A deep insight into the suppression mechanism of Sedum alfredii root exudates on Pseudomonas aeruginosa based on quorum sensing.

Quorum sensing (QS) plays an important role in the intensive communication between plants and microbes in the rhizosphere during the phytoremediation. This study explored the influence of the root exudates of hyperaccumulator Sedum alfredii on Pseudomonas aeruginosa based on QS. The effects of the components of root exudates, genes expression and transcription regulation of QS system (especially the las system) in Pseudomonas aeruginosa wild-type strain (WT) and rhl system mutant strain (ΔrhlI) were systematically analyzed and discussed. The WT and ΔrhlI exposed to gradient root exudates (0×, 1×, 2×, 5× and 10×) showed a concentration-corrective inhibition on protease production, with the inhibition rates of 51.4-74.5% and 31.2-50.0%, respectively. Among the components of the root exudates of Sedum alfredii, only thymol had an inhibition effects to the root exudates on the activity of protease and elastase. The inhibition rates of 50 µmol/L thymol on protease and elastase in WT were 44.7% and 24.3%, respectively, which was consistent with the variation in ΔrhlI. The gene expression of lasB declined 36.0% under the 1× root exudate treatment and 73.0% under the 50 µmol/L thymol treatment. Meanwhile, there was no significant impact on N-3-oxo-dodecanoyl-L-homoserine lactone signal production and the gene expression of lasI and lasR. Therefore, thymol from Sedum alfredii root exudates could inhibit the formation of protease and elastase in Pseudomonas aeruginosa by suppressing the expression of lasB, without any significant influence on the main las system as a potential natural QS inhibitor.

Garlic Allelochemical Diallyl Disulfide Alleviates Autotoxicity in the Root Exudates Caused by Long-Term Continuous Cropping of Tomato.

Continuous cropping obstacles seriously affect the sustainable production of tomatoes (Solanum lycopersicum L.). Researchers have found that intercropping with garlic (Allium sativum L.) could alleviate tomato continuous cropping obstacles. Diallyl disulfide (DADS) is the main allelochemical in garlic. However, the mechanism of DADS in alleviating tomato continuous cropping obstacles is still unknown. In this research, aqueous extracts of tomato continuous cropping soil were used to simulate the continuous cropping condition of tomato. Our results showed that DADS increased root activity and chlorophyll content and improved the activity of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and phenylalanine ammonia-lyase (PAL)) and the metabolism of nonenzymatic antioxidants (glutathione (GSH) and oxidized glutathione (GSSG)) in tomato plants. DADS treatment reduced the content of fatty acid esters in tomato root exudates (e.g., palmitate methyl ester, palmitoleic acid methyl ester, oleic acid methyl ester) and increased the level of substances such as dibutyl phthalate and 2,2'-methylenebis(6-tert-butyl-4-methylphenol). The higher concentrations of palmitate methyl ester inhibited tomato hypocotyl growth, while oleic acid methyl ester inhibited tomato root growth. Moreover, the application of DADS significantly inhibited the secretion of these esters in the root exudates. Therefore, it suggests that DADS may increase tomato resistance and promote tomato plant growth by increasing root activity and photosynthetic capacity and development to reduce autotoxicity of tomato.

Maize (Zea mays L.) root exudates modify the surface chemistry of CuO nanoparticles: Altered aggregation, dissolution and toxicity.

Copper oxide nanoparticles (CuO NPs), as an antimicrobial nanomaterial, have found many applications in agriculture. Ubiquitous and complex root exudates (RE) in the plant root zone motivates the determination of how specific components of RE interact with CuO NPs. This work aims to reveal the role of maize (Zea mays L.)-derived RE and their components on the aggregation and dissolution of CuO NPs in the rhizosphere. We observed that RE significantly inhibited the aggregation of CuO NPs regardless of ionic strength and electrolyte type. In the presence of RE, the CCC of CuO NPs in NaCl shifted from 30 to 125 mM and the value in CaCl2 shifted from 4 to 20 mM. Furthermore, this inhibition was correlated with molecular weight (MW) of RE fractions. Higher MW fraction (>10 kDa) reduced the aggregation most. We also discovered that RE significantly promoted the dissolution of CuO NPs and lower MW fraction (<3 kDa) RE mainly contributed to this process. Additionally, phytotoxicity of CuO NPs in the presence of RE and different fractions of RE was evaluated. The addition of 20 mg/L RE reduced the seedlings growth rate to 1.89% after 7 days exposure to 25 mg/L CuO NPs, which were significantly lower than the control group (4.82%). Notably, Cu accumulation in plant root tissues was significantly enhanced by 20 mg/L RE. This study provides useful insights into the interactions between RE and CuO NPs, which is of significance for the safe use of CuO NPs-based antimicrobial products in agricultural production.

Biological properties and pharmacological potential of plant exudates.

