Regulatory roles of RpoS in the biosynthesis of antibiotics 2,4-diacetyphloroglucinol and pyoluteorin of Pseudomonas protegens FD6

The alternative sigma factor RpoS regulates Pseudomonas aeruginosa quorum sensing response by repressing the pqsABCDE operon and activating vfr

Pseudomonas aeruginosa is an important opportunistic pathogen. Several of its virulence-related processes, including the synthesis of pyocyanin (PYO) and biofilm formation, are controlled by quorum sensing (QS). It has been shown that the alternative sigma factor RpoS regulates QS through the reduction of lasR and rhlR transcription (encoding QS regulators). However, paradoxically, the absence of RpoS increases PYO production and biofilm development (that are RhlR dependent) by unknown mechanisms. Here, we show that RpoS represses pqsE transcription, which impacts the stability and activity of RhlR. In the absence of RpoS, rhlR transcript levels are reduced but not the RhlR protein concentration, presumably by its stabilization by PqsE, whose expression is increased. We also report that PYO synthesis and the expression of pqsE and phzA1B1C1D1E1F1G1 operon exhibit the same pattern at different RpoS concentrations, suggesting that the RpoS-dependent PYO production is due to its ability to modify PqsE concentration, which in turn modulates the activation of the phzA1 promoter by RhlR. Finally, we demonstrate that RpoS favors the expression of Vfr, which activates the transcription of lasR and rhlR. Our study contributes to the understanding of how RpoS modulates the QS response in P. aeruginosa, exerting both negative and positive regulation.

Biofilms formation in plant growth-promoting bacteria for alleviating agro-environmental stress

Biofilm formation represents a pivotal and adaptable trait among microorganisms within natural environments. This attribute plays a multifaceted role across diverse contexts, including environmental, aquatic, industrial, and medical systems. While previous research has primarily focused on the adverse impacts of biofilms, harnessing their potential effectively could confer substantial advantages to humanity. In the face of escalating environmental pressures (e.g., drought, salinity, extreme temperatures, and heavy metal pollution), which jeopardize global crop yields, enhancing crop stress tolerance becomes a paramount endeavor for restoring sufficient food production. Recently, biofilm-forming plant growth-promoting bacteria (PGPB) have emerged as promising candidates for agricultural application. These biofilms are evidence of microorganism colonization on plant roots. Their remarkable stress resilience empowers crops to thrive and yield even in harsh conditions. This is accomplished through increased root colonization, improved soil properties, and the synthesis of valuable secondary metabolites (e.g., ACC deaminase, acetin, 2,3-butanediol, proline, etc.). This article elucidates the mechanisms underpinning the role of biofilm-forming PGPB in bolstering plant growth amidst environmental challenges. Furthermore, it explores the tangible applications of these biofilms in agriculture and delves into strategies for manipulating biofilm formation to extract maximal benefits in practical crop production scenarios.