Tripartite symbiosis – Soybean, Bradyrhizobium, Arbuscular Mycorrhizal Fungi: effect of formononetin and diversity of Bradyrhizobium in soils from different Brazilian regions

Abstract

This thesis is divided in two parts. The first, contains general introduction and theoretical background. In the second part, two articles are presented. The first article refers to the effect of formononetin associated with phosphorus fertilization on mycorrhizal colonization, nitrogen fixation and soybean productivity. The goal of the present study was to evaluate the effect of the isoflavone formononetin on soybean in combination with three levels of phosphorus (P) fertilization. Two experiments were conducted in 2010/2011 and 2012/13, consisting in a 3 x 4 factorial scheme (three levels of phosphorus: 0, 60 and 120 kg hectare (ha)-1 P2O5 and four doses of formononetin: 0, 25, 50 and 100 g ha-1) was used with five and four replicates, respectively. Formononetin had a positive effect on mycorrhizal colonization only at lower levels of P addition (0 and 60 kg ha-1). Application of formononetin together with 60 kg ha-1 P2O5 increased soybean productivity to values equivalent to those observed when the highest level of P was applied. This indicates that seed treatment with formononetin doses higher than 50 g ha-1 decreases the need for phosphorus fertilization by up to 50% of the recommended level of fertilizer. The second paper refers to the study of the genetic diversity of Bradyrhizobium strains that nodulate soybeans in soils from different Brazilian regions. The Bradyrhizobium genus includes species distributed throughout different geographic regions. However, 16S rRNA gene diversity is generally low. In the present study, we analyzed 46 Bradyrhizobium strains from soils collected in different regions of Brazil (Midwest, Northeast, Southeast, and South), from areas of the implementation of the previous experiments similar this work, performed in other Institutions participating of the public notice 69/2009, by analyzing the 16S rRNA gene and five housekeeping genes (atpD, gyrB, dnaK, recA, and rpoB) and their symbiotic efficiency with soybean plants grown under axenic conditions in Leonard jars. In our study, the phylogenetic analysis of housekeeping genes revealed greater diversity compared to the analysis of the 16S rRNA gene, indicating the possibility of novel species, some of which exhibited high symbiotic efficiency with soybean plants. Our results emphasize the great diversity of Bradyrhizobium genus strains native to Brazilian soil.