Siderophore production byBradyrhizobium spp. strains nodulating groundnut

Abstract

Eighteen Bradyrhizobium spp. strains, four Rhizobium spp. strains and one Azorhizobium caulinodans strain were grown under Fe limitation and assayed for siderophore production. It was further assessed if Fe accumulation in two groundnut cultivars was influenced by inoculant strain or nitrate fertilisation. Growth of Bradyrhizobium spp. strains nodulating groundnut was slow with mean generation times from 11-24 h. All strains, except MAR 967, showed a reduced growth rate when deprived of Fe; none of the strains showed starvation at 1/tM Fe. In the CAS (chrome azurol S)-agar assay, all strains, which formed colonies, produced siderophores as visualised by orange halos around the colonies on blue plates. Bradyrhizobium strains produced much smaller halos than the reference Rhizobium meliloti strain. In the CAS-supernatant assay, all strains, except MAR 967, gave positive responses (measured as absorbance at 630 nm) when supernatants of Fe-depleted cultures were assayed with CAS-indicator complex in comparison with Fe-supplemented cultures. Responses of all four Rhizobium spp. strains were large, while responses of all Bradyrhizobium strains, except B. japonicum MAR 1491 (USDA 110), were small and mostly insignificant. A small response, i.e. a low Fe-scavenging ability, implies either the production of small quantities of siderophores or the production of low affinity siderophores. Among the Bradyrhizobium strains, MAR 1574 and MAR 1587 gave the largest responses taken over the two assays. Fe accumulation in groundnut cultivar Falcon was seven times larger than in cultivar Natal Common. No correlation was found between the quantity of nodule tissue and Fe accumulation, making it unlikely that bacteroids are involved in Fe acquisition by groundnuts. Nitrate-fertilised plants accumulated significantly more Fe, suggesting involvement of nitrate reductase in Fe assimilation in groundnut. The two most successful Fe-scavenging Bradyrhizobium spp. strains were also the most effective in nodulating groundnut, the reverse also being true. Strain MAR 967, with the lowest Fe requirement, produced the largest nodule dry weight. These data indicate that improved Fe scavenging properties and/or reduced Fe requirement improve rhizospheric growth and with that nodulation effectiveness.

RFLP analysis of the rRNA operon of a Brazilian collection of bradyrhizobial strains from 33 legume species

“…van Rossum et al, 1995;Moreira, 1991Moreira, , 2000Moreira et al, 1993;Boddey & Hungria, 1997; Doignon-Bourcier et al, 1999;Chen et al, 2002). However, the diversity observed among bradyrhizobial strains is usually not reflected in 16S rRNA gene sequence analysis; therefore, few Bradyrhizobium species have been described so far.…”

“…Partial or complete 16S rRNA gene sequences, which have also been used extensively for studying the phylogeny of rhizobia (e.g. Young et al, 1991;Oyaizu et al, 1992;Yanagi & Yamasato, 1993;van Rossum et al, 1995;Urtz & Elkan, 1996;Moreira et al, 1998;Vinuesa et al, 1998;Wang et al, 1999;Chen et al, 2000; JaraboLorenzo et al, 2000), have contributed to the recent descriptions of four new genera and several rhizobial species. However, there are reports showing that, despite a high level of diversity in morphological, physiological and genetic properties, diversity is low in the 16S rRNA gene sequences of strains of Bradyrhizobium investigated so far (So et al, 1994, Urtz & Elkan, 1996Molouba et al, 1999;Vinuesa et al, 1998;Chen et al, 2000;van Berkum & Fuhrmann, 2000;Willems et al, 2001;Qian et al, 2003).…”

Bradyrhizobium spp. (TGx) isolates nodulating the new soybean cultivars in Africa are diverse and distinct from bradyrhizobia that nodulate North American soybeans.

“…Glycine max Nyankpala, Ghana McMillin (1996) and the two GC-rich primers, 5h-GAC GAC GAC GAC GAC-3h and 5h-TGC GCC GAA TTA TGC GG-3h, were used by Van Rossum et al (1995) to differentiate Bradyrhizobium isolates. The PCR reactions were carried out as for the 16S rRNA gene, except that the primer annealing temperature was 40 mC, the primer concentration was 1n0 µM and the number of cycles was 40.…”

Bradyrhizobium canariense sp. nov., an acid-tolerant endosymbiont that nodulates endemic genistoid legumes (Papilionoideae: Genisteae) from the Canary Islands, along with Bradyrhizobium japonicum bv. genistearum, Bradyrhizobium genospecies alpha and Bradyrhizobium genospecies beta

“…+, >95 % of isolates were positive; 2, >95 % of the isolates were negative; +/2, variable; NR, not reported. 1995; Yao et al, 2002), and it is well documented that phenotypic and genotypic clustering of Bradyrhizobium strains correlates poorly (Dupuy et al, 1994;So et al, 1994;van Rossum et al, 1995;Zhang et al, 1999).…”