Recognition and Infection of Clover Root Hairs by Rhizobium Trifolii. In: Evans HJ, Bottomley PJ, Newton WE, editors. Nitrogen fixation research progress. Dordrecht: Springer Netherlands; 1985. pp. 239-45. [DOI: 10.1007/978-94-009-5175-4_32]

Rhizobium meliloti host range nodH gene determines production of an alfalfa-specific extracellular signal

The Rhizobium meliloti nodH gene is involved in determining host range specificity. By comparison with the wild-type strain, NodH mutants exhibit a change in host specificity. That is, although NodH mutants lose the ability to elicit root hair curling (Hac-), infection threads (Inf-), and nodule meristem formation (Nod-) on the homologous host alfalfa, they gain the ability to be Hac+ Inf+ Nod+ on a nonhomologous host such as common vetch. Using root hair deformation (Had) bioassays on alfalfa and vetch, we have demonstrated that sterile supernatant solutions of R. meliloti cultures, in which the nod genes had been induced by the plant flavone luteolin, contained symbiotic extracellular signals. The wild-type strain produced at least one Had signal active on alfalfa (HadA). The NodH- mutants did not produce this signal but produced at least one factor active on vetch (HadV). Mutants altered in the common nodABC genes produced neither of the Had factors. This result suggests that the nodABC operon determines the production of a common symbiotic factor which is modified by the NodH product into an alfalfa-specific signal. An absolute correlation was observed between the specificity of the symbiotic behavior of rhizobial cells and the Had specificity of their sterile filtrates. This indicates that the R. meliloti nodH gene determines host range by helping to mediate the production of a specific extracellular signal.

Expression of Nodulation Genes in Rhizobium leguminosarum biovar trifolii Is Affected by Low pH and by Ca and Al Ions

Early stages in the infection of leguminous plants by Rhizobium spp. are restricted at low pH and are further influenced by the presence of Ca and Al ions. In the experiments reported here, transcriptional and translational fusions of the Escherichia coli lacZ gene to Rhizobium leguminosarum biovar trifolii nodulation ( nod ) genes were used to investigate the effects of pH and of Ca and Al ions on nod gene expression. The regulatory nodD gene in R. leguminosarum biovar trifolii was constitutively expressed at a range of pH levels between 4.8 and 6.5, and expression was not affected by the addition of 22.5 μM Al or 1,250 μM Ca. Induction of expression of nodA, nodF, and region II nodulation genes in the presence of 5 × 10 −7 M 7,4′-dihydroxyflavone was restricted at a pH of <5.7 and was extremely poor at pH 4.8. Induction of nodA expression was further restricted by 22.5 μM Al over a range of pH levels but was increased in the presence of Ca. The addition of Ca, however, only slightly alleviated the Al-mediated inhibition of nodA induction. Induction of expression of nodA was equally sensitive to low pH in three strains of R. leguminosarum biovar trifolii (ANU845, ANU815, and ANU1184), which exhibited contrasting growth abilities in solution culture at a pH of <5.0. Aluminum, however, differentially affected the induction of nodA in these three strains, with the most Al-tolerant strain for growth being the most Al-sensitive strain for nod gene induction. Poor induction of expression of nodulation genes in R. leguminosarum biovar trifolii was considered to be an important factor contributing to the acid-sensitive step of legume root infection.

Lectin-enhanced accumulation of manganese-limited Rhizobium leguminosarum cells on pea root hair tips

The ability of Rhizobium leguminosarum 248 to attach to developing Pisum sativum root hairs was investigated during various phases of bacterial growth in yeast extract-mannitol medium. Direct cell counting revealed that growth of the rhizobia transiently stopped three successive times during batch culture in yeast extract-mannitol medium. These interruptions of growth, as well as the simultaneous autoagglutination of the bacteria, appeared to be caused by manganese limitation. Rhizobia harvested during the transient phases of growth inhibition appeared to have a better attachment ability than did exponentially growing rhizobia. The attachment characteristics of these manganese-limited rhizobia were compared with those of carbon-limited rhizobia (G. Smit, J. W. Kijne, and B. J. J. Lugtenberg, J. Bacteriol. 168:821-827, 1986, and J. Bacteriol. 169:4294-4301, 1987). In contrast to the attachment of carbon-limited cells, accumulation of manganese-limited rhizobia (cap formation) was already in full progress after 10 min of incubation; significantly delayed by 3-O-methyl-D-glucose, a pea lectin haptenic monosaccharide; partially resistant to sodium chloride; and partially resistant to pretreatment of the bacteria with cellulase. Binding of single bacteria to the root hair tips was not inhibited by 3-O-methyl-D-glucose. Whereas attachment of single R. leguminosarum cells to the surface of pea root hair tips seemed to be similar for both carbon- and manganese-limited cells, the subsequent accumulation of manganese-limited rhizobia at the root hair tips is apparently accelerated by pea lectin molecules. Moreover, spot inoculation tests with rhizobia grown under various culture conditions indicated that differences in attachment between manganese- and carbon-limited R. leguminosarum cells are correlated with a significant difference in infectivity in that manganese-limited rhizobia, in contrast to carbon-limited rhizobia, are infective. This growth-medium-dependent behavior offers and explanation for the seemingly conflicting data on the involvement of host plant lectins in attachment of rhizobia to root hairs of leguminous plants. Sym plasmid-borne genes do not play a role in manganese-limitation-induced attachment of R. leguminosarum.

Correlation between infection by Rhizobium leguminosarum and lectin on the surface of Pisum sativum L. roots

The lectin on the surface of 4- and 5-dold pea roots was located by the use of indirect immunofluorescence. Specific antibodies raised in rabbits against pea seed isolectin 2, which crossreact with root lectins, were used as primary immunoglobulins and were visualized with fluorescein- or tetramethylrhodamine-isothiocyanate-labeled goat antirabbit immunoglobulin G. Lectin was observed on the tips of newly formed, growing root hairs and on epidermal cells located just below the young hairs. On both types of cells, lectin was concentrated in dense small patches rather than uniformly distributed. Lectin-positive young hairs were grouped opposite the (proto)xylematic poles. Older but still-elongating root hairs presented only traces of lectin or none at all. A similar pattern of distribution was found in different pea cultivars, as well as in a supernodulating and a non-nodulating pea mutant. Growth in a nitrate concentration which inhibits nodulation did not affect lectin distribution on the surface of pea roots of this age. We tested whether or not the root zones where lectin was observed were susceptible to infection by Rhizobium leguminosarum. When low inoculum doses (consisting of less than 10(6) bacteria·ml(-1)) were placed next to lectin-positive epidermal cells and on newly formed root hairs, nodules on the primary roots were formed in 73% and 90% of the plants, respectively. Only a few plants showed primary root nodulation when the inoculum was placed on the root zone where lectin was scarce or absent. These results show that lectin is present at those sites on the pea root that are susceptible to infection by the bacterial symbiont.