Competition Problem

Rhizobium and Legume Root Nodulation
Leguminous Plants
The Discovery of Symbiotic Nitrogen Fixation
Rhizobium Classification
Cultural Characteristics
Genetics
Flavonoids Induce Nodualation
Novel Approaches to Extend Root Nodulation
Ecology
Infection
Lectins
Structure of the Nodule
Function of the Nodule
Nitrogenase Hydrogenase Interrelationship
Stem Nodulating Legumes
Factors Affecting Nodulation
Temperature and Light
Hydrogen Ion Concentration
Mineral Nutrition

Competition Problem

	Competition Problem Because legumes have been grown in soil for many years repeatedly, it is logical to expect a plethora of rhizobial strains living saprophytically in soil waiting for occupying sites on the root system to produce nodules that are efficient or inefficient in N₂ fixation. In many soils the inefficient strains dominate leading to stiff competition when a superior efficient strain is introduced in bulk into the soil as an inoculant.
If soils have fewer native strains, the chances are bright for the superior introduced strain to occupy space on the root system leading to improvement in N₂ fixation and grain yield in the legume under cultivation. The reverse is true if the soil has multitude of native inefficient strains which begin to compete and dislodge the introduced strain for root occupancy. This situation is commonly referred to as the competition problem, more serious .with the promiscuous Rhizobium spp. (cowpea miscellany) inhabiting tropical soils. It is always said that the competition problem is the number one limiting factor in maximizing yields of legitmes. A solution to this problem has not been easily forthcoming but there have been several leads to the issue under consideration and they are outlined below:
1. Field experiments have shown that in soybean (Glycine max) and red clover (Trifolium subterraneum) cultivation, massive inoculation with a superior strain many times over a period of several years (nearly 1000 times the. soil population of native rhizobia) may help to dislodge the native strains. 2. Bulk inoculation of soil rather than seed inoculation with superior rhizobia offers better dispersion in the entire root zone thereby offering greater chances for root occupancy by the introduced strain.
3. A strategy to develop mutants of rhizobia to alter the competitive ability of a strain has been attempted but so far has yielded negative results. However, some information has been gained regarding the mutated locus providing a lead to further, investigations. 4. By the use of reporter genes inserted into a Rhizobium sp., the introduced strain in soil can be monitored. Some examples are the insertion of luciferase gene (luc) into R. metiloti and R. leguminosarum by trifolii rendering the strain bioluminescent, the construction of lac Z fusions in Rhizobium sp. facilitating detection of the lac Z gene products, namely the enzyme-B-galactosidase on X-gal plates [X-gal is a chromogenic substrate (5 bromo-4-chloro-3-indolyl-B-D-galactosidase)] and the construction of gus A fusions (for expression of B-glucoronidase) and estimating the tagged strains by simple colorimetry.
	5. Using genetically engineered strains for better nodulation and yield In commercial Rhizobium inoculants. Some examples of success in this direction to improve competitiveness of a strain and produce better yield in the field are: (a) the insertion of the trifolitoxin gene from the inefficient strain T 24 of R. leguminosarum by trifolii into an effective strain so that the potent antirhizobial compound trifoliotoxin can minimize the numbers of native rhizobia,

(b) by insertion of additional copies of nif A and dct ABD genes in R. melitoli strain RMBPC-2, (c) the construction of R. leguminosarum and R. meliloti strains by incorporating the Bacillus thuringiensis sub-sp tenebrionis endotoxin gene (Cry III) so as to protect the nodule from nodule eating larvae of the insect Sitona, (d) the construction of strains of Rhizobium with the ability to oxidize the H₂ produced in the N₂ fixation reaction and funnel or recycle these wasteful electrons to the productive main reaction of converting N₂ into NH₃, as exemplified by the success of this venture in the case of soybean inoculated with H₂ positive strains of Badyrhizobium japonicum, and (e) by cloning the genes for antibiotic production and resistance into efficient rhizobia to impart competitive advantage for nodulation over indigenous native strains.

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