Genetical Factors of the Host

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

Genetical Factors of the Host

	Genetical Factors of the Host Different varieties of the same legume