Surfactin Stimulated by Pectin Molecular Patterns and Root Exudates Acts as a Key Driver of the Bacillus-Plant Mutualistic Interaction.

Bacillus velezensis is considered as a model species belonging to the so-called Bacillus subtilis complex that evolved typically to dwell in the soil rhizosphere niche and establish an intimate association with plant roots. This bacterium provides protection to its natural host against diseases and represents one of the most promising biocontrol agents. However, the molecular basis of the cross talk that this bacterium establishes with its natural host has been poorly investigated. We show here that these plant-associated bacteria have evolved a polymer-sensing system to perceive their host and that, in response, they increase the production of the surfactin-type lipopeptide. Furthermore, we demonstrate that surfactin synthesis is favored upon growth on root exudates and that this lipopeptide is a key component used by the bacterium to optimize biofilm formation, motility, and early root colonization. In this specific nutritional context, the bacterium also modulates qualitatively the pattern of surfactin homologues coproduced in planta and forms mainly variants that are the most active at triggering plant immunity. Surfactin represents a shared good as it reinforces the defensive capacity of the host. IMPORTANCE Within the plant-associated microbiome, some bacterial species are of particular interest due to the disease protective effect they provide via direct pathogen suppression and/or stimulation of host immunity. While these biocontrol mechanisms are quite well characterized, we still poorly understand the molecular basis of the cross talk these beneficial bacteria initiate with their host. Here, we show that the model species Bacillus velezensis stimulates the production of the surfactin lipopeptide upon sensing pectin as a cell surface molecular pattern and upon feeding on root exudates. Surfactin favors bacterial rhizosphere fitness on one hand and primes the plant immune system on the other hand. Our data therefore illustrate how both partners use this multifunctional compound as a unique shared good to sustain a mutualistic interaction.

The subtilisin-like protease SBT3 contributes to insect resistance in tomato.

Subtilisin-like proteases (SBTs) constitute a large family of extracellular plant proteases, the function of which is still largely unknown. In tomato plants, the expression of SBT3 was found to be induced in response to wounding and insect attack in injured leaves but not in healthy systemic tissues. The time course of SBT3 induction resembled that of proteinase inhibitor II and other late wound response genes suggesting a role for SBT3 in herbivore defense. Consistent with such a role, larvae of the specialist herbivore Manduca sexta performed better on transgenic plants silenced for SBT3 expression (SBT3-SI). Supporting a contribution of SBT3 to systemic wound signaling, systemic induction of late wound response genes was attenuated in SBT3-SI plants. The partial loss of insect resistance may thus be explained by a reduction in systemic defense gene expression. Alternatively, SBT3 may play a post-ingestive role in plant defense. Similar to other anti-nutritive proteins, SBT3 was found to be stable and active in the insect's digestive system, where it may act on unidentified proteins of insect or plant origin. Finally, a reduction in the level of pectin methylesterification that was observed in transgenic plants with altered levels of SBT3 expression suggested an involvement of SBT3 in the regulation of pectin methylesterases (PMEs). While such a role has been described in other systems, PME activity and the degree of pectin methylesterification did not correlate with the level of insect resistance in SBT3-SI and SBT3 overexpressing plants and are thus unrelated to the observed resistance phenotype.

Arylnaphthalene and aryltetralin-type lignans in hairy root cultures of Linum perenne, and the stereochemistry of 6-methoxypodophyllotoxin and one diastereoisomer by HPLC-MS and NMR spectroscopy.

INTRODUCTION: Hairy root cultures of Linum sp. are an alternative for the high production of lignans. Linum perenne is known to produce arylnaphthalene-type lignans such as justicidin B, isojusticidin and diphyllin. OBJECTIVE: To elucidate the presence of aryltetralin-type lignan diastereoisomers, besides the known arylnaphthalene-type lignans, in hairy roots of Linum perenne, and to determine the configurations of one diastereoisomer of 6-methoxypodophyllotoxin (6-MPTOX). METHODS: Lignans from hairy root cultures of Linum perenne were extracted and separated by HPLC. Arylnaphthalene-type lignans were identified by LC-MS, according to the literature. Two diastereoisomers of aryltetralin-type lignans were analysed by mass spectrometry and NMR spectroscopy. RESULTS: Numerous arylnaphthalene-type lignans (diphyllin-2-hexose-pentose, diphyllin-3-pentose and diphyllin-hexose) were identified in hairy root cultures. Methoxypodophyllotoxin, an aryltetralin-type lignan, was also identified, as well as one diastereoisomer. This aryltetralin-type lignan could be derived via 7-hydroxymatairesinol as a hypothetical biosynthetic pathway. The stereochemical configurations of aryltetralin isomers were determined. CONCLUSION: Arylnaphthalene and two diastereoisomers of aryltetralin-type lignans are produced in Linum perenne hairy root cultures. Matairesinol, the precursor of justicidin B, also seems to be converted into 6-MPTOX via 7-hydroxymatairesinol. This is the first report of the stereochemical configurations of an aryltetralin-type lignan other than podophyllotoxin (PTOX).

