Attachment of soil bacteria to plant cells is supposedly the very early step required in plant-microbe interactions. Attachment also is an initial step for the formation of microbial biofilms on plant roots. For the rhizobia-legume symbiosis, various mechanisms and diverse surface molecules of both partners have been proposed to mediate in this process. The first phase of attachment is a weak, reversible, and unspecific binding in which plant lectins, a Ca(+2)-binding bacterial protein (rhicadhesin), and bacterial surface polysaccharide appear to be involved. The second attachment step requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. Cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment of Agrobacterium to plant cells. Attachment of Azospirillum brasilense to cereals roots also can be divided in two different steps. Bacterial surface proteins, capsular polysaccharide and flagella appear to govern the first binding step while extracellular polysaccharide is involved in the second step. Outer cell surface proteins and pili are implicated in the adherence of Pseudomonas species to plant roots.

http://igem-victoria.pbworks.com/f/rhizobial+attachment+to+plant+roots.pdf

Attachment of bacteria to the roots of higher plants.

Spontaneous mutants of Rhizobium meliloti L5-30 defective in motility or chemotaxis were isolated and compared against the parent with respect to symbiotic competence. Each of the mutants was able to generate normal nodules on the host plant alfalfa (Medicago sativa), but had slightly delayed nodule formation, diminished nodulation in the initially susceptible region of the host root, and relatively low representation in nodules following co-inoculation with equal numbers of the parent. When inoculated in growth pouches with increasing dosages of the parental strain, the number of nodules formed in the initially susceptible region of the root increased sigmoidally, with an optimum concentration of about 105 to 106 bacteria/plant. The dose-response behavior of the nonmotile and nonchemotactic mutants was similar, but they required 10- to 30-fold higher concentrations of bacteria to generate the same number of nodules. The distribution frequencies of nodules at different positions along the primary root were very similar for the mutants and parent, indicating that reduced nodulation by the mutants in dose-response experiments probably reflects reduced efficiency of nodule initiation rather than developmentally delayed nodule initiation. The number of bacteria that firmly adsorbed to the host root surface during several hours of incubation was 5- to 20-fold greater for the parent than the mutants. The mutants were also somewhat less effective than their parent as competitors in root adsorption assays. It appears that motility and chemotaxis are quantitatively important traits that facilitate the initial contact and adsorption of symbiotic rhizobia to the host root surface, increase the efficiency of nodule initiation, and increase the rate of infection development.

/pdf/role-of-motility-and-chemotaxis-in-efficiency-of-nodulation-2izz7ci19i.pdf

Role of Motility and Chemotaxis in Efficiency of Nodulation by Rhizobium meliloti.

The lipopolysaccharide (LPS) from a Rhizobium phaseoli mutant, CE109, was isolated and compared with that of its wild-type parent, CE3. A previous report has shown that the mutant is defective in infection thread development, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that it has an altered LPS (K. D. Noel, K. A. VandenBosch, and B. Kulpaca, J. Bacteriol. 168:1392-1462, 1986). Mild acid hydrolysis of the CE3 LPS released a polysaccharide and an oligosaccharide, PS1 and PS2, respectively. Mild acid hydrolysis of CE109 LPS released only an oligosaccharide. Chemical and immunochemical analyses showed that CE3-PS1 is the antigenic O chain of this strain and that CE109 LPS does not contain any of the major sugar components of CE3-PS1. CE109 oligosaccharide was identical in composition to CE3-PS2. The lipid A's from both strains were very similar in composition, with only minor quantitative variations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of CE3 and CE109 LPSs showed that CE3 LPS separated into two bands, LPS I and LPS II, while CE109 had two bands which migrated to positions similar to that of LPS II. Immunoblotting with anti-CE3 antiserum showed that LPS I contains the antigenic O chain of CE3, PS1. Anti-CE109 antiserum interacted strongly with both CE109 LPS bands and CE3 LPS II and interacted weakly with CE3 LPS I. Mild-acid hydrolysis of CE3 LPS I, extracted from the polyacrylamide gel, showed that it contained both PS1 and PS2. The results in this report showed that CE109 LPS consists of only the lipid A core and is missing the antigenic O chain.

/pdf/characterization-of-the-lipopolysaccharide-from-a-rhizobium-1dtlgsao6g.pdf

Characterization of the lipopolysaccharide from a Rhizobium phaseoli mutant that is defective in infection thread development.

