Browsing by Subject "Bacillus amyloliquefaciens"

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    Transcriptional profiling of Bacillus amyloliquefaciens FZB42 in response to seed and root exudates collected under different nutrient regimes
    (2010) Carvalhais Costa, Lilia; von Wirén, Nicolaus
    Plant growth-promoting rhizobacteria (PGPR) live in close association with plants and improve their growth. Bacillus amyloliquefaciens strain FZB42 is a prominent plant root-colonizing bacterium that is able to stimulate the growth of maize. To decipher the molecular cross-talk between B. amyloliquefaciens and crop plants, an exploratory analysis of the effect of seed and root exudates on the transcriptome of Bacillus amyloliquefaciens FZB42 was performed. Root exudates were collected from maize plants grown in an axenic hydroponic system under nutrient sufficiency or under deficient supply of nitrogen (N), phosphorus (P), iron (Fe) or potassium (K). An analysis of primary metabolites in the exudates was carried out, compared between treatments, and correlated with the transcriptional profiles of Bacillus amyloliquefaciens FZB42 that were gained after incubation of the bacterial culture with the root exudates. Higher exudation rates of citrate were found under Fe deficiency and greater release of ã-amino butyric acid under P deficiency. Based on a negative correlation observed between the average diffusion coefficient of N, P, K, and Fe in soils and the exudation rates of primary metabolites under conditions of N, P, K, or Fe deficiency, it was hypothesized that the exudation of sugars, amino acids and organic acids may reflect the availability and mobility of plant nutrients in soils. In the presence of seed and root exudates collected from nutrient-sufficient plants, genes involved in spore germination, transport and utilization of nutrients, biosynthesis pathways, multidrug transporters, motility and competence development were differentially expressed. In comparison to P, Fe and K, N-deficient maize root exudates caused a more distinguished change in the transcriptome of bacteria when they were in the logarithmic growth phase. During this growth phase, a number of genes coding for ribosomal proteins were down-regulated by N-deficient maize root exudates, indicating that bacterial activity was repressed. Exclusively in the presence of P-deficient maize root exudates, several genes associated to bacterial motility were induced. Moreover, a gene involved in the biosynthesis of the auxin precursor tryptophan was up-regulated by all deficiency treatments. In the transitional growth phase of Bacillus amyloliquefaciens FZB42, several genes were commonly down-regulated in different deficiency treatments. This finding is in agreement with previous studies showing that quorum-sensing and starvation-sensing are integrated to regulate cell entry into the transient phase. Taken together, this is the first study comparing the effect of different nutrient deficiencies on the composition of primary metabolites in root exudates of one plant species and evaluating systematically the transcriptional response of a Gram-positive PGPR to seed and root exudates collected from plants grown under different nutrient regimes. This analysis provides new information about the early communication between plant roots and PGPR and points to involved genes and processes that merit further investigation.
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    Understanding the role of plant growth promoting bacteria on sorghum growth and biotic suppression of striga infestation
    (2014) Mounde, Lenard Gichana; Sauerborn, Joachim
    Witchweeds (Striga sp.) are parasitic weeds of great agricultural significance, parasitizing the roots of their hosts. Striga, like all other root parasitic weeds, drain essential organic and inorganic resources from their hosts leading to poor crop development and low yield. In Africa, about 50 million ha in over 30 countries are infested by Striga spp. causing grain loss of cereals. Estimated yield losses of maize, sorghum, millets and upland rice are between 30 and 90%. The parasite, therefore, is ranked as the leading biotic constraint to cereal production in the continent. Plant growth promoting rhizobacteria (PGPR) are promising components for integrated solutions to agro-environmental problems because inoculants possess the capacity to promote crop growth and reduce the population of deleterious microbes in the rhizosphere. Although there are numerous studies on crop growth promotion and biological control of diseases, weeds, nematodes and parasitic weeds using PGPR, little is known about the potential of some Bacillus subtilis, B. amyloliquefaciens and Burkholderia phytofirmans strains in sorghum growth promotion and resistance against Striga infection. The main objective of the study was to assess the effect of B. subtilis Bsn5, B. subtilis GBO3, B. amyloliquefaciens FZB42 and Burkholderia phytofirmans PsJN on growth promotion of sorghum crop and suppression of Striga development, thus providing a basic understanding on the sorghum-PGPR-Striga interaction. This study opens with an elaborate review of the state-of-the-art knowledge on the tripartite interactions between Striga, sorghum and different species of PGPR. Prior to this, bipartite relationship between sorghum and Striga, PGPR-sorghum and PGPR-Striga are reviewed with a focus on understanding Striga impact on sorghum, sorghum defence responses to infection, plant growth and disease suppression benefits by PGPR on sorghum, and the effect of PGPR on Striga development. Knowledge gaps in both bipartite and tripartite relationships are described, and future research recommendations given. A key recommendation from the review