Soybean interactions with soil microbes, agronomical and molecular aspects

Abstract

Soybean, Glycine max (L.) Merrill, is one of the most important food crops in the world. High soybean yields require large amounts of N fertilizers, which are expensive and can cause environmental problems. The industrial fixation of nitrogen accounts for about 50% of fossil fuel usage in agriculture. In contrast, biological fixation of N 2 is a low-cost source of N for soybean cropping through the symbiotic association between the plant and soil bacteria belonging to the genera Bradyrhizobium and Sinorhizobium, which are collectively called "soybean rhizobia". In general, symbiotic nitrogen fixation in crop legumes not only reduces fertilizer costs but also improves soil fertility through crop rotation and intercropping. Biological nitrogen fixation is due to symbioses between leguminous plants and species of Rhizobium bacteria. Replacing this natural N source by synthetic N fertilizers would cost around 10 billion dollars annually. Moreover, legume seed and foliage have a higher protein content than that of non-legumes, and this makes them desirable protein crops. There is a wide knowledge of the industrial elaboration and use of commercial soybean inoculants based on bradyrhizobia strains. At present, the technology to prepare different types of inoculants, either solid or liquid, is sufficiently developed to meet market requirements, although further research and investments are still required to improve the symbiotic efficacy of rhizobial inoculants. Inoculation of soybeans under field conditions has been successful in the USA, Brazil and Argentina, which are the world leaders in soybean cultivation in terms of acreage and grain yields. There are, however, limitations to a wider use of rhizobial inoculants: the size of indigenous soil rhizobial populations can prevent the successful use of inoculants in some particular areas. For example, many Chinese soils contain more than 10 5 soybean rhizobia per gram of soil, which imposes a serious barrier for nodule occupancy by the soybean rhizobia used as an inoculant. The use of inoculants based on soil bacteria other than rhizobia has also increased in the last decades. An example is the genus Azospirillum, which can be used for its capacity to increase plant growth and seed yields through different mechanisms, such as the production of plant hormones and the increase in phosphate uptake by roots. In addition, co-inoculation with Azospirillum and rhizobia enhances nodulation and nitrogen fixation. Although less developed, it is expected that inoculants based on mycorrhizal fungi will also play a relevant role in sustainable agriculture and forestry. In spite of any possible limitations, the use of inoculants appears compulsory in a frame of sustainable agriculture, which seeks to increase crop yields and nutrient-use efficiency while reducing the environmental costs associated with agriculture intensification. This review also summarizes some of the most relevant genetic aspects of soybean rhizobia in relation to their symbiosis with soybeans. They can be listed a...

Organically Grown Soybean Production in the USA: Constraints and Management of Pathogens and Insect Pests

“…Plant-growth promoting rhizobacteria (PGPR) are characterized by their ability to enhance plant health by increasing nutrient update, fixing nitrogen, suppressing disease, stimulating phytohormone production, and eliciting defense responses [40][41][42]. Overall, PGPR aid in keeping the plant healthy, allowing the plant to better withstand abiotic and biotic stresses.…”

The Production and Potential of Biofertilizers to Improve Crop Yields

“…In 2000, the area of legumes treated with commercial biofertilizers was more than 40 million hectares annually (Phillips 2004), with about half of this figure being represented by soybean production (Catroux et al 2001). The commercial production of rhizobial inoculants has thus been in place for a number of decades, partially replacing the need for mineral N fertilizers on legume crops in many countries (Rodríguez-Navarro et al 2011). In some regions and some crops (e.g., peas in North America), the proportion of inoculated acreage can reach almost 100 % of the total acreage for the particular crop in that region, whereas for most crops, the figures are below 50 % of the planted acreage.…”

Identification and Diagnostics of Plant-Symbiotic and Phytopathogenic Bacteria

“…This genus contains closely-related species, and some of them are prospective for soybean biofertilizer production [29]. Identification and characterization of this genus are necessary to reveal and select most efficient symbiotic bacteria, create most productive plant-symbiont combination and thus increase crop capacity of fields.…”