The inoculation of seeds with nitrogen-fixing and plant-growth promoting bacteria is a well-established agricultural practice that has been increasingly adopted worldwide, decreasing costs and environmental impacts of food production. Most of the globally commercialized inoculants are for the soybean crop, and a method for recovery of Bradyrhizobium cells from inoculated soybean seeds for subsequent counting has been adopted by several laboratories of South America, especially to investigate the bacterial survival on seeds treated with pesticides. However, the use of inoculants containing Azospirillum brasilense in cereal crops has exponentially increased, requiring investigation about the recovery and counting of cells from inoculated seeds. We first verified that the method used for recovery and counting of viable cells of Bradyrhizobium from soybean seeds was not applicable for maize seeds inoculated with A. brasilense. We then modified several steps of the method, aiming at succeeding in recovering Azospirillum viable cells. The main limitation was identified in the nature of the seed tegument, dry and poor in nutrients, resulting in A. brasilense cell aggregation. Pre-hydration of seeds for 2 h in sterile distilled water, followed by shaking for 30 min in sterile distilled water with Tween 80 allowed proper counting of A. brasilense cells recovered from maize seeds. The method was successfully applied to count Azospirillum cells recovered from pre-inoculated maize seeds, and to estimate the impact of seed treatment with pesticides on cell survival.
The use of inoculants carrying diazotrophic and other plant growth–promoting bacteria plays an essential role in the Brazilian agriculture, with a growing use of microorganism-based bioproducts. However, in the last few years, some farmers have multiplied microorganisms in the farm, known as “on farm” production, including inoculants of Bradyrhizobium spp. for soybean (Glycine max L. Merrill.) and Azospirillum brasilense for corn (Zea mays L.) or co-inoculation in soybean. The objective was to assess the microbiological quality of such inoculants concerning the target microorganisms and contaminants. In the laboratory, 18 samples taken in five states were serial diluted and spread on culture media for obtaining pure and morphologically distinct colonies of bacteria, totaling 85 isolates. Molecular analysis based on partial sequencing of the 16S rRNA gene revealed 25 genera of which 44% harbor species potentially pathogenic to humans; only one of the isolates was identified as Azospirillum brasilense, whereas no isolate was identified as Bradyrhizobium. Among 34 isolates belonging to genera harboring species potentially pathogenic to humans, 12 had no resistance to antibiotics, six presented intrinsic resistance, and 18 presented non-intrinsic resistance to at least one antibiotic. One of the samples analyzed with a shotgun-based metagenomics approach to check for the microbial diversity showed several genera of microorganisms, mainly Acetobacter (~ 32% of sequences) but not the target microorganism. The samples of inoculants produced on farm were highly contaminated with non-target microorganisms, some of them carrying multiple resistances to antibiotics.
Farmers sometimes sow soybean (Glycine max (L.) Merrill) in dry soil in the expectation of rain in the short time. However, the forecast may not confirm, letting the inoculated seeds in the dry soil indefinitely. We assessed the survival of inoculated Bradyrhizobium and physiological quality of soybean seeds sown in dry soil. In the first experiment, irrigation was applied with 2 h, 1, 4, 11, 18, or 21 days after sowing; in the second experiment, sowing was carried out 2 h, 1, 5, 12, 14, or 20 days before irrigation. Each time represented a treatment in a completely randomized design. Bradyrhizobium cells dropped from ~8-9 × 104 colony forming units per seed soon after inoculation to -60% at 2 h after sowing in dry soil, and decreased to close to zero with time in both experiments. Although there was no effect on germination (59% and 81% in the first and second experiments, respectively), the exposure to dry soil reduced the emergence speed index from 19.5 (2 h) to 12.0 (21 days) in the first experiment and from 37.8 (2 h) to 13.8 (21 days) in the second. In the first experiment, the number of abnormal seedlings increased from 7% (2 h) to 24% (21 days); in the second, cotyledons showed cracks, which increased from 1% (2 h) to ~50% (≥ 5 days). Sowing in dry soil negatively affects not only the inoculated Bradyrhizobium, but also the physiological quality of soybean seeds.
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