Soil Microbial Ecology - Soil Molecular Ecology Laboratory

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E X E R C I S E
6
SOIL MICROBIAL ASSOCIATION
2. RHIZOBIA
OBJECTIVE:
Examine root nodulation, isolate rhizobium bacteria, and inoculate new plants with and
without nitrogen source.
INTRODUCTION
Bacteria of the genera Rhizobium and Bradyrhizobium are capable of inducing the formation
of specialized structures called root nodules on the roots of many leguminous plants.
The legumes can be subdivided into what are known as cross inoculation groups. These
are groups of legumes which can be nodulated by the same rhizobial strain. There are 20
cross-inoculation groups; however, in practice only six of these receive much attention.
The genus Rhizobium is currently divided into species based on the legume crossinoculation
group which the particular strain is able to nodulate. The Rhizobium species
and the cross-inoculation groups which they nodulate are as follows: Rhizobium meliloti,
alfalfa group; R. trifolii, clover group; R. leguminosarum, pea group; R. phaseoli, bean
group, R. lupini, lupine group; B. japonicum, soybean group, and the “cowpea
miscellany,” the cowpea group.
The following exercise introduces several aspects of the rhizobia-legume symbiosis.