CHARACTERIZATION OF LOCAL ISOLATES OF BRADYRHIZOBIUM SPP. RECOVERED FROM A SOIL UNDER CONVENTIONAL TILLAGE AND A PRISTINE SOIL

BRIGNOLI, DAMIÁN; FRICKEL CRITTO EMILIA; MONGIARDINI ELÍAS JAVIER; CASTELLS CECILIA; PEREZ GIMENEZ JULIETA; LODEIRO ANÍBAL ROBERTO

Chapadmalal

Congreso; XVIII Congreso de la Sociedad Argentina de Microbiología General; 2023

SAMIGE

Soybean crops are routinely inoculated with elite strains of Bradyrhizobium spp. to obtain most of their N-nutrient from atmospheric N2-fixation. However, elite strains are seldom adapted to local edaphic and climatic conditions. Therefore, N2 fixation efficiency may be increased by formulating new inoculants with strains from soybean-nodulating allochthonous populations (SNAPs) obtained from the same soil environment where the soybean crop will be planted. Our hypothesis is that strains adapted to the local conditions might have better symbiotic performance in its environment than the foreign elite strain B. japonicum E109, widely used in inoculants in Argentina.SNAPs were isolated from a soil with more than 10 yr of soybean cultivation (S-soil), and from a pristine soil chosen on the basis of its good edaphic and plant growth-promoting characteristics (P-soil) using soybean as trap-plants. By calculating the most probable number, we estimated the SNAP size in 7,6x103 soybean-nodulating rhizobia g-1 soil in the S-soil and in 6x101 rhizobia g-1 soil in P-soil. DNA fingerprinting coupled to 16S rRNA, atpD, recA, and glnII sequencing showed that the isolates were not similar to the type and reference strains, and that several of them were identical to each other. The SNAP isolates were classified in three groups, namely B. diazoefficiens, B. elkanii, and B. japonicum. None of the strains solubilized phosphate or produced siderophores. Only B. elkanii strains produced indoleacetic acid in concentrations ranging 0.5-2.0 g ml‒1 by comparison to Azospirillum argentinense Az39, which produced 5.0 g ml‒1.To assess symbiotic performance, we measured total nodule dry weight (NDW), total N contents in shoots by the Kjeldahl method (SN), and chlorophyll contents in leaves using a portable chlorophyllometer (ChL), including B. japonicum E109 as reference strain and uninoculated controls. All three variables were significantly correlated. The two best isolates belonged to B. diazoefficiens, with relative values of 1.6 for NDW, 1.4 for SN, and 1.1 for ChL with respect to E109 as 1.0. In addition, all strains surpassed the negative control in all variables. These results constitute the first step to select local adapted strains for the development of new inoculants for soybean.