Isolation and Characterization of Plant Growth Promoting Rhizosphere Bacteria and Arbuscular Mycorrhizal Fungi Associated with Coffee Rhizosphere from Qellem Wallaga, Ethiopia

Abstract

Coffee is one of the most consumed and exported crop for income in the agriculture-based country like Ethiopia. However, the use of chemical fertilizers are not encouraged for coffee production due its cost, availability and environmental concerns. The uses of biofertilizers such as PGPR and AMF could serve as an alternative for enhancement of plant growth and sustainable agricultural production in eco-friendly ways. The purpose of this study was to isolate and characterize the PGPR and AMF associated with coffee rhizosphere. Rhizosphere soil samples and fine fibrous roots were collected from four different coffee variety of Coffea arabica species: 74110, Black Coffee (Buna Gurracha), Mana Sibu and Non-Shading coffee (Buna Aduu). The samples were collected from two different cultivation system, monoculture and agroforestry of Gawo Kebe District, Qellem Wallaga Zone, Ethiopia. The soil samples and fine coffee roots were utilized for isolation of rhizobacteria and determination of AMF root colonization potential. The isolates were tested for phosphate solubilisation ability, IAA production and antagonism against coffee wilt disease (Gibberella xylarioides) under in-vitro condition. From the total of five composite coffee rhizosphere soil samples, thirty-five bacteria (sixteen from monoculture and nineteen from agroforestry) were isolated. From the total rhizobacterial isolates, 42.9% were Gram positive while 57.1% of them were Gram negative bacteria. Based on their morphological and biochemical characteristics the rhizobacterial isolates were identified as genus Bacillus, Pseudomonas, Enterobacter, Corynebacterium, Citrobacter, Serratia, Brevibacterium, Micrococcus, Gordonia, Erwinia, Klebsiella and Burkholderia. The in-vitro screening of isolates showed that 40% exhibited phosphate solubilisation and 48.6% showed antagonism against Gibberella xylarioides. However, all the isolates failed to produce IAA without supplement of precursor Lysogeny-tryptophan. AMF spore density ranging from 1.66 to 3.68 g-1 of soil were recorded. The highest spore density was recorded from coffee rhizosphere soil of agroforestry. All of the coffee plant roots were colonized with AMF structures (hyphae, arbuscule and vesicle) with variable percentage. Total root length colonization was between 87.6% to 95% for agroforestry while monoculture was 61.9% to 81.5%. Generally, most of the coffee rhizobacterial isolates showed plant growth promotion ability. Therefore, the use of PGPR and AMF as bioinoculant should be the future direction for agricultural productivity improvement of coffee.