Theory and Practice in Experimental Bacteriology.

Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium are collectively termed rhizobia. They share the ability to penetrate legume roots and elicit morphological responses that le...

The Genetic and Biochemical Basis for Nodulation of Legumes by Rhizobia

Bacteria adsorbed in low numbers to alfalfa or clover root surfaces were counted after incubation of seedlings in mineral solution with very dilute inocula (less than 105 bacteria per ml) of an antibiotic-resistant strain under defined conditions. After specified washing, bacteria which remained adsorbed to roots were selectively quantitated by culturing the roots embedded in yeast extract-mannitol-antibiotic agar and counting the microcolonies along the root surface; the range was from about 1 bacterium per root (estimated as the most probable number) to 50 bacteria per cm of root length (by direct counting). This simple procedure can be used with any pair of small-rooted plant and antibiotic-resistant bacterium, requires bacterial concentrations comparable to those frequently found in soils, and yields macroscopic localization and distribution data for adsorbed bacteria over the root surface. The number of adsorbed bacteria was proportional to the size of the inoculum. One of every four Rhizobium meliloti cells adsorbed in very low numbers to alfalfa roots resulted in the formation of a nodule. Overall adsorption of various symbiotic and nonsymbiotic bacterial strains to alfalfa and clover roots did not reflect the specificities of these legumes for their respective microsymbionts, R. meliloti and R. trifolii.

Quantitation of adsorption of rhizobia in low numbers to small legume roots.

Early (4 h) adsorption of Rhizobium meliloti L5-30 in low numbers to alfalfa roots in mineral solution was examined for competition with other bacterial strains. All tested competitor strains decreased the adsorption of L5-30 by extents which depended on the strain and its concentration. The decrease of adsorption by R. meliloti competitors (all of them infective in alfalfa) was nearly complete at saturation (97 to 99% decrease). All other heterologous rhizobia and Agrobacterium tumefaciens at saturating concentrations (106 to 107 per ml) decreased adsorption of L5-30 only partially, less than 60%. The differential effects of homologous and heterologous competitors indicate that initial adsorption of R. meliloti to the root surface of its host occurs in symbiont-specific as well as nonspecific modes and suggest the existence of binding sites on roots which are highly selective for the specific microsymbiont in the presence of other heterologous bacteria even in very unfavorable (less than 10−4) symbiont-competitor concentration ratios.

Host-Symbiont Specificity Expressed during Early Adsorption of Rhizobium meliloti to the Root Surface of Alfalfa.

bacterium, requires bacterial concentrations comparable tothose frequently foundinsoils, andyields macroscopic localization anddistribution datafor adsorbed bacteria overtherootsurface. Thenumberofadsorbed bacteria was proportional tothesizeofthe inoculum. OneofeveryfourRhizobium meliloti cells adsorbed invery lownumberstoalfalfa rootsresulted in theformation ofanodule. Overall adsorption ofvarious symbiotic andnonsymbiotic bacterial strains toalfalfa andclover rootsdidnotreflect thespecificities ofthese legumes fortheir respective microsymbionts, R.meliloti andR.trifolii. Interactions between soil bacteria andplant roots leadtoa variety ofassociative phenomena, amongwhichsymbiotic aswell asparasitic orpathogenic relationships areimportant toplant life andproductivity. Manyofthese associations are characterized bybacterial penetration andinvasion ofthe roottissues. Inthose cases, anearly stage inthedevelopmental pathway isattraction andadsorption ofsoil bacteria totherootsurface. Onesuchinstance isfoundduring the preinfection phaseinthenitrogen-fixing symbiosis between rhizobia andleguminous roots. Inthis process, soil rhizobia areinitially attracted tothedeveloping roots, colonize the rhizosphere, attach totheroothairs, andcausetheir deformation andcurling. Subsequent rhizobial infection ofthe roottissues viapenetration oftheroothairs leads tothe

Gustavo Caetano Anollés

Papers

Quantitation of adsorption of rhizobia in low numbers to small legume roots.

Host-Symbiont Specificity Expressed during Early Adsorption of Rhizobium meliloti to the Root Surface of Alfalfa.

Quantitation ofAdsorption ofRhizobia inLow NumberstoSmall LegumeRoots