Bacillus cereus UW85 inoculation effects on growth, nodulation, and N accumulation in grain legumes: Controlled-environment studies

Abstract

Treating seed with Bacillus cereus strain UW85 stimulates nodulation in soybean, but the underlying mechanisms of this stimulation are poorly understood. In this study we assessed the effects of inoculation on nodulation, nitrogenase activity and dry matter partitioning in soybean (cv. Maple Ridge), common bean (cv. OAC Rico) and pea (cv. Express) under controlled, gnotobiotic conditions. Plants were grown for 34 d under controlled-environment conditions without a mineral N source, at low two levels of (brady)rhizobia inoculation. Soybean and common bean were grown at a single temperature regime and pea was grown at two temperature regimes. Each trial consisted of five treatments (noninoculated control, low (brady)rhizobia inoculation, low (brady)rhizobia plus UW85 inoculation, high (brady)rhizobia inoculation, and high (brady)rhizobia plus UW85 inoculation) with six plants as replicates per treatment in a completely randomized design. Inoculation of soybean with UW85 increased growth of roots, shoots, and nodules across both levels of bradyrhizobia inoculation and increased plant N accumulation by 12%. In oculation with UW85 also increased whole-plant nodulation (nodules plant-1) by 16%, but there were no effects on specific nodulation [nodules g-1 root dry weight (DW)], individual nodule DW, nitrogenase activity (μmol H2 g-1 nodule DW h-1) or N2 fixation efficiency (g plant-N g-1 nodule DW). Inoculation of pea with UW85 at the low temperature regime increased whole-plant nodulation by 19%, but had no positive effects on any other growth parameters. Inoculation of common bean with UW85 had no positive effects on any aspects of growth, nodulation or nitrogenase activity. Our data indicate that under gnotobiotic conditions, UW85 inoculation increases nodulation in soybean indirectly by increasing root growth and not stimulating the nodulation process per se. The bacterium had little to no positive effects on pea and common bean symbioses. Key words: Bacillus cereus UW85, Glycine max, growth-promotion, nodulation, Phaseolus vulgaris, Pisum sativum