The Rhizobium-Legume Symbiosis: Co-opting Successful Stress Management

The interaction of bacteria with plants can result in either a positive, negative, or neutral association. The rhizobium-legume interaction is a well-studied model system of a process that is considered a positive interaction. This process has evolved to require a complex signal exchange between the host and the symbiont. During this process, rhizobia are subject to several stresses, including low pH, oxidative stress, osmotic stress, as well as growth inhibiting plant peptides. A great deal of work has been carried out to characterize the bacterial response to these stresses. Many of the responses to stress are also observed to have key roles in symbiotic signaling. We propose that stress tolerance responses have been co-opted by the plant and bacterial partners to play a role in the complex signal exchange that occurs between rhizobia and legumes to establish functional symbiosis. This review will cover how rhizobia tolerate stresses, and how aspects of these tolerance mechanisms play a role in signal exchange between rhizobia and legumes.

Prestimulation of wheat seedlings with gibberellic acid followed by application of an agitated high dilution of the same hormone

Background In previous multicentre studies, the influence of a homeopathic ultra-high dilution of gibberellic acid on wheat growth was scrutinized. Data showed that this test dilution slowed down stalk growth when experiments were performed in the autumn season. Aim To test the hypothesis that pretreatment of grains with high concentrations of gibberellic acid would enhance the growth-inhibiting effect of the ultra-high dilution of the plant hormone Methods Grains of winter wheat (Triticum aestivum, 500 or 1000 per group) were pretreated with (non-agitated) gibberellic acid 10-5, 10-4 and 10-3 or with water (“W”) for control prior to treatment with G30x or W30x. Grains were observed under the influence of extremely diluted gibberellic acid (10-30) prepared by stepwise dilution and agitation according to a protocol derived from homeopathy (“G30x”). Analogously prepared water was used for control (“W30x”). Seedlings were allowed to develop under standardized conditions for 7 days; plants were harvested and stalk lengths were measured. Results Of the four pretreatment variants under study, Ge-3 yielded most growth, followed by Ge-4 , Ge-5 and finally W. This outcome was modulated by the application of G30x and W30x in that the inhibition obtained with G30x as compared to W30x was the greater the lower the pretreatment concentration of G had been. The hypothesis that pretreatment of grains with high concentrations of gibberellic acid would enhance the growth inhibiting effect of G30x had to be rejected. Rather, G30x slowed down stalk growth most in the W group with p < 0.001, only moderately in the Ge-5 and Ge-4 group and not at all in the Ge-3 group.

A noble extended stochastic logistic model for cell proliferation with density-dependent parameters

Cell proliferation often experiences a density-dependent intrinsic proliferation rate (IPR) and negative feedback from growth-inhibiting molecules in culture media. The lack of flexible models with explanatory parameters fails to capture such a proliferation mechanism. We propose an extended logistic growth law with the density-dependent IPR and additional negative feedback. The extended parameters of the proposed model can be interpreted as density-dependent cell-cell cooperation and negative feedback on cell proliferation. Moreover, we incorporate further density regulation for flexibility in the model through environmental resistance on cells. The proposed growth law has similarities with the strong Allee model and harvesting phenomenon. We also develop the stochastic analog of the deterministic model by representing possible heterogeneity in growth-inhibiting molecules and environmental perturbation of the culture setup as correlated multiplicative and additive noises. The model provides a maximum sustainable stable cell density (MSSCD) and a new fitness measure for proliferative cells. The proposed model shows superiority to the logistic law after fitting to real cell culture datasets. We illustrate both MSSCD and the new cell fitness for a range of parameters. The cell density distributions reveal the chance of overproliferation, underproliferation, or decay for different parameter sets under the deterministic and stochastic setups.

A new, reliable, and high-throughput strategy to screen bacteria for antagonistic activity against Staphylococcus aureus

Background Antibiotic-resistant Staphylococcus aureus clones have emerged globally over the last few decades. Probiotics have been actively studied as an alternative to antibiotics to prevent and treat S. aureus infections, but identifying new probiotic bacteria, that have antagonistic activity against S. aureus, is difficult since traditional screening strategies are time-consuming and expensive. Here, we describe a new plasmid-based method which uses highly stable plasmids to screen bacteria with antagonistic activity against S. aureus. Results We have created two recombinant plasmids (pQS1 and pQS3) which carry either gfpbk or mCherry under the control of a S. aureus quorum-sensing (QS) promoter (agrP3). Using this recombinant plasmid pair, we tested 81 bacteria isolated from Holstein dairy milk to identify bacteria that had growth-inhibiting activity against S. aureus and suggest potential explanations for the growth inhibition. The stability test illustrated that pQS1 and pQS3 remained highly stable for at least 24 h in batch culture conditions without selection pressure from antibiotics. This allowed co-culturing of S. aureus with other bacteria. Using the newly developed pQS plasmids, we found commensal bacteria, isolated from raw bovine milk, which had growth-inhibiting activity (n = 13) and quorum-quenching (QQ) activity (n = 13) towards both S. aureus Sa25 (CC97) and Sa27 (CC151). The pQS-based method is efficient and effective for simultaneously screening growth-inhibiting and QQ bacteria against S. aureus on agar media. Conclusions It was shown that growth-inhibiting and QQ activity toward pQS plasmid transformants of S. aureus can be simultaneously monitored by observing the zone of growth inhibition and reporter protein inhibition on agar plates. Newly identified antagonistic bacteria and their functional biomolecules are promising candidates for future development of probiotic drugs and prophylactics/therapeutics for bacterial infections including S. aureus. Furthermore, this new approach can be a useful method to find bacteria that can be used to prevent and treat S. aureus infections in both humans and animals.

