An Osmoregulatory Mechanism Operating through OmpR and LrhA Controls the Motile-Sessile Switch in the Plant Growth-Promoting Bacterium Pantoea alhagi

Abstract

Adaptation to osmotic stress is crucial for bacterial growth and survival in changing environments. Although a large number of osmotic stress response genes have been identified in various bacterial species, how osmotic changes affect bacterial motility, biofilm formation, and colonization of host niches remains largely unknown. In this study, we report that the LrhA regulator is an osmoregulated transcription factor that directly binds to the promoters of the flhDC, eps, and opgGH operons and differentially regulates their expression, thus inhibiting motility and promoting exopolysaccharide (EPS) production, synthesis of osmoregulated periplasmic glucans (OPGs), biofilm formation, and root colonization of the plant growthpromoting bacterium Pantoea alhagi LTYR-11Z. Further, we observed that the LrhAregulated OPGs control RcsCD-RcsB activation in a concentration-dependent manner, and a high concentration of OPGs induced by increased medium osmolarity is maintained to achieve the high level of activation of the Rcs phosphorelay, which results in enhanced EPS synthesis and decreased motility in P. alhagi. Moreover, we showed that the osmosensing regulator OmpR directly binds to the promoter of lrhA and promotes its expression, while lrhA expression is feedback inhibited by the activated Rcs phosphorelay system. Overall, our data support a model whereby P. alhagi senses environmental osmolarity changes through the EnvZ-OmpR two-component system and LrhA to regulate the synthesis of OPGs, EPS production, and flagellum-dependent motility, thereby employing a hierarchical signaling cascade to control the transition between a motile lifestyle and a biofilm lifestyle. IMPORTANCE Many motile bacterial populations form surface-attached biofilms in response to specific environmental cues, including osmotic stress in a range of natural and host-related systems. However, cross talk between bacterial osmosensing, swimming, and biofilm formation regulatory networks is not fully understood. Here, we report that the pleiotropic regulator LrhA in Pantoea alhagi is involved in the regulation of flagellar motility, biofilm formation, and host colonization and responds to osmotic upshift. We further show that this sensing relies on the EnvZ-OmpR two-component system that was known to detect changes in external osmotic stress. The EnvZ-OmpR-LrhA osmosensing signal transduction cascade is proposed to increase bacterial fitness under hyperosmotic conditions inside the host. Our work proposes a novel regulatory mechanism that links osmosensing and motile-sessile lifestyle transitions, which may provide new approaches to prevent or promote the formation of biofilms and host colonization in P. alhagi and other bacteria possessing a similar osmoregulatory mechanism.

Osmoregulated Periplasmic Glucans Transmit External Signals Through Rcs Phosphorelay Pathway in Yersinia enterocolitica

“…A recent study showed that Pantoea alhagi senses environmental osmolarity changes through the EnvZ/OmpR system and LrhA to regulate synthesis of OPGs, EPS production and flagella-dependent motility. OPGs control the Rcs activation in a concentrationdependent manner, and a high concentration of OPGs induced by increased medium osmolarity is maintained to achieve the high level of activation of the Rcs phosphorelay which results in enhanced EPS synthesis and decreased motility in P. alhagi (Li et al, 2019). As motility and biofilm formation are critical for bacterial colonization of the host, these findings indicate that OPGs associated with Rcs and EnvZ/OmpR systems play a key role in shaping the pathogenic phenotype of bacteria.…”

Ensifer aridi LMR001T Symbiosis and Tolerance to Stress Do Not Require the Alternative Sigma Factor RpoE2

“…Indeed, nearly 60% of genes with COG assigned to that class were repressed upon stress. Hyperosmotic stresses have already been shown to repress motility gene transcription in many bacteria [32][33][34][35][36] including rhizobia [37]. Interestingly the repression of motility is often associated to an induction of genes involved in the synthesis of biofilm components such as surface polysaccharides or proteins [32,35,38,39].…”

Massively parallel transposon mutagenesis identifies temporally essential genes for biofilm formation in Escherichia coli

“…The similarities in phenotypes could indicate both genes influence biofilms in a similar manner. The role of lrhA in motility regulation has been well documented [43,44,73], and expression of tomB has been seen to reduce motility through repression of fliA [74]. Although ΔlrhA and ΔtomB deletion mutation strains shared many similar phenotypes, TraDIS-Xpress data predicted that tomB was beneficial and lrhA was detrimental to biofilm development at 12, 24 and 48 h. Therefore, these genes may regulate the same pathways but in different ways.…”