Quorum sensing and other communication strategies in A. brasilense

GUALPA, JOSÉ; NIEVAS, SOFÍA; LOPEZ, GASTÓN; HALLIDAY, NIGEL; CAMARA, MIGUEL; CASSÁN, FABRICIO

Foz do Iguazú

Congreso; XVI Symposium on Biological Nitrogen Fixation with Non-Legumes, IV Latin-American workshop on PGPR and XIX RELARE.; 2018

EMBRAPA

The mechanism known as quorumsensing (QS) based on N-acyl-L-homoserine lactones (AHL) allows cell­-to-cellcommunication to control the expression of certain genes at high cell density. QSsystems are found in Gram-negative bacteria and used for controling variousphenotypes that are often essential for interaction with a eukaryotic host,notably for plant-associated bacteria. Azospirillum brasilense Az39 is a strain comprisingversatile plant growth-promoting rhizobacteria (PGPR), with several reports ofstrains showing remarkable capacity to benefit plant growth in a variety ofplant species. In Argentina, A. brasilense strains Az39 have beenexponentially used in commercial inoculants for grain crops including maize (Zea mays) and wheat (Triticumaestivum L.) and also for other legumes and non-legumes.  Az39may indirectly regulate several processes by QS mechanisms. However,the complete AHL-mediated QS system seems unusual in genus Azospirillum mainly in the species brasilense.On the other hand, quorum quenching (QQ) refers to all processes involved inthe interruption of QS, QQ is a natural microbial mechanism, either to degradetheir own QS signals, or to establish competitive relationships with othermicroorganisms. The QQ enzymes AHL-lactonase and AHL-acylase play strategicroles in the bacterial interactions, including bacterium?plant associations. Despitethe agronomic importance of A. brasilense strain Az39, few studies havebeen performed on QS in that genus and none with this important strain. Weexamined in silico and in vitro bacterial mechanisms related tothe QS and QQ mechanisms in Az39. The results obtained showed that although Az39 does not producemolecules of the AHL type, it is capable of degrading them. Interestingly, the QQ activity is due to the action of an AHL-acylaseand not to an AHL-lactonase, and would also be associated with the cell and notwith the culture supernatant. Moreover, the Az39 genome encodes a single keyLuxR solo for bacterial communicationat the rhizospheric level. These results suggest that Az39 in the environmentcan potentially degrade the exogenous AHLs of the bacteria with which it sharesniche and also under certain conditions respond to chemical structure moleculessimilar to AHLs, thus modulating the interaction between microbial communitiesand plants.