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Eleven extra-slow-growing strains were isolated from nodules of the relict legume Vavilovia formosa growing in North Ossetia (Caucasus) and Armenia. All isolates formed a single rrs cluster together with the type strain Tardiphaga robiniae LMG 26467(T), while the sequencing of the 16S-23S rDNA intergenic region (ITS) and housekeeping genes glnII, atpD, dnaK, gyrB, recA and rpoB divided them into three groups. North Ossetian isolates (in contrast to the Armenian ones) were clustered separately from the type strain LMG 26467(T). However, all isolates were classified as T. robiniae because the DNA-DNA relatedness between them and the type strain LMG 26467(T) was 69.6 % minimum. Two symbiosis-related genes (nodM and nodT) were amplified in all isolated Tardiphaga strains. It was shown that the nodM gene phylogeny is similar to that of ITS and housekeeping genes. The presence of the other symbiosis-related genes in described Tardiphaga strains, which is recently described genus of rhizobia, as well as their ability to form nodules on any plants are under investigation.

The ability of wild indigenous legumes to form root nodules capable of biological nitrogen (N 2) fixation has rarely been demonstrated for species in natural ecosystems in large parts of Europe. In ...

Background and aims Legumes of the genus Lessertia have recently been introduced to Australia in an attempt to increase the range of forage species available in Australian farming systems capable of dealing with a changing climate. This study assessed the diversity and the nodulation ability of a collection of Lessertia root nodule bacteria isolated from different agro-climatic areas of the Eastern and Western Capes of South Africa.

Nineteen strains of root nodule bacteria were grown under various iron regimes (0.1, 1.0 and 20 microM added iron) and tested for catechol and hydroxamate siderophore production and the excretion of malate and citrate. The growth response of the strains to iron differed markedly. For 12 strains (Bradyrhizobium strains NC92B and 32H1, B. japonicum USDA110 and CB1809, B. lupini WU8, cowpea Rhizobium NGR234, Rhizobium meliloti strains U45 and CC169, Rhizobium leguminosarum bv viciae WU235 and Rhizobium leguminosarum bv trifolii strains TA1, T1 and WU95) the mean generation time showed no variation with the 200-fold increase in iron concentration. In contrast, in Bradyrhizobium strains NC921, CB756 and TAL1000, B. japonicum strain 61A76 and R. leguminosarum bv viciae MNF300 there was a 2-5 fold decrease in growth rate at low iron. R. meliloti strains WSM419 and WSM540 showed decreased growth at high iron. All strains of root nodule bacteria tested gave a positive CAS (chrome azurol S) assay for siderophore production. No catechol-type siderophores were found in any strain, and only R. leguminosarum bv trifolii T1 and bv viciae WU235 produced hydroxamate under low iron (0.1 and 1.0 microM added iron). Malate was excreted by all strains grown under all iron regimes. Citrate was excreted by B. japonicum USDA110 and B. lupini WU8 in all iron concentrations, while Bradyrhizobium TAL1000, R. leguminosarum bv viciae MNF300 and B. japonicum 61A76 only produced citrate under low iron (0.1 and/or 1.0 microM added iron) during the stationary phase of growth.

Significant correlations exist between the ability of root-nodule bacteria to fix molecular nitrogen and some of their properties (accumulation of riboflavin, cobalamine, ATP, cell pyrolysis, effect on plant vegetation characteristics) which can be used for quickly assessing the activity of the process in individual strains of rhizobia.

Rhizobium trifolii organisms associated with eleven species ofTrifolium were isolated and studied by means of antigen-antibody agglutination and immune diffusion techniques. Ten serologic types were differentiated from the eleven species of nativeTrifolium.

Eighteen strains of Rhizobium lupini were shown to form effectively nitrogen-fixing root nodules in Lotus uliginosus whose rhizobia appeared culturally similar to R. lupini. Six strains formed effective or (in one case) semi-effective nodules in Ornithopus sativus.

