Abstract
Ethiopia remains a net importer of soybean partly due to low average yields which may be improved by inoculation with rhizobia and/or plant growth promoting rhizobacteria (PGPR). The functional characterisation of 231 rhizobacteria isolated from the rhizosphere of soybean grown in 102 soils collected from different pedo-climatic regions of Ethiopia was carried out. Isolates were initially characterised by Gram staining and then functionally for: indole-3-acetic acid production; phosphate solubilisation; growth on a nitrogen-free medium; and, resistance to the pathogenic fungus Fusarium oxysporum. A sub-set of 72 of the best performing isolates were tested in vitro for: production of bioprotectants; polysaccharide degradation; and their relative capacity to maintain growth in response to extremes of: temperature; pH; salinity; antibiotics; pesticides; and, heavy metals. Twenty isolates with the best PGPR potential were identified via 16S rRNA gene sequencing. Seventeen isolates were Gram-negative: Pseudomonas (7); Stenotrophomonas (5); Acinetobacter (3); Enterobacter (1); and Achromobacter (1). Gram-positive types were: Bacillus (2); and, Microbacterium (1). Of the six of the most promising PGPR tested on soybean plants, Achromobacter and Acinetobacter significantly enhanced soybean seed germination, seedling growth and plant vigour index compared to non-inoculated plants. Key words: Soybean, Ethiopia, plant growth promoting rhizobacteria, plant growth promoting rhizobacteria (PGPR), seedling vigour.
Highlights
Soybean (Glycine max L.) is an important source of high protein grain in Ethiopia, having been introduced in the early 1950 s (Shurtleff and Aoyagi, 2009)
Of the 231 soybean rhizobacterial isolates initially screened for their potential PGP traits, 198 isolates produced indole-3-acetic acid (IAA), 30 isolates solubilised bound calcium phosphate, 38 isolates survived on N-free medium and 29 isolates inhibited growth of the fungal pathogen F. oxysporum (Table 1 and Supplementary Table 1)
Of the twenty 16S rRNA gene sequenced isolates, the highest Ca3(PO4)2 Phosphate solubilisation index (PSI) were recorded for SR40 (Acinetobacter sp.), SR99B (Pseudomonas sp.), SR20B (Acinetobacter sp.), SR10A (Acinetobacter sp.) and SR20A (Achromobacter mucicolens) with values varying from 2.8 to 2.2 (Table 2)
Summary
Soybean (Glycine max L.) is an important source of high protein grain in Ethiopia, having been introduced in the early 1950 s (Shurtleff and Aoyagi, 2009). Soybean has mainly been used for processing into baby food, edible oils, common cultural dishes and animal feeds (Abebe et al, 2015).
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