The Catabolite Repressor/Activator Cra Is a Bridge Connecting Carbon Metabolism and Host Colonization in the Plant Drought Resistance-Promoting Bacterium Pantoea alhagi LTYR-11Z

Abstract

Efficient root colonization is a prerequisite for application of plant growth promoting (PGP) bacteria in improving health and yield of agricultural crops. We have recently identified an endophytic bacterium LTYR-11Z with multiple PGP properties that effectively colonizes the root system of wheat and improves its growth and drought tolerance. To identify novel regulatory genes required for wheat colonization, we screened a LTYR-11Z transposon (Tn) insertion library and found to be a colonization-related gene. By using RNA-seq analysis, we found that transcriptional levels of an operon, the gene encoding an anti-FlhDC factor and the gene encoding an enzyme for synthesis of cyclic dimeric GMP (c-di-GMP) were significantly downregulated in the mutant Δ Further studies demonstrated that Cra directly binds to the promoters of the operon, and and activates their expression, thus inhibiting motility and promoting exopolysaccharides (EPS) production and biofilm formation. Consistent with previous findings that Cra plays a role in transcriptional regulation in response to carbon source availability, the activating effects of Cra were much more pronounced when LTYR-11Z was grown within a gluconeogenic environment than when it was grown within a glycolytic environment. We further demonstrate that the ability of LTYR-11Z to colonize wheat roots is modulated by the availability of carbon sources. All together, these results uncover a novel strategy utilized by LTYR-11Z to achieve host colonization in response to carbon nutrition in the environment, in which Cra bridges a connection between carbon metabolism and colonization capacity of LTYR-11Z. Rapid and appropriate response to environmental signals is crucial for bacteria to adapt to competitive environments and to establish interactions with their hosts. Efficient colonization and persistence within the host is controlled by various regulatory factors that respond to specific environmental cues. The most common is nutrient availability. In this work, we unraveled the pivotal role of Cra in regulation of colonization ability of LTYR-11Z in response to carbon source availability. Moreover, we identified three novel members of the Cra regulon involved in EPS synthesis, regulation of flagellar biosynthesis, and synthesis of c-di-GMP, and propose a working model to explain the Cra-mediated regulatory mechanism that links carbon metabolism to host colonization. This study elucidates the regulatory role of Cra in bacterial attachment and colonization of plants, which raises the possibility to extend our studies to other bacteria associated with plant and human health.

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

“…Notably, many bacteria are able to develop two different lifestyles, the planktonic mode (characterized by single motile cells) and the biofilm mode (characterized by sessile multicellular communities that are attached to each other and a surface) (4,5). While the transition between motile and sessile lifestyles of bacteria is crucial for their ecological success, complex regulatory networks are integrated to achieve the control of the motile-sessile switch in response to a variety of environmental signals, including nutrient availability, oxygen, temperature, salinity, metal ions, calcium, and phosphate (6)(7)(8)(9)(10). Although considerable efforts are being made to understand the link between environmental signals and bacterial endogenous regulatory pathways that govern the lifestyle switch, the underlying mechanisms remain largely unknown.…”

“…The rhizosphere is a critical interface between plants and their associated soil environment and thus the primary site of complex plant-microbe interactions (16,17). PGPB recruited from the soil are widely known to offer benefits to plants in the rhizosphere, while some root surface colonizers are able to transcend the endodermis barrier and establish endophytic populations (10,18). Efficient colonization and persistence on/within plant roots is considered a prerequisite for bacteriummediated improvement of crop performance (10,19).…”

“…PGPB recruited from the soil are widely known to offer benefits to plants in the rhizosphere, while some root surface colonizers are able to transcend the endodermis barrier and establish endophytic populations (10,18). Efficient colonization and persistence on/within plant roots is considered a prerequisite for bacteriummediated improvement of crop performance (10,19). While chemotaxis, motility, and adhesins, such as exopolysaccharides (EPS) and cell surface proteins, are well-known bacterial traits involved in the colonization of plant root surface, plant polymerdegrading enzymes have been shown to play a role in the colonization of the root interior by endophytic bacteria (20,21).…”

“…While chemotaxis, motility, and adhesins, such as exopolysaccharides (EPS) and cell surface proteins, are well-known bacterial traits involved in the colonization of plant root surface, plant polymerdegrading enzymes have been shown to play a role in the colonization of the root interior by endophytic bacteria (20,21). In addition, functions relevant for rhizosphere competence and/or plant colonization also include efficient iron acquisition, detoxification, and degradation of organic compounds, secretion systems, and signaling systems (10,20,22).…”

“…For both animal-and plant-associated bacteria, biofilm formation has evolved as an adaptive strategy to achieve host colonization as well as to establish beneficial or antagonistic interactions with their host (23,24). While biofilm formation is a complex multistage process under the control of a variety of cell surface components and regulators, the inverse regulation of EPS production and motility appears to be widely conserved during both initiation of biofilm formation and dispersal of preformed biofilms (10,25). In general, flagellum formation is turned off and EPS production is turned on during the switch from planktonic to biofilm growth, whereas biofilm dispersal is preceded by decreased EPS production and increased flagellar motility (23,25,26).…”