The failure of pea (Pisum sativum) to grow in ‘goradu’ soil of Central Gujarat without artificial inoculation of the seed with the specific root nodule bacteria has been investigated. The study of a large number of soil fungi isolated from ‘goradu’ soils failed to show that they had any selective antibiotic activity against the pea rhizobia. It has been explained that the high sand content of this soil and the lower thermal death point of the pea rhizobia are the probable causes for the failure. The high sand content does not give any protection to the pea rhizobia against the high temperatures of the summer months with the result that their number gets so reduced and are not able to nodulate plants sufficiently.

Trichloro-, dichloro- and monochloroacetic and α-dichloro-, α-monochloro-and β-monochloropropionic acids as sodium salts in 0.10–1.0mm concentration accelerated the growth of a strain of Rhizobium leguminosarum in a liquid synthetic medium with biotin as sole accessory growth factor. The strongest effect was shown by β-monochloropropionate. Concentrations of 10mm and above were inhibitory. α-Monochlorobutyrate did not accelerate growth.

Growth of Lotus nodule bacteria (Strain LC 296) was temporarily suppressed in a yeast-extract mineral-salts medium containing 2,4-DB at 50 and 100 µg/ml. However, the 2,4-DB concentration needed to completely suppress the growth of the bacteria appeared to be above 500 µg/ml. When added during the logarithmic phase of growth 2,4-DB at 50 and 100 µg/ml did not inhibit the growth of the cells, so that the growth-inhibiting effect of the herbicide apparently was directed primarily against lag phase cells. Pre-incubation of rhizobia in a medium containing 2,4-DB at concentrations up to 10 µg/ml did not affect the capacity of the bacteria to effectively nodulate their host plant. Attempts to induce the rhizobia to utilize 2,4-DB as a sole carbon source failed. Nevertheless, when theLotus nodule bacteria (LC 296) were grown in a medium containing 2,4-DB-1-C14, C14O2 was released. Similar results were obtained with other strains of the Lotus rhizobia and other species of Rhizobium. C14O2 evolution was also demonstrated when radioactive 2,4-DB was added to both intact (excised) and crushed trefoil nodules. The results indicate that the nodule bacteria are capable of degrading 2,4-DB; although the mechanism is still obscure it does not appear to be via cleavage of the ether link.

THE root nodule bacteria of leguminous plants (Rhizobium spp.) have repeatedly been observed to survive as long as 15–16 yr. in various media1,2. Bisset3 has explained this remarkable longevity as due to formation of endospores in a complicated life-cycle.

IN a recent paper, Chen, Nicol and Thornton1 present results from which they conclude that “the production as a result of infection of soluble substances affecting the growth of the bacteria affords an explanation of those differences in nodule growth that determine the effectiveness or ineffectiveness of the different strains of bacteria as regards nitrogen fixation within the host”.

MR. VINCENT, who courteously sent me a copy of this letter before publication, suggests that the juice from roots bearing effective nodules is more stimulating to the growth of Rhizobium as a result of effective association. This is in agreement with our views. We state in our paper1 (p. 489) that there is evidence of “an increased stimulating effect from juices of roots bearing effective nodules, possibly connected with the products of nitrogen fixation”.

IT has been suggested by Thimann1, Link, Wilcox and Link2, and Link3 that β-indole-acetic acid (heteroauxin) is the agent partially or entirely responsible for the formation of nodules on the roots of leguminous plants. Thimann1 advances the hypothesis that the root nodule bacteria convert tryptophane into heteroauxin, (a) by stimulating elongation of a lateral root and (b) by arresting root elongation through continued auxin production, resulting in the formation of a nodule. If this condition exists, there may be differences in the rate of formation and in total amounts of heteroauxin derived from tryptophane, between efficient and inefficient strains of Rhizobia. (Efficient: Rhizobia which infect the host plant and benefit the latter by the symbiosis; inefficient: Rhizobia which infect the host plant but fail to aid in fixing atmospheric nitrogen.) Nodules produced by efficient strains are generally characterized as being large and usually located on the crown of the tap-root, indicating early invasion, while inefficient strains produce very small nodules which are distributed over the secondary root system, indicative of relatively late invasion.