Exudates released from plants, consist of complex mixtures of organic and inorganic molecules that have been used in traditional medicine from several years. They may vary among genera, species or within a genus and mainly include latex, sap, gums, resins, seed or root exudates. Plant exudates are known to possess several biological activities including, antimicrobial, anti-inflammatory, antioxidant, wound healing and anti-nociceptive. Exudates oozed out from plants have also been used as ingredients in medicines, food, perfumes and cosmetics. The present review provides brief overview about the exudates released from plants, their biological properties and beneficial effects for human beings. Due to the presence of various compounds, different methodologies and procedures have been employed for their collection and analyses. Literature studies suggest that plant exudates have extensive therapeutic potential for curing diseases with minimal toxic effects. This aspect could be taken into account in prospective studies regarding the search of new products derived from plant exudates with pharmaceutical value.

Study on the release routes of allelochemicals from Pistia stratiotes Linn., and its anti-cyanobacteria mechanisms on Microcystis aeruginosa.

Allelochemicals in Pistia stratiotes Linn. have a strong anti-cyanobacteria effect on Microcystis aeruginosa. To further determine the release routes of allelochemicals in P. stratiotes and understand their anti-cyanobacteria mechanisms, we aimed to systematically investigate the allelopathic effects of leaf leachates, leaf volatilization, root exudates, and residue decomposition of P. stratiotes on M. aeruginosa. The influences of P. stratiotes allelochemicals on the physiological properties of M. aeruginosa were also studied. Root exudates of P. stratiotes exhibited the strongest inhibitory effect on M. aeruginosa growth. The residue decomposition and leaf leachates exhibited a relatively strong inhibitory effect on M. aeruginosa growth. By contrast, the leaf volatilization stimulated M. aeruginosa growth. Therefore, root exudation was determined to be the main release route of allelochemicals from P. stratiotes. The mixed culture experiment of P. stratiotes root exudates and M. aeruginosa showed that the allelochemicals released from root exudation had no effect on the electron transfer of M. aeruginosa photosynthetic system II. However, it reduced the phycocyanin (PC) content and phycocyanin to allophycocyanin (PC/APC) ratio in the photosynthetic system. As the root exudates concentration increased, the electrical conductivity (EC) and superoxide anion radical (O2(*-)) values in the M. aeruginosa culture fluid increased significantly, indicating that the allelochemicals released from the root of P. stratiotes inhibited algae growth by affecting the PC and PC/APC levels in photosynthesis, destroying the cell membrane, and increasing O2(*-) content to result in oxidative damage of M. aeruginosa.

Allelochemicals in the rhizosphere soil of Euphorbia himalayensis.

Weed infestation has been known to cause considerable reductions in crop yields, thereby hindering sustainable agriculture. Many plants in genus Euphorbia affect neighboring plants and other organisms by releasing chemicals into the environment. In view of the serious threat of weeds to agriculture, the allelochemicals of Euphorbia himalayensis and their allelopathic effects were investigated. The extract of root exudates from rhizosphere soil exhibited allelopathic activities against crops (wheat, rape, and lettuce) and grasses (Poa annua, Festuca rubra, and red clover). Bioassay-guided fractionation and isolation from the root extract of E. himalayensis led to the characterization of two ellagic acid derivatives and a jatrophane diterpene, which observably showed phytotoxic activities against lettuce, Festuca arundinacea, and F. rubra. They were further confirmed by ultra-performance liquid chromatography-tandem mass spectrometry to have concentrations of 3.6, 3.8, and 8.99 nmol/g in the rhizospere soil, respectively. Bioassay indicated that the combination of the allelochemicals could be selective plant growth regulator in agriculture.

Novel weapons and invasion: biogeographic differences in the competitive effects of Centaurea maculosa and its root exudate (+/-)-catechin.

Recent studies suggest that the invasive success of Centaurea maculosa may be related to its stronger allelopathic effects on native North American species than on related European species, one component of the "novel weapons" hypothesis. Other research indicates that C. maculosa plants from the invasive range in North America have evolved to be larger and better competitors than conspecifics from the native range in Europe, a component of the "evolution of increased competitive ability" hypothesis. These hypotheses are not mutually exclusive, but this evidence sets the stage for comparing the relative importance of evolved competitive ability to inherent competitive traits. In a competition experiment with a large number of C. maculosa populations, we found no difference in the competitive effects of C. maculosa plants from North America and Europe on other species. However, both North American and European C. maculosa were much better competitors against plants native to North America than congeners native to Romania, collected in areas where C. maculosa is also native. These results are consistent with the novel weapons hypothesis. But, in a second experiment using just one population from North America and Europe, and where North American and European species were collected from a broader range of sites, competitive interactions were weaker overall, and the competitive effects of C. maculosa were slightly stronger against European species than against North American species. Also consistent with the novel weapons hypothesis, (+/-)-catechin had stronger effects on native North American species than on native European species in two experiments. Our results suggest that the regional composition of the plant communities being invaded by C. maculosa may be more important for invasive success than the evolution of increased size and competitive ability.