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Characterization and diversity of rhizobia nodulating selected tree legumes in Ghana
M
Characterization and diversity of rhizobia nodulating selected tree legumes in Ghana
Emmanuel Yaw Boakye 0
Innocent Yao Dotse Lawson 0
Seth Kofi Akyea Danso 0
Samuel Kwame offei 0
0 College of Agriculture and Consumer Sciences, Department of Soil Science, University of Ghana Legon , P. O. Box 245, Legon , Ghana
1 Emmanuel Yaw Boakye
The study was conducted to assess the characteristics and diversity of the rhizobia that nodulate some prominent tree legumes in three soils of Ghana. Five introduced and/or indigenous tree legumes were initially assessed for nodulation in three Ghanaian soils. After 12 weeks of growth in nursery pots the 200 rhizobial strains isolated from their nodules were characterized culturally, metabolically and phenotypically. Sixty of these isolates were selected randomly and their genotypic characteristics determined using PCR-RFLP of 16S rRNA and intergenic spacer (ITS) genes. Each tree legume was nodulated by isolates classified as fast or very fastgrowers or by isolates classified as slow- or very slowgrowers with 54 % of all the 200 isolates belonging to fastor very fast-growers. Morphologically, eighty five percent of the colonies formed on yeast extract mannitol agar were wet and gummy while 70 % were acid tolerant, i.e. they were able to grow at a pH of 3.5. Combined restriction of the 16S rRNA genes of the 60 rhizobial isolates with five restriction enzymes clearly distinguished seven different clusters at 80 % similarity level. The majority of A. lebbeck isolates were distinct from those of the Acacias and L. leucocephala. The M. thonningii isolates were related to L. leucocephala isolates. Simple PCR of the ITS DNA provided several distinct band sizes indicating great variation among the isolates and restriction of the ITS with three different enzymes did not yield many further differences. Molecular techniques revealed a great diversity among the rhizobia that nodulate tree legumes in the tropics
Characterization; Diversity; Rhizobium; Tree legumes
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and this may explain why many introduced and/or indigenous
trees are able to form nodules with indigenous rhizobia in this
region.
1 Introduction
Rhizobia are usually defined as nitrogen-fixing soil bacteria
capable of inducing the formation of root or stem nodules on
leguminous plants in which atmospheric nitrogen is reduced
to ammonia for the benefit of the plant. Although the majority
of legumes form symbiosis with members of genera that
belong to the class Alphaproteobacteria (Allorhizobium,
Azorhizobium, Blastobacter, Bradyrhizobium, Devosia,
Ensifer, Mesorhizobium, Methylobacterium, Rhizobium and
Sinorhizobium), some legumes, such as those in the large
genus Mimosa, are nodulated predominately by members of the
class Betaproteobacteria in the genera Burkholderia and
Cupriavidus
(Gyaneshwar et al. 2011)
. However, a recent
report by
Bontemps et al. (2015)
indicated that the endemic
Mexican mimosas unlike their Brazillian counterparts were
nodulated predominantly by Alphaproteobacteria from the
genera Rhizobium and Ensifer.
In the last few years, many studies investigating rhizobia
isolated from tree legumes in Kenya and Sudan have revealed
considerable phenotypic and genotypic diversity among
strains, and several distinct groups have been identified and
novel species described
(Zhang et al. 1991; Odee et al. 1997,
2002; Nick et al. 1999; McInroy et al. 1999)
. These studies
concluded that there is a large heterogeneity among the strains
(Crow et al. 1981; De Lajudie et al. 1994; Dupuy et al. 1994;
Jarvis 1983; Moreira et al. 1998; Zhang et al. 1991)
. This
indicates that trees can form nodules and fix nitrogen with
several different groups of rhizobia
(Crow et al. 1981; Jarvis
1983; Lindstrom et al. 1983; Padmonabhan et al. 1990)
.
Of late, the assessment of diversity within rhizobial natural
populations in various regions of the world has received
increasing attention
(Amann et al. 1995; Batzli et al. 1992;
Brunel et al. 1996; Cartwright et al. 1995)
. Many attempts
have been made to determine the actual composition and
characteristics of indigenous strains isolated from different
cultivated legumes
(Chen et al. 1991; Crow et al. 1981)
and also
from less explored plants
(Felsenstein 1985)
. The
development of molecular genetic methods and the availability of
sensitive and accurate PCR- based fingerprinting methods
(Galtier et al. 1996; Gibson 1980; Gurtler 1993)
has enabled
the differentiation of closely related bacterial strains and the
detection of high rhizobial diversity
(Gurtler and Stanisich
1996; Jarvis 1983; Jarvis et al. 1982)
.
In Ghana, with the exception of studies on cowpea rhizobia
(Fening 1999)
there is little knowledge of the diversity of
rhizobia that nodulate other legumes especially tree legumes.
The aim of the present study was to assess the phenotypic and
genotypic characteristics of the rhizobia including their
diversity that nodulate some indigenous and introduced
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