Response of soybean (Glycine Max CV. Williams) to co-inoculation with bradyrhizobium and azospirillum

Abstract

Azospirillum brasilense is a PGPR with a substantial potential as biofertiliser for leguminous and non-leguminous plants. Agriculturally important crops, especially sugar cane, have been often inoculated with A. brasilense producing increase in growth parameters and yield in different soils and climatic regions. Furthermore, sugar cane inoculated with A. brasilense has been grown in intercropping with legumes like beans and soybean. In field experiments of intercropping of sugarcane inoculated with A. brasilense 8-I and soybean, carried out in Cuban soils, an increase in most of the growth parameters of sugar cane have been observed, but it is not known the effects produced by Azospirillum inoculation on the other plant of the intercropping. This work analyzes the variations in plant and nodule development, nitrogenase activity and colonization of roots in soybean inoculated with A. brasilense and Bradyrhizobium japonicum. Soybean seeds were inoculated with A. brasilensis 8-I and B. japonicum USDA 110 and let grown under controlled environmental conditions for 5 weeks. A. brasilense inoculation did not produced changes in the height and biomass of aerial part of the plant or in nitrogenase activity. However increases in root and nodule biomass were detected. The root colonization by Azospirillum was studied using light and electron microscopy and immunocytochemistry. A. brasilense colonized root surface by adhesion to mucilaginous material and afterwards the bacteria take the advantage of the break caused by the emergence of a secondary root to enter in the intercellular spaces of plant root parenchyma and between epidermal root cells, allowing soybean root colonization by Azospirillum. Root morphology change as a consequence of A. brasilense colonization, producing a matrix between two layers of parenchyma root cells. This matrix facilitates bacteria multiplication and the anaerobic environment to express the nitrogenase activity. Inmunolocalization with the specific antibody made against A. brasilense 8-I demonstrated his presence in the break caused by the emergence of a secondary root, in the intercellular spaces and in the intercellular matrix. These results suggested that A. brasilense 8-I inoculation could be suitable candidate as biofertilizer and/or inoculant of leguminous with agricultural purposes.