Abstract
The biological nitrogen fixation constitutes a strategy to accelerate soil reclamation and the symbiotic systems Rhizobium-legume is the major N2-fixing in which the enzyme carboxymethyl cellulase plays a key role. As many rhizobia species are cellulase negative, the association with cellulolytic bacteria can be a strategy for the recovery of degraded ecosystems. It has been hypothesized that the sharing of resources should mostly be prevalent among phylogenetically and metabolically different species. Accordingly, twenty-seven actinobacteria isolates from Actinobacteria phyla and twenty-six rhizobia isolates from Proteobacteria phyla were selected from the bacterial collection of the Laboratory of Environmental Microbiology of the Federal University of Ceará. The presence of cellulolytic activity was observed for the rhizobia isolates at 28 °C and for actinobacteria isolates at 28, 39, 41, 43 and 45 °C. Rhizobia isolates deficient in cellulase and actinobacteria isolates with enzymatic activity detected at higher temperature were selected and characterized. The antagonism between isolates of two groups was tested and the pairs antagonistic were eliminated. The cross-feeding test between actinobacteria and rhizobia isolates was realized in a chemically defined medium containing carboxymethyl-cellulose as the only carbon and energy source. Growth of rhizobia strains in 50% of the pairwise indicated that the cellulose hydrolyzed by actinobacteria was used as substrate for the growth of the rhizobia. The Bradyrhizobium strain R10 associated with Streptomyces strains A09 and A18 and Nocardia A11 are promissory inoculants for recovery of semi-arid regions.
Highlights
Semiarid environments are characterized by high temperatures, low water availability, high salinity and low nutrient content in the soil which may be attributed to low nitrogen fixing activity (Divito & Sadras, 2014).Brazil has one of the largest semi-arid areas in the world and about 94% of this region is characterized as having moderate to high susceptibility to desertification resulting from the climatic variations and human activities (Vieira et al, 2015).Cowpea (Vigna unguiculata (L) Walp) is a typical legume of the Brazilian northeast because of its adaptability to severe climatic conditions
The actinobacteria were isolated in casein dextrose agar (CDA) (Clark, 1965) and the isolates from the two groups were reactivated in the same liquid culture media and inoculated in Petri dishes to verify the purity
The actinobacteria isolates A06, A09, A11, A12, A13 and A18 were selected for posterior cross feeding test with rhizobia isolates cellulase negative
Summary
Cowpea (Vigna unguiculata (L) Walp) is a typical legume of the Brazilian northeast because of its adaptability to severe climatic conditions The root of this plant is nodulated by a bacterial group generically known as rhizobia (Zilli, Valisheski, Freire Filho, Neves, & Rumjanek, 2004), classified in the Proteobacteria phyla and that act as the primary symbiotic fixer of nitrogen (Seishiro, Motomi, & Wataru, 2013). Nitrogen fixation is energetically an expensive process, dependent on the available energy from plant residues that contain a high percentage of cellulose. For this reason, the production of cellulolytic enzymes has been considered an indirect mechanism to promote plant growth (Gupta, Parihar, Ahirwar, Snehi, & Singh, 2015). The cellulolytic activity of rhizobia is very low and there are few species that can fix nitrogen (N) and use cellulose as their main energy and carbon source (Jiménez-Zurdo, Mateos, Dazzo, & Martínez-Molina, 1996)
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