxis. The strains were also characterized according to indole acetic acid production, phosphate solubilization and potential for plant growth promotion. Results showed that 15 strains produced high levels of IAA and all 20 showed potential for solubilizing inorganic phosphate. Plant growth promotion evaluated under greenhouse conditions revealed the ability of the strains to enhance the root number, length and dry weight and also the leaf number, petiole length and dry weight of the aerial portion. Seven Bacillus spp. strains promoted root development and one strain of Sphingopyxis sp. promoted the development of plant shoots. The plant growth promotion showed to be correlated to IAA production and phosphate solubilization. The data also suggested that bacterial effects could potentially be harnessed to promote plant growth during seedling acclimatization in strawberry.

/pdf/isolation-of-micropropagated-strawberry-endophytic-bacteria-3ycde9ua7z.pdf

Isolation of micropropagated strawberry endophytic bacteria and assessment of their potential for plant growth promotion

– The objectives of this work were to isolate, characterize and identify the endophytic bacterial community of soybean seeds, and to test the biotechnological potential of this community. Seeds from 12 soybean cultivars were used. The endophytic bacterial isolates were evaluated for in vitro antagonism against phytopathogenic fungi, synthesis of indoleacetic acid (IAA), and capacity to solubilize phosphate. Isolation techniques, ampliﬁ ed ribosomal DNA restriction analysis (ARDRA) grouping, and identiﬁ cation by means of partially sequencing the 16S rDNA were used in community characterization. The isolates with best biotechnological potential were inoculated in seeds to evaluate their ability to promote plant growth. Twelve ribotypes were identify by means of ARDRA and classiﬁ ed as: Acinetobacter , Bacillus , Brevibacterium , Chryseobacterium , Citrobacter , Curtobacterium , Enterobacter , Methylobacterium , Microbacterium , Micromonospora , Pantoea , Paenibacillus , Pseudomonas

/pdf/diversidade-e-potencial-biotecnologico-da-comunidade-rg4d0sx2my.pdf

Laura de Castro Assumpção

Papers

Isolation of micropropagated strawberry endophytic bacteria and assessment of their potential for plant growth promotion

Diversidade e potencial biotecnológico da comunidade bacteriana endofítica de sementes de soja