Study of the potential use of selenium accumulating rhizobacteria for biófortificatión of wheat grown in volcanic soils

Abstract

Selenium (Se) is an essential micronutrient for humans with antioxidant properties. Selenium deficiency in the human diet affected around 15% of world population increasing the probability of ocurrence of diverse disease such as cancer, thyroid dysfunction and the reduction of immune functions among others. In Chile, cereal crops production (wheat, oat and barley) is one of the most important sources of nutrition for the population. This crop frequently has suboptimal levels of Se due to diverse factors associated with plant species, soil type, fertilization and the nature of selenium compounds available. In this context, the rhizosphere harbors a wide variety of bacterial species which play important role in the biogeochemical cycle of nutrients and micronutrients such as Se. Microbiological transformations of Se (methylation, oxidation, and reduction) have been suggested as biotechnological tools for bioremediation of Se-contaminated soils. However, there are not studies focused to the potential application of Se-utilizing bacteria in agriculture to increase the content of Se available in the rhizosphere. For this reason, the general aim of this Doctoral thesis was to study the bioaccumulation of Se by native bacteria present in the rhizosphere of cereals grown in Chilean acid soils (Andisols), in order to develop a biotechnological tool for Se-biofortification of wheat. Firstly, we present a general vision of the problematic of Se in human health and plant by a critical review of worldwide research in Chapter II . From a biofortification perspective, we described that Se-accumulating rhizobacteria can be used for Se enrich plants. In this context, the Chapter III shown the occurrence of Se tolerant bacteria in the rhizosphere and its contributions to enhance Se content in plants. The results described that selenobacteria selected; belong to the genera Stenotrophomonas, Bacillus, Enterobacter and Pseudomonas, similar genera to the ones previously reported in seleniferous soils. The Selenobacteria have a great ability to tolerate and accumulate Se intra- and extracellular in micro- and nanospherical Se0 deposits. Furthermore, effectively the inoculation of wheat plantlets with selenobacteria inocula showed increased Se content in plant tissues. In addition, the results here obtained suggest that selenobacteria inocula can be used as a biotechnological tool for Se biofortification in plant. In fact, the effectiveness to enhance Se content in grain by the co-inoculation of selenobacteria strains and mycorrhizal arbuscular fungus (Glomus claroideum) demonstrated a great potential of these rhizosphere microorganisms for biofortification of wheat and derivates foods (i.e. Se enriched flour). This microbial association enhanced the Se content of grains in 23.5% compared with non-mycorrizal plants, associated with a higher biodiversity on the rhizosphere (Chapter IV). Thus, our results showed that Se biosynthesized by selenobacteria can be translocated inside the plants toward the grain. This results support the hypothesis that selenobacteria have a great potential for Se-biofortification of cereals (Chapter V). In summary, Se-biofertilizer based on selenobacteria isolated from volcanic soils are more effective than Se inorganic source for uptake and translocation in wheat plants and representing a promising strategy for Se biofortification to be used at large-scale for intensive cereal crops.