Artificial selection of ecosystems for improvement soybean inoculants

BRIGNOLI, DAMIÁN; MONGIARDINI, E.J.; PÉREZ GIMÉNEZ, JULIETA; LODEIRO, A.R.

Los Cocos

Congreso; XVII Congreso Argentino de Microbiología General; 2022

Sociedad Argentina de Microbiología General (SAMIGE)

Bradyrhizobium spp. are widely used as inoculants for soybean cultivation to profit from its symbiotic N2-fixation capacity. However, the efficiency of inoculants often is low due to the competition exerted by the resident soybean-nodulating rhizobia in the soil. This might be due to the inoculation of all soybean crops with one or a few elite strains, which could be a wrong strategy when soybean-nodulating populations adapted to local conditions are present. Instead, the possibility of increasing N2 fixation by harnessing the capacity of these soil rhizobial populations using novel strategies that conceive the inoculant as part of a larger microbiological complex that incorporate plant growth-promoting microorganisms (PGPM) could be explored.We started an artificial ecosystem selection with the aim of establishing new soil bacteria consortia. To this end, we selected bacteria from two soils: a soil under inoculated soybean cultivation (S-soil), and a pristine soil, chosen on the basis of its good edaphic and plant growth-promoting characteristics (P-soil). Our hypothesis is that the good properties of P-soil were, at least in part, modeled by its microbiota. Therefore, isolates from this soil might contribute to improve the properties of S-soil and at the same time, the N2-fixing performance of the soybean-nodulating rhizobia present there.Isolation of microorganisms was carried out by seeding aliquots of serial dilutions of both soil extract on Yeast Extract Manitol medium, Tryptic Soy Agar medium and Sabourad Agar medium at 28°C and 37°C. PGPM properties such as phosphate solubilization, iron chelation and IAA production were analyzed, and we observed PGPM activity in 8 isolates. 16S rRNA sequencing was performed to ascertain whether dangerous species were present in our sample. From 36 isolates, we determined 6 genera in S-soil (Bradyrhizobium, Bacillus, Cellulosimicrobium, Herminiimonas, Microbacterium and Oerskovia) and 6 genera in P-soil (Bacillus, Ensifer, Microbacterium, Pseudomonas, Brevibacterium and Bradyrhizobium). Furthermore, isolates of Microbacterium and Cellulosimicrobium genera were discarded for further developments due to the risk associated with them. Likewise, soybean-nodulating rhizobia were isolated with trap-plants, and the population size in S-soil was estimated in 7,6x103 rhizobia g-1 soil by the MPN method. DNA fingerprinting and 16S rRNA sequencing consistently shown that most of the isolates were B. elkanii, while a few were B. japonicum.We are starting to combine rhizobial isolates with PGPM clusters from both soils with the aim of improve N2-fixation traits such as nodulation, nodule mass, and shoot/root ratio as a first step to select consortia for the development of new-generation inoculants.