RESUMO
A wide variety of plant-associated soil bacteria (rhizobacteria) communicate with each other by quorum sensing (QS). Plants are able to detect and produce mimics and inhibitor molecules of the QS bacterial communicative process. Arachis hypogaea L. (peanut) establishes a nitrogen-fixing symbiosis with rhizobia belonging to the genus Bradyrhizobium. These bacteria use a QS mechanism dependent on the synthesis of N-acyl homoserine lactones (AHLs). Given the relevance that plant-rhizobacteria interactions have at the ecological level, this work addresses the involvement of peanut in taking part in the QS mechanism. By using biosensor bacterial strains capable of detecting AHLs, a series of standard and original bioassays were performed in order to determine both (i) the production of QS-like molecules in vegetal materials and (ii) the expression of the QS mechanism throughout plant-bacteria interaction. Mimic QS-like molecules (mQS) linked to AHLs with long acyl chains (lac-AHL), and inhibitor QS-like molecules (iQS) linked to AHLs with short acyl chains (sac-AHL) were detected in seed and root exudates. The results revealed that synthesis of specific signaling molecules by the plant (such as mQS and iQS) probably modulates the function and composition of the bacterial community established in its rhizosphere. Novel bioassays of QS detection during peanut-Bradyrhizobium interaction showed an intense production of QS signals in the contact zone between root and bacteria. It is demonstrated that root exudates stimulate the root colonization and synthesis of lac-AHL by Bradyrhizobium strains in the plant rhizosphere, which leads to the early stages of the development of beneficial plant-bacteria interactions.
Assuntos
Acil-Butirolactonas/metabolismo , Arachis/metabolismo , Percepção de Quorum , Rizosfera , Bactérias/crescimento & desenvolvimento , BradyrhizobiumRESUMO
The aim of this work was to study the antimicrobial activity of essential oils obtained from Thymus vulgaris (thyme) and Origanum vulgare (oregano) on phytopathogenic Pseudomonas species isolated from soybean. Strains with characteristics of P. syringae were isolated from leaves of soybean plants with blight symptoms. Ten of these could be identified in Group Ia of LOPAT as P. syringae. Six of these were confirmed as P. syringae using 16S rRNA, indicating the presence of these phytopathogenic bacteria in east and central Argentina. All the phytopathogenic bacteria were re-isolated and identified from the infected plants. MIC values for thyme were 11.5 and 5.7 mg·ml(-1) on P. syringae strains, while oregano showed variability in the inhibitory activity. Both essential oils inhibited all P. syringae strains, with better inhibitory activity than the antibiotic streptomycin. The oils were not bactericidal for all pseudomonads. Both oils contained high carvacrol (29.5% and 19.7%, respectively) and low thymol (1.5%). Natural products obtained from aromatic plants represent potential sources of molecules with biological activity that could be used as new alternatives for the treatment of phytopathogenic bacteria infections.