Characterization and diversity of rhizobia nodulating selected tree legumes in Ghana.

Emmanuel Yaw Boakye,Seth Kofi Akyea Danso,Samuel Kwame offei,Innocent Yao Dotse Lawson

doi:10.1007/s13199-016-0383-1

Abstract

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 fast-growers or by isolates classified as slow- or very slow-growers with 54 % of all the 200 isolates belonging to fast- or 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 and this may explain why many introduced and/or indigenous trees are able to form nodules with indigenous rhizobia in this region.

Highlights

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
The Alajo series with a clay loam texture had the highest values for all parameters measured, while the Hatso series, a sandy soil was lowest in total N, organic carbon and available P, whereas the Toje series, a sandy clay loam was lowest in pH and cation exchange capacity (CEC) values
The results indicate that, native rhizobia capable of nodulating each of the five tree legumes are present in all the three soils used for the study with the native rhizobia population ranging from 22 per gram of soil in Alajo soil to 5200 per gram of soil in either Hatso or Toje soil

Summary

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. 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)

Objectives

Methods

Results

Conclusion