Aspectos ecológicos e funcionais do gênero Bradyrhizobium associado com cana-de-açúcar

Abstract

Many species of the genus Bradyrhizobium have the ability to associate symbiotically with plants of the Fabaceae family (legumes), benefiting plant growth through the process of biological nitrogen fixation (BNF). However, the genus Bradyrhizobium is also found in other types of environments, often in the free-living conditions, such as in the endosphere and rhizosphere of non-legume plants. In sugarcane, independent cultivation methods have shown that the genus Bradyrhizobium can play an important role in the BNF process. This thesis aimed to better understand the functions and ecology of Bradyrhizobium spp. associated with sugarcane. The thesis contains four research chapters. In the first, a diverse collection of 103 strains of Bradyrhizobium spp. From sugarcane is characterized by means of two molecular fingerprinting techniques, by sequence analysis of the recA housekeeping gene and by morphocultural characteristics. In the second chapter, using a comparative and functional genomics approach, we investigate genetic factors (nif gene clusters) in the recently described strain Bradyrhizobium sacchari, which is atypical for its ability to fix nitrogen in a mutualistic way with various species and leguminous plants, but which also has the ability of free-living BNF. In chapter III, it was studied the potential of using strains of Bradyrhizobium spp. to improve the growth of sugarcane plants. These studies were conducted first under greenhouse conditions and then at field level. Chapter IV deals with a more ecological aspect and seeks to assess how the recent history of soil cultivation (uncultivated, with Brachiaria grass, or with leguminous peanuts and soybeans) influences the bacterial community found in the sugarcane rhizosphere. The results indicated that there is a diversity of bacterial groups of the genus Bradyrhizobium associated with sugarcane that have not been explored yet. Furthermore, joint analysis of BOX and ERIC PCR was able to efficiently group the strains with correspondence between the phenotypic characteristics and phylogeny of the recA gene. The atypical situation was revealed that B. sacchari possesses two complete clusters of genes for BNF, designated as symbiotic (SC) and non-symbiotic (NSC). RT-qPCR analysis showed that the expression level of the two nifH genes (nitrogenase reductase) varies according to environmental conditions. Inoculation tests under greenhouse conditions allowed the identification of three Bradyrhizobium strains that positively affect aspects of plant growth, such as root system size, chlorophyll content and total leaf surface. Field-level trials were inconclusive but indicated that inoculation of Bradyrhizobium strains has the potential to increase plant leaf area during the first few months of development. Furthermore, it was found that previous soil cultivation with peanuts and soybeans increased the abundance of rhizobia and some other genera of growth-promoting bacteria in the sugarcane rhizosphere. Thus, the results presented in this thesis represent an advance in the understanding of the functionality, applicability and ecology of Bradyrhizobium spp. in the cultivation of sugar cane.