is to conduct experiments under controlled environmental conditions using Bacillus subtilis, B. amyloliquefaciens and Burkhoderia phytofirmans strains in order to understand their relationship with sorghum and Striga at bipartite and tripartite levels. Petri dish bioassays and root chamber experiments under controlled conditions were conducted at the Institute of Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim between 2012 and 2014. B. subtilis Bsn5, B. subtilis GBO3, B. amyloliquefaciens FZB42 and Burkholderia phytofirmans strain PsJN inocula and their corresponding cell culture supernatants were evaluated for their growth promotion potential on sorghum and suppressiveness on Striga development. Sorghum root exudates and synthetic stimulant GR24 were used to induce Striga seed germination. Bacillus subtilis Bsn5 supernatant, which showed the greatest inhibitory activity on Striga germination and radicle elongation, was separated by ethyl acetate into lipophilic and hydrophilic phases. The purpose of this extraction was to try and identify the polarity of the inhibitor. Protein composition by mass spectrometry (MS) was also done on the supernatant with a view of establishing the presence of peptides because peptides have been associated with Orobancheceae germination and radicle inhibition in previous studies. In addition, determination of plant growth hormones in bacteria supernatants was also conducted using Radio-Immuno-Assay (RIA) in order to relate PGPR hormone production and sorghum growth enhancement. Burkholderia phytofirmans PsJN significantly (<0.05) induced a higher vigor index (VI) on sorghum seedlings (>18,000) compared to other PGPR and control treatments. The lowest VI (7626) was recorded in seeds inoculated with Bacillus amyloliquefaciens FZB42. Complete Striga germination inhibition (0% germination) occurred in seeds exposed to all PGPR inocula suspended while the highest germination (>60%) occurred in control treatments (10% Luria Bertani (LB) + GR24 and sterile distilled water (SDW) + GR24). The effect of bacterial supernatants on the germination percentage and radicle length of Striga seeds was also significantly (<0.05) different among treatments. The least germination (7.4 %) was observed in Bacillus subtilis Bsn5 + GR24 while the highest (66 %) was observed in SDW + GR24 control. Bacillus subtilis Bsn5 supernatant produced the lowest mean radicle lengths (0.1 mm) while the highest radicle lengths were observed in SDW + GR24 (2.2 mm). Therefore, Bacillus subtilis Bsn5 supernatant was selected for further investigation of compounds causing inhibition of Striga germination and preventing radicle elongation. The supernatant was separated into hydrophilic and hydrophobic fractions using ethyl acetate. Each fraction was then prepared in 1%, 25%, 50%, 75% and 100% concentrations before being evaluated for their inhibitory activity in Striga germination and radicle elongation. The highest germination percentage (63%) and radical length (2.9 mm) was observed in SDW + GR24 control treatment. The ethyl acetate (lipophilic) fraction at both 100% and 1% concentration + GR24 produced a germination percentage of >40% which was similar to 10% LB + GR24 and ethyl acetate + GR24 controls. There was complete inhibition of Striga seed germination after exposure to either Bacillus subtilis Bsn5 supernatant + GR24 or 100% hydrophilic fraction of the supernatant + GR24. However, at 25% and 1% concentration + GR24, Striga germination percentage increased to 34% and 49%, respectively. Light microscopy examination of Striga radicles exposed to Bacillus subtilis Bsn5 supernatant + GR24 revealed that stunting of the radicles was due to reduction in cell sizes at the radicle elongation zone. Extended agar gel assays (EAGA) experiments showed a similar trend of results with B. subtilis Bsn5 showing the highest inhibitory activity on Striga germination and radicle elongation compared to other PGPR and control treatments. Results from root chamber experiments demonstrated significant (p<0.05) differences in biomass production between Striga-free and Striga-infected sorghum. Total biomass yield in uninoculated Striga-free plants was 40% higher than uninoculated Striga-infected sorghum plants. Bacillus amyloliquefaciens FZB42, B. subtilis GBO3 and Burkholderia phytofirmans PsJN inoculated Striga-free sorghum showed a 75%; 142% and 158% increase in biomass yield, respectively, compared to uninoculated Striga-free sorghum. There were no significant differences in biomass yield observed between inoculated and uninoculated Striga-infected plants. All PGPR supernatants and 10% LB media showed production of phytohormones cytokinin, IAA, GAs and ABA. Cytokinin content in PGPR supernatants was significantly (>0.05) higher than blank 10% LB control media. There was a significant negative correlation (r= -0.96) between IAA and cytokinins. However, there was no significant positive correlation between any phytohormone and sorghum plant height, SPAD values, biomass production, Striga germination, attachment and tubercle death. Finally, this study shows that Bacillus subtilis Bsn5, B. subtilis GBO3, B. amyloliquefaciens FZB42 and Burkholderia phytofirmans PsJN might accelerate sorghum growth and suppress key stages of Striga development under laboratory conditions. Greenhouse and field experiments are recommended to better understand these interactions under natural conditions where other biotic and abiotic factors come into play. These findings could contribute to a better understanding of sorghum and beneficial bacteria interactions and provide novel information of the long-term effects of a PGPR on sorghum development, opening new avenues for Striga control and sustainable, ecofriendly sorghum production.