Promotion of infection thread formation by substances from Rhizobium

Symbiotic root nodule bacteria isolated from yam bean (Pachyrhizus erosus)

A total of 25 isolates from root nodules of yam bean (Pachyrhizus erosus L. Urban), a tuber-producing leguminous plant, were characterized. All isolates formed effective nodules mainly on lateral roots while edible tubers were developed on the taproot. The root nodules formed were identified as the typical determinate type. By an analysis of the partial sequences of the 16S rRNA gene (approximately 300 bp) of 10 strains which were selected randomly, the isolated root nodule bacteria of yam bean were classified into two different genera, Rhizobium and Bradyrhizobium. Two strains, YB2 (Bradyrhizobium group) and YB4 (Rhizobium group) were selected and used for further analyses. The generation time of each strain was shown to be 22.5 h for strain YB2 and 0.8 h for strain YB4, respectively. Differences between strains YB2 and YB4 were also reflected in the bacteroid state in the symbiosome. Symbiosome in nodule cells for the strain YB4 contained one bacteroid cell in a peribacteroid membrane, whereas a symbiosome for strain YB2 contained several bacteroid cells.

Transfer of the symbiotic plasmid from Rhizobium leguminosarum biovar trifolii to Agrobacterium tumefaciens

This study examined the symbiotic properties of Agrobacterium transconjugants isolated by transferring a Tn5-mob-marked derivative of the 315 kb megaplasmid pRt4Sa from Rhizobium leguminosarum bv. trifolii 4S (wild-type strain) to Agrobacterium tumefaciens A136 as the recipient. The genetic characteristics of the AT4S transconjugant strains were ascertained by random amplified polymorphic DNA (RAPD) analyses and Southern hybridization using Tn5-mob and nod genes as probes. Several of these AT4S transconjugants carrying pRt4Sa were able to nodulate roots of the normal legume host, white clover. In addition, some AT4S transconjugant strains were able to induce nodules on other leguminous plants, including alfalfa and hairy vetch. A characteristic bacteroid differentiation was observed in clover and alfalfa nodules induced by the AT4S-series strains, although nitrogen-fixing activity (acetylene reduction) was not found. Furthermore, strain H1R1, obtained by retracing transfer of the pRt4Sa::Tn5-mob from strain AT4Sa to strain H1 (pRt4Sa cured derivative of 4S), induced Fix(+) nodules on clover roots. These results indicate the evidence that only nod genes can be expressed in the Agrobacterium background.

Nitrogen limitation of chemostat-grown Rhizobium etli elicits higher infection-thread formation in Phaseolus vulgaris

The symbiotic association between rhizobia and legume roots is a complex process involving many steps. An infection thread is a tubular structure of host origin formed during the infection of legume roots by rhizobia. Previous studies with batch cultures have reported that optimal attachment of rhizobia to root hairs coincides with nutrient limitation. In this study, the ability of chemostat-grown, nutrient-limited Rhizobium etli cells to form infection threads with its symbiotic partner Phaseolus vulgaris was investigated. Rhizobia were grown in a chemostat in synthetic media under C- or N-limiting conditions. Infection-thread formation was examined after inoculation of seedlings with a rhizobial cell suspension from each treatment. The number of infection threads was estimated by light microscopy after staining root sections with o-toluidine. Exopolysaccharide (EPS) production was also measured, and the cellular content and electrophoretic pattern of lipopolysaccharide (LPS) determined semiquantitatively. N-limited cells showed a markedly higher infectivity (measured as infection-thread formation) than C-limited cells. With one of the two bean cultivars used, the number of infection threads produced by N-limited cells was higher than that produced by exponentially growing cells in batch cultures. The higher infectivity of N-limited cells was correlated with higher EPS production. Electrophoretic analysis of LPS showed that C- and N-limited cells shared a common profile but the relative concentration of short LPS forms differed.

Phosphoglycerol substituents present on the cyclic beta-1,2-glucans of Rhizobium meliloti 1021 are derived from phosphatidylglycerol

The synthesis of periplasmic cyclic beta-1,2-glucans is a property unique to species of the family Rhizobiaceae. For this reason, it is generally believed that these molecules may play an important role in the plant infection process. In the present study, we determined that the cyclic beta-1,2-glucans produced by Rhizobium meliloti 1021 were predominantly anionic in character and contained both phosphoglycerol and succinic acid substituents. In addition, we demonstrated that phosphatidylglycerol was the source of the phosphoglycerol substituents present on these oligosaccharides and that greater than 60% of the total phospholipid turnover in this organism involved this substitution reaction.

Cyclic glucans produced by Agrobacterium tumefaciens are substituted with sn-1-phosphoglycerol residues

In a previous study (Miller, K.J., Kennedy, E.P. and Reinhold, V.N. (1986) Science 231, 48-51) it was reported that the biosynthesis of periplasmic cyclic beta-1,2-glucans by Agrobacterium tumefaciens is strictly osmoregulated in a pattern closely similar to that found for the membrane-derived oligosaccharides of Escherichia coli (Kennedy, E.P. (1982) Proc. Natl. Acad. Sci. USA 79, 1092-1095). In addition to the well-characterized neutral cyclic glucan, the periplasmic glucans were found to contain an anionic component not previously reported. Biosynthesis of the anionic component is osmotically regulated in a manner indistinguishable from that of the neutral cyclic beta-1,2-glucan. We now find that the anionic component consists of cyclic beta-1,2-glucans substituted with one or more sn-1-phosphoglycerol residues. The presence of sn-1-phosphoglycerol residues represents an additional, striking similarity to the membrane-derived oligosaccharides of E. coli.

Exopolysaccharide-deficient mutants of Rhizobium meliloti that form ineffective nodules

By screening with the fluorescent stain Calcofluor, we have isolated 26 independent transposon Tn5 insertion mutants of Rhizobium meliloti that are deficient in the production of a known extracellular polysaccharide (Exo-). The mutants belonged to six distinct genetic groups based on the ability of their Exo- phenotype to be complemented by different recombinant plasmids from a R. meliloti clone bank. With few exceptions, all of the mutants formed ineffective (non-nitrogen-fixing) nodules on alfalfa. For all but one group, the complementing plasmids restored effective nodulation. These results establish a firm and extensive correlation between the ability of Rhizobium to produce a particular polysaccharide and symbiotic proficiency. The ineffective nodules appeared to contain no bacteroids and to form without shepherds' crooks or infection threads; this symbiotic phenotype matches that described for a set of independently isolated mutants that belong phenotypically and genetically to the group B exopolysaccharide mutants described previously [Finan et al. (1985) Cell 40, 869-877]. Apparently the exopolysaccharide, although not required for nodule formation, is involved in wild-type nodule invasion.

Bacterial attachment as related to cellular recognition in the Rhizobium-legume symbiosis

Bacterial attachment is viewed as a cellular recognition event during the infection of legumes by the nitrogen-fixing symbiont, Rhizobium. Studies on the biochemical basis of selective attachment are reviewed, and suggest that this recognition process is accomplished by specific glycoprotein lectin-polysaccharide interactions on the surfaces of the symbionts. An understanding of host specificity may lead to ways to broaden the host range of nitrogen-fixing symbioses.