Variability in Bradyrhizobium japonicum and B. elkanii Seven Years after Introduction of both the Exotic Microsymbiont and the Soybean Host in a Cerrados Soil

Abstract

The plasticity of rhizobial genomes is far greater than previously thought, with complex genomic recombination events that may be accelerated by the often stressful environmental conditions of the tropics. This study aimed at evaluating changes in soybean rhizobia due to adaptation to inhospitable environmental conditions (high temperatures, drought, and acid soils) in the Brazilian Cerrados. Both the host plant and combinations of four strains of soybean Bradyrhizobium were introduced in an uncropped soil devoid of rhizobia capable of nodulating soybean. After the third year, seeds were not reinoculated. Two hundred and sixty-three isolates were obtained from nodules of field-grown soybean after the seventh year, and their morphological, physiological, serological, and symbiotic properties determined, followed by genetic analysis of conserved and symbiotic genes. B. japonicum strain CPAC 15 (same serogroup as USDA 123) was characterized as having high saprophytic capacity and competitiveness and by the seventh year represented up to 70% of the cultivable population, in contrast to the poor survival and competitiveness of B. japonicum strain CPAC 7 (same serogroup as CB 1809). In general, adapted strains had increased mucoidy, and up to 43% of the isolates showed no serological reaction. High variability, presumably resulting from the adaptation to the harsh environmental conditions, was verified in rep-PCR (polymerase chain reaction) profiles, being lower in strain CPAC 15, intermediate in B. elkanii, and higher in CPAC 7. Restriction fragment length polymorphism (RFLP)-PCR types of the 16S rDNA corresponded to the following: one type for B. elkanii species, two for B. japonicum, associated to CPAC 15 and CPAC 7, and unknown combinations of profiles. However, when nodC sequences and RFLP-PCR of the nifH region data were considered, only two clusters were observed having full congruence with B. japonicum and B. elkanii species. Combining the results, variability was such that even within a genetically more stable group (such as that of CPAC 15), only 6.4% of the isolates showed high similarity to the inoculant strain, whereas none was similar to CPAC 7. The genetic variability in our study seems to result from a variety and combination of events including strain dispersion, genomic recombination, and horizontal gene transfer. Furthermore, the genetic variability appears to be mainly associated with adaptation, saprophytic capacity, and competitiveness, and not with symbiotic effectiveness, as the similarity of symbiotic genes was higher than that of conserved regions of the DNA.

Soybean [Glycine max (L.) Merrill] rhizobial diversity in Brazilian oxisols under various soil, cropping, and inoculation managements

“…In addition, horizontal gene transfer between one inoculant strain of Mesorhizobium loti and indigenous strains of Mesorhizobium has been demonstrated under field conditions in New Zealand Sullivan et al , 1996. Increasing evidences of high rates of horizontal gene transfer in the soils of the Cerrados have also been reported (Galli-Terasawa et al 2003;Batista et al 2006) and might help to explain the high genetic diversity observed in several isolates of this study that were positioned in clusters showing very low genetic similarity in comparison to the inoculant strains (Fig. 1).…”

“…After some years of soybean cropping, a high percentage of strains belonging to unknown serogroups or showing highly dissimilar DNA profiles have been detected in Brazil (Santos et al 1999;Ferreira et al 2000;GalliTerasawa et al 2003;Hungria et al 2006;Batista et al 2006). For example, in the South Region, Ferreira et al (2000) reported that 38% of the strains belonged to unknown serogroups after 17 years of soybean cropping, and in the Cerrados that percentage was about 50% after some years of cropping (Galli-Terasawa et al 2003;Mendes et al 2004).…”

“…However, dispersal of inoculant strains did occur (Ferreira and Hungria 2002;Mendes et al 2004) and Andrade and Hungria (2002) demonstrated that viable cells on the soybean seed coat can be transported to new cropping areas. After introduction through inoculation and some years of adaptation to the soil conditions, a high level of diversity in morphological, physiological, genetic, and symbiotic properties has been reported especially under the stressful conditions of the Cerrados (Hungria et al , 1998Nishi et al 1996;Boddey and Hungria 1997;Santos et al 1999;Hungria and Vargas 2000;Galli-Terasawa et al 2003;Batista et al 2006). In the USA, Streeter (1994) has also noted substantial "genetic adjustment" during the period of adaptation, leading to wide genetic diversity in field populations.…”

“…In previous studies, we have shown that a level of similarity of 70% of the rep-PCR profiles analyzed in the Bionumerics program with a tolerance of 5% is adequate for the use in studies about rhizobial diversity in Brazilian soils (Grange and Hungria 2004;Alberton et al 2006;Kaschuk et al 2006a,b;Batista et al 2006); therefore, profiles showing similarity higher than 70% were also considered to be similar in this study.…”

Diversidade morfológica de rizóbios isolados de caupi cultivado em solos do Estado do Rio Grande do Norte

“…A produção de muco altera a permeabilidade das células, tornando as estirpes mais resistentes a fatores bióticos de competição no solo, como a presença de microrganismos produtores de antibióticos. Batista et al (2007) Zhang et al (2007) obtiveram isolados de caupi de diferentes regiões da China e observaram, na coleção, por análises moleculares do gene ribossomal 16S, isolados com similaridade a Bradyrhizobium, Rhizobium e Sinorhizobium. Os autores identificaram, ainda, isolados com similaridade a Bradyrhizobium, mas que diferiram das espécies atualmente descritas no gênero, sinalizando para a possibilidade de pertencerem a uma espécie ainda não caracterizada.…”