Apt (Adenine Phosphoribosyltransferase) Mutation in Laboratory-Selected Vancomycin-Intermediate Staphylococcus aureus

Comparative genomic sequencing of laboratory-derived vancomycin-intermediate Staphylococcus aureus (VISA) (MM66-3 and MM66-4) revealed unique mutations in both MM66-3 (in apt and ssaA6), and MM66-4 (in apt and walK), compared to hetero-VISA parent strain MM66. Transcriptional profiling revealed that both MM66 VISA shared 79 upregulated genes and eight downregulated genes. Of these, 30.4% of the upregulated genes were associated with the cell envelope, whereas 75% of the downregulated genes were associated with virulence. In concordance with mutations and transcriptome alterations, both VISA strains demonstrated reduced autolysis, reduced growth in the presence of salt and reduced virulence factor activity. In addition to mutations in genes linked to cell wall metabolism (ssaA6 and walK), the same mutation in apt which encodes adenine phosphoribosyltransferase, was confirmed in both MM66 VISA. Apt plays a role in both adenine metabolism and accumulation and both MM66 VISA grew better than MM66 in the presence of adenine or 2-fluoroadenine indicating a reduction in the accumulation of these growth inhibiting compounds in the VISA strains. MM66 apt mutants isolated via 2-fluoroadenine selection also demonstrated reduced susceptibility to the cell wall lytic dye Congo red and vancomycin. Finding that apt mutations contribute to reduced vancomycin susceptibility once again suggests a role for altered purine metabolism in a VISA mechanism.

Phytohormonal activity of lactone-structure brassinosteroids under chemical stress in plants

The dependence of the action of lactone-structure brassinosteroids in composition with a growth-inhibiting dose of glyphosate on the root system and the aerial part of fiber flax (Linum usitatissimum L.) and spring barley (Hordeum vulgare L.) seedlings was determined. The species specificity of growth responses to seed incrustation by such mixtures was identified. The concentration ranges of brassinosteroids, in which their interaction with the herbicide is most pronounced, were detected. In this case, there is an increase (on spring barley seedlings) or weakening (on fiber flax seedlings) of the inhibitory effect of glyphosate on the root system.

Ultrasound-Targeted Microbubble Destruction Enhances Inhibitory Effect of Apatinib on Angiogenesis in Triple Negative Breast Carcinoma Xenografts

Ultrasound-targeted microbubble destruction (UTMD) has been proven as an effective technique to assist drugs to cross the vascular wall and cell membrane. This study was aimed at evaluating the synergistic antiangiogenic and growth-inhibiting effects of apatinib (APA) and UTMD on the triple negative breast cancer (TNBC). The TNBC xenograft model was established in nude mice ( n = 40 ) which were then randomly divided into the APA plus UTMD (APA-U) group, UTMD group, APA group, and model control (M) group ( n = 10 per group). Corresponding treatment was done once daily for 14 consecutive days. The general condition and body weight of tumor-bearing nude mice were monitored. Routine blood test and detection of liver and kidney function were done after treatments. The tumor size and microcirculation were examined by two-dimensional ultrasonography (2DUS) and contrast-enhanced ultrasonography (CEUS), respectively. Then, the tumor tissues were harvested for the detection of vascular endothelial growth factor (VEGF) by immunohistochemistry and for CD31-PAS double staining to assess microvessel density (MVD) and heterogeneous vascular positivity rate. After treatments, the tumor growth and angiogenesis were significantly inhibited in the APA group and the APA-U group, and these effects were more obvious in the APA-U group. The tumor volume, CEUS parameters, VEGF expression, and MVD in the APA-U group were significantly lower than those in the APA group ( P < 0.05 ), while there were no marked differences in the heterogeneous vascular positivity rate, body weight, and blood parameters between the two groups ( P > 0.05 ). In the UTMD group, the tumor growth and angiogenesis were not significantly inhibited, and all the parameters were similar to those in the M group ( P > 0.05 ). During the experiment, all mice survived and generally had good condition. In conclusion, APA combined with UTMD may exert synergistic antiangiogenic and growth-inhibiting effects on the TNBC and not increase the heterogeneous vasculature and the severity of APA-related systemic side effects.

Plumbagin, a Potent Naphthoquinone from Nepenthes Plants with Growth Inhibiting and Larvicidal Activities

Some plant species are less susceptible to herbivore infestation than others. The reason for this is often unknown in detail but is very likely due to an efficient composition of secondary plant metabolites. Strikingly, carnivorous plants of the genus Nepenthes show extremely less herbivory both in the field and in green house. In order to identify the basis for the efficient defense against herbivorous insects in Nepenthes, we performed bioassays using larvae of the generalist lepidopteran herbivore, Spodoptera littoralis. Larvae fed with different tissues from Nepenthes x ventrata grew significantly less when feeding on a diet containing leaf tissue compared with pitcher-trap tissue. As dominating metabolite in Nepenthes tissues, we identified a naphthoquinone, plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone). When plumbagin was added at different concentrations to the diet of S. littoralis larvae, an EC50 value for larval growth inhibition was determined with 226.5 µg g−1 diet. To further determine the concentration causing higher larval mortality, sweet potato leaf discs were covered with increasing plumbagin concentrations in no-choice-assays; a higher mortality of the larvae was found beyond 60 µg plumbagin per leaf, corresponding to 750 µg g−1. Plant-derived insecticides have long been proposed as alternatives for pest management; plumbagin and derivatives might be such promising environmentally friendly candidates.