Genetic diversity of Rhizobia and Rhizobacteria from Soybean [Glycine max (L) Merr.]: Implication for the Commercial Production and Application to Enhance Soybean Production under Low Input Agriculture in Ethiopia
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Date
2017-06
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Addis Ababa Universty
Abstract
Soybean [Glycine max (L) Merr.] is a nutritious crop used as food, feed and a raw material for
manufacturing various products. Soybean improves soil fertility due to its association with
symbiotic bacterial groups known as Bradyrhizobium, Rhizobium/Sinorhizobium and
Agrobacterium species. It is also associated with diverse plant growth promoting rhizobacteria
(PGPR) that enhance its health, growth and productivity. Soybean is widely grown in the
lowlands regions of Ethiopia with average yield of about 2.0 tons ha-1 compared to 2.70 tons ha-1
of world average. The low yield of soybean in the country is predominantly attributed to low soil
fertility associated with the absence of effective indigenous rhizobia that nodulate and fix enough
nitrogen to the host. Attempts to inoculate the crop with exotic rhizobia showed inconsistent and
unsatisfactory results that necessitated the search for effective local rhizobia adapted to
ecological conditions of the country. To this end, 140 soil samples were collected from various
sites of Ethiopia to screen for symbiotically effective soybean rhizobia, and plant growth
promoting rhizobacteria (PGPR). The rhzobial isolates were trapped, authenticated and tested for
their symbiotic effectiveness using three soybean varieties (Cheri, Ethio-Yugoslavia and Jalele)
under greenhouse conditions. The PGPR were screened in vitro for their multiple plant growth promoting traits and potential ecological adaptations. The diversity of the selected rhizobia and
PGPR was studied using their phenotypic (numerical taxonomy), and genotypic characters via
sequence analysis of 16S rRNA (and some other genes of rhizobia). The most effective rhizobia
and the most versatile PGP Achromobacter were inoculated on a soybean cultivar (Jalele) to
evaluate their effect on nodulation, growth and yield of the crop against a standard soybean
inoculant Bradyrhizobium japonicum SBTAL379 under field conditions. The result showed that
only 18 soil samples (13%) induced nodulation on the host variety from which 21 bacterial
isolates were authenticated as soybean rhizobia. The isolates were equally distributed into fast
growing (11) and slow growing (10), and grouped under the genus Rhizobium and
Bradyrhizobium, respectively as classified previously. Based on genetic characters, a fast
growing isolate (SNB 41) was identified as Rhizobium/Agrobacterium sp. whereas three slow
growing isolates (SNB57B, SNB70 and SNB120A) were identified as Bradyrhizobium spp.
Likewise, the representative PGPR isolates were also classified into seven genera; six under
Proteobacteria (Gram negative): Achromobacter, Acinetobacter, Enterobacter, Microbacterium,
Pseudomonas and Stenotrophomonas; and one under the Firmicutes (Gram positive): Bacillus.
The isolates under the genera Pseudomonas and Stenotrophomonas were the most diverse group
among the PGPR. With regard to their plausible ecological adaptations tested under in vitro, the
fast growing soybean rhizobia were more tolerant to pesticides, higher temperature and higher
NaCl concentrations and more versatile to utilize different carbon and nitrogen sources than the
slow growing isolates which were better in their inherent antibiotic resistance (IAR). The
majority of the rhizobacteria were grown at 40oC, 4% NaCl and showed multiple antibiotic and
heavy metal resistance. Some of the soybean rhizobia and rhizobacteria also demonstrated
multiple PGP traits (2 to 9). The data also showed the overall better performance of gram negative rhizobacteria and fast growing rhizobia in terms of the number of PGP traits and
tolerated stresses. The nodulation and symbiotic effectiveness tests of the rhizobia showed that
SNB57B, SNB120A, SNB120C, SNB125A, SNB125B and SNB140 nodulated all the three
soybean varieties with prolific nodulation (54-173 nodules plant-1; 1.76-2.33 mg of nodule dry
weight plant-1) and shoot dry weight (1.10-2.27 g plant-1) showing highly effective symbiosis
(80-100%) in relation to the nitrogen-fertilized control plants under greenhouse experiment. The
isolates showed similar pattern of relatively high nodulation parameters and symbiotic
performance on Jalele and Cheri varieties compared to the Ethio-Yugoslavia variety. The
findings also showed co-inoculation of rhizobia and the PGP Achromobacterium significantly
increased more growth and yield parameters of soybean at Dembi station of Debrezeit
Agricultural Research Center (DDARC) field site with low population of indigenous soybean
rhizobia and where maximum nodule number (168 plant-1) and dry matter (1.96 g plant-1), shoot
dry matter (25 g plant-1) and total nitrogen (4 %), number of pods (114 plant-1) and seeds (214
plant-1) and grain yield (4.01 tons ha-1) were recorded. There were highly significant (p≤0.05)
effects of the rhizobial isolates on most growth, nodulation and yield parameters. Indigenous
soybean rhizobia performed much better than the exotic Bradyrhizobium japonicum SBTAL379
and control treatments under greenhouse and field conditions so that they can be further
validated and recommended as inoculant (together with the PGP bacterium) to improve growth
and productivity of the crop in the country.
Key words: soybean, rhizobia, PGPR, diversity, nodulation, yield
Description
Keywords
Soybean, Rhizobia, PGPR, Diversity, Nodulation, Yield