Improved stability of TMS derivatives for the robust quantification of plant polar metabolites by gas chromatography-mass spectrometry.

Plant metabolite profiling is commonly carried out by GC-MS of methoximated trimethylsilyl (TMS) derivatives. This technique is robust and enables a library search for spectra produced by electron ionization. However, recent articles have described problems associated with the low stability of some TMS derivatives. This limits the use of GC-MS for metabolomic studies that need large sets of qualitative and quantitative analyses. The aim of this work is to determine the experimental conditions in which the stability of TMS derivatives could be improved. This would facilitate the analysis of the large-scale experimental designs needed in the metabolomics approach. For good repeatability, the sampling conditions and the storage temperature of samples during analysis were investigated. Multiple injections of one sample from one vial led to high variations while injection of one sample from different vials improved the analysis. However, before injection, some amino acid TMS derivatives were degraded during the storage of vials in the autosampler. Only 10% of the initial quantity of glutamine 3 TMS and glutamate 3 TMS and 66% of α-alanine 2 TMS was detected 48 h after derivatization. When stored at 4 °C until injection, all TMS derivatives remained stable for 12 h; at -20 °C, they remained stable for 72 h. From the integration of all these results, a detailed analytical procedure is thus proposed. It enables a robust quantification of polar metabolites, useful for further plant metabolomics studies using GC-MS.

Biosynthesis of benzylisoquinoline alkaloids in Corydalis bracteata: compartmentation and seasonal dynamics.

Numerous species of the genus Corydalis (Papaveraceae) produce a large spectrum of benzylisoquinoline alkaloids (BIA), some of which are of potential therapeutic value, but no information on sites of their biosynthesis and compartmentation is available. This study focuses on the biosynthesis, compartmentation and seasonal dynamics of BIA in Corydalis bracteata (Steph. ex Willd) Pers., a geophyte with a very short spring vegetation period, which for the rest of the year is represented by underground tubers with buds. It was found that all organs of C. bracteata contained high levels of BIA, the highest concentrations being detected in underground tuber buds in early autumn. Neither xylem nor phloem sap contained alkaloids throughout the year but BIA were present in the apoplastic wash fluid of the tuber. The absence of long-distance transport of alkaloids was confirmed by the experiment using an isotopically labeled tracer, [ring-(13)C6]-tyramine: when whole plants were fed with the tracer with via the roots, the alkaloids became labeled in the roots only and not in other organs. However, when detached roots, leaves, tubers and stems were exposed to [ring-(13)C6]-tyramine, the label was incorporated into alkaloids in all organs. We conclude that no long-distance translocation of alkaloids occurs between organs of C. bracteata, while in the tuber the cell-to-cell transport of alkaloids could occur via the apoplast. In contrast to other BIA-producing species, every organ of C. bracteata was found to be capable of de novo biosynthesis of the full complement of alkaloids.

Structural features and bioremediation activity of an exopolysaccharide produced by a strain of Enterobacter ludwigii isolated in the Chernobyl exclusion zone.

The bacterium Enterobacter ludwigii Ez-185-17, member of the family Enterobacteriaceae, was isolated from the root nodules of plants harvested in the nuclear power region of Chernobyl. Under batch culture conditions, the bacteria produce a high-molecular-mass exopolysaccharide (EPS). After purification, the structure of this EPS was determined using a combinatory approach including monosaccharide composition (GC-FID, HPAEC-PAD) and branching structure determination (GC-MS), as well as 1D/2D NMR ((1)H, (13)C) and ESI-MS (HR, MS/MS) studies of oligosaccharides obtained from mild acid hydrolysis. The EPS was found to be a charged hexasaccharide with a repeating unit composed of d-galactose, d-glucose, l-fucose, d-glucuronic acid (2:1:2:1) and substituted with acyl and pyruvyl groups. The metal-binding properties of the exopolysaccharide were then investigated, and the results seem to indicate that the EPS decreased Cd sequestration in flax seeds.

Exopolysaccharide production by nitrogen-fixing bacteria within nodules of Medicago plants exposed to chronic radiation in the Chernobyl exclusion zone.

Nitrogen-fixing bacteria isolated from root nodules of Medicago plants growing in the 10 km zone around the Chernobyl nuclear power plant were screened for the production of new water-soluble acidic exopolysaccharides (EPSs). The different strains belonged to the Enteriobacteriaceae family (Enterobacter ludwigii, Raoultella terrigena, Klebsiella oxytoca), except for one which belonged to the Rhizobiaceae family (Sinorhizobium meliloti). All of the bacteria produced highly viscous EPS with an average molecular weight comprised between 1 x 10(6) and 3 x 10(6) Da. Five different compositions of EPS were characterized by physico-chemical analyses and (1)H NMR spectroscopy: galactose/mannose (2/1), galactose/glucose (1/1), galactose/glucose/mannose (1/2/1), fucose/galactose/glucose (2/1/1) and fucose/galactose/glucose/mannose (2/2/1/1 or 1/1/2/4). Glucuronic acid, a charged monosaccharide, was also recovered in most of the different EPSs.