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Measuring the spermosphere colonizing capacity (spermosphere competence) of bacterial inoculants
Abstract:
Spermosphere establishment by bacteria which were coated onto seeds was studied using soybean seeds treated with four bacterial strains at levels of log10 1 to 4 colony-forming units (cfu) per seed planted in a field soil mix, and incubated 48 h. Each strain at every inoculum level developed spermosphere population densities of log10 4 to 8 cfu/seed, demonstrating an average multiplication of log10 3 cfu/seed. An alternative method was developed to differentially rank bacteria for spermosphere colonizing capacity, based upon incorporation of bacteria into a soil and monitoring the resulting spermosphere population densities around noninoculated seeds after 4 days at 14 degrees C. Fifty-seven bacterial strains which were isolated from soybean roots or from water samples, including Pseudomonas putida, P. putida biovar B, P. fluorescens, Serratia liquefaciens, Enterobacter aerogenes, and Bacillus spp. were tested in the spermosphere colonization assay. Average spermosphere population densities for the 57 strains ranged from 0 to log10 7.0 cfu/seed. Strains of a given taxon demonstrated marked diversity with ranges from 0 to log10 6.0 cfu/seed for Bacillus spp. and from log10 1.4 to 7.0 cfu/seed for Pseudomonas putida. The relative ranking of representative strains was consistent in repeating experiments. The potential usefulness of the assay for efforts to develop competitive bacterial inoculants for crop seeds is discussed.
Insights
Bacterial strains effectively establish spermosphere populations on soybean seeds, multiplying significantly. A new assay effectively ranks bacterial colonization capacity, aiding the development of competitive crop seed inoculants.
Area of Science:
- Agricultural Microbiology
- Plant-Microbe Interactions
- Soil Science
Background:
- The spermosphere, the microenvironment around a seed, is crucial for plant-microbe interactions.
- Understanding bacterial colonization of the spermosphere is key for developing effective microbial inoculants.
Purpose of the Study:
- To investigate spermosphere establishment by bacteria coated onto soybean seeds.
- To develop and validate an assay for ranking bacterial colonization capacity in the spermosphere.
Main Methods:
- Soybean seeds were treated with four bacterial strains at varying inoculum levels (log10 1-4 colony-forming units/seed) and incubated.
- An alternative method involved incorporating bacteria into soil and monitoring populations around non-inoculated seeds after 4 days at 14°C.
- Fifty-seven bacterial strains, including Pseudomonas, Serratia, Enterobacter, and Bacillus, were tested in the colonization assay.
Main Results:
- All tested strains established spermosphere populations, with densities ranging from log10 4 to 8 colony-forming units/seed, showing an average multiplication of log10 3.
- The new assay demonstrated a wide range of spermosphere population densities (0 to log10 7.0 colony-forming units/seed) among the 57 strains.
- Significant diversity in colonization capacity was observed within taxa, such as Bacillus spp. (0 to log10 6.0) and Pseudomonas putida (log10 1.4 to 7.0).
Conclusions:
- Bacterial strains exhibit varying capacities for spermosphere colonization on soybean seeds.
- The developed assay is a reliable tool for assessing and ranking bacterial colonization potential.
- This assay can aid in selecting competitive bacterial strains for developing enhanced crop seed inoculants.