In greenhouse trials, 21 strains of slow-growing rhizobia were tested for symbiotic effectiveness and ability to nodulate three varieties of cowpea, Vigna unguiculata L. Walp. in two Ultisol subsoil samples, each adjusted to pH 4.6 and pH 6.0-6.2. The results confirmed that cowpea rhizobia contain a large and perhaps continuous variation in symbiotic tolerance of soil acidity. Some strains comb...

Nodulation in legumes involves the coordination of epidermal infection by rhizobia with cell divisions in the underlying cortex. During nodulation, rhizobia are entrapped within curled root hairs to form an infection pocket. Transcellular tubes called infection threads then develop from the pocket and become colonized by rhizobia. The infection thread grows toward the developing nodule primordi...

Soybean rhizobia were isolated from two soils with different cropping histories from Hubei province in central China. The first, from Honghu county, has been under soybean cultivation for decades. All of the isolates obtained from nodules on soybeans growing in this soil were fast-growing, acid-producing rhizobia. However, slow-growing, alkali-producing isolates were obtained at higher dilution...

Initiation of symbiotic nodules in legumes requires cytokinin signaling, but its mechanism of action is largely unknown. Here, we tested whether the failure to initiate nodules in the Medicago truncatula cytokinin perception mutant cre1 (cytokinin response1) is due to its altered ability to regulate auxin transport, auxin accumulation, and induction of flavonoids. We found that in the cre1 muta...

The inoculation of soybean (Glycine max L.) roots with Bradyrhizobium japonicum produces a regulatory response that inhibits nodulation in the younger regions of the roots. By exposing the soybean roots to live homologous bacteria for only a short period of time, the question of whether or not early interactions of rhizobia with root cells, prior to infection, elicit this regulatory response ha...

Experiments were undertaken to test whether peat-based legume seed inoculants, which are prepared with liquid cultures that have been deliberately diluted, can attain and sustain acceptable numbers of viable rhizobia. Liquid cultures of Rhizobium japonicum and Rhizobium phaseoli were diluted to give 10, 10, or 10 cells per ml, using either deionized water, quarter-strength yeast-mannitol broth,...

The DNA content of bacteroids from 22 different Rhizobium-legume associations was compared to that of the corresponding free living Rhizobium species using laser flow microfluorometry. In all 18 effective associations, the bacteroids had either similar or higher DNA content than the free living rhizobia. Bacteroid populations isolated from effective clover (Trifolium repens) and alfalfa (Medica...

The rhizobium-legume symbiosis has been widely studied as the model of mutualistic evolution and the essential component of sustainable agriculture. Extensive genetic and recent genomic studies have led to the hypothesis that many distinct strategies, regardless of rhizobial phylogeny, contributed to the varied rhizobium-legume symbiosis. We sequenced 26 genomes of Sinorhizobium and Bradyrhizob...

AIMS Our aim was to evaluate the effect of acid and alkaline pH on chickpea rhizobia, and on chickpea-rhizobia symbiosis. METHODS AND RESULTS Forty-seven rhizobia isolates obtained from 12 Portuguese soils were grown at pH 5, 7 and 9. Among these, 26 grew more at pH 5 than at 7, suggesting the existence of acidophiles. All isolates were identified as mesorhizobia by 16S rDNA partial sequence ...