Mutations in lipopolysaccharide biosynthetic genes impair maize rhizosphere and root colonization of Rhizobium tropici CIAT899

Abstract

Three transposon mutants of Rhizobium tropici CIAT899 affected in lipopolysaccharide (LPS) biosynthesis were characterized and their maize rhizosphere and endophytic root colonization abilities were evaluated. The disrupted genes coded for the following putative products: the ATPase component of an O antigen ABC-2 type transporter (wzt), a nucleotide-sugar dehydratase (lpsbeta2) and a bifunctional enzyme producing GDP-mannose (noeJ). Electrophoretic analysis of affinity purified LPS showed that all mutants lacked the smooth LPS bands indicating an O antigen minus phenotype. In the noeJ mutant, the rough LPS band migrated faster than the parental band, suggesting a truncated LPS core. When inoculated individually, the wzt and noeJ mutants colonize the rhizosphere and root to a lower extent than the parental strain while no differences were observed between the lpsbeta2 mutant and the parental strain. All mutants were impaired in competitive rhizosphere and root colonization. Pleiotropic effects of the mutations on known colonization traits such as motility and growth rate were observed, but they were not sufficient to explain the colonization behaviours. It was found that the LPS mutants were sensitive to the maize antimicrobial 6-methoxy-2-benzoxazolinone (MBOA). Only the combined effects of altered growth rate and susceptibility to maize antimicrobials could account for all the observed colonization phenotypes. The results suggest an involvement of the LPS in protecting R. tropici against maize defence response during rhizosphere and root colonization.

Gut and Root Microbiota Commonalities

“…Similarly, among root microbiotas, there are plant-pathogenic bacteria that normally would not affect the plants when kept in low numbers by other plant community strains or plant antimicrobials. Lipopolysaccharide Rhizobium mutants that were more sensitive to maize antimicrobial benzoxazinones had reduced rhizospheric colonization (152). Antimicrobial peptides constitute a line of defense in plants as effectors of innate immunity and regulate not only bacteria but also methanogenic archaea in guts (153).…”

Maize Root Lectins Mediate the Interaction with Herbaspirillum seropedicae via N-Acetyl Glucosamine Residues of Lipopolysaccharides

“…This may suggest an important role of LPS against plant basal defenses. In other rhizobacteria such as Pseudomonas fluorescens, Azospirillum brasilense and R. tropici CIAT899 alterations in the LPS structure also reduced rhizospheric and endophytic root colonization [15]-[18], [9], but the role of LPS in such interactions is not clear.…”

“…Molecules of the bacterial envelope such as flagella proteins, exopolysaccharides, adhesins and lipopolysaccharides are the first structures that contact the host, and may be required for bacterial attachment on root surfaces. In particular, several roles have been demonstrated for the LPS in plant-microbe interactions, including communication, protective barrier against plant antimicrobials and modulation of plant defense responses [6]–[9]. The lipopolysaccharides (LPS) are highly complex macromolecules found exclusively as a monolayer on the outer membrane of gram-negative bacteria [10].…”

“…Studies with associative bacteria showed that modification of LPS reduces plant colonization and competitiveness [15]–[18], [9]. In a previous work we constructed H. seropedicae mutant strains in the genes rfbB and rfbC , which are involved in dTDP-rhamnose biosynthesis, the major LPS monosaccharide [19].…”

“…Such rearrangements led to diminished attachment of the bacteria to maize roots, suggesting that interaction between LPS and plant receptors was required for adhesion onto roots. Mutations in lipid A biosynthesis genes are often lethal for gram-negative bacteria [21], while mutations in genes responsible for biosynthesis of monosaccharides can produce pleiotropic effects, such as decreased growth rate and altered exopolysaccharide production [9], [19]. In the present work, we showed activation of the expression of LPS biosynthesis gene waaL during colonization of maize root by H. seropedicae ; we constructed a waaL (codes for the O-antigen ligase) mutant strain defective in the synthesis of the O-antigen portion of LPS, and showed that it is impaired in epiphytic and endophytic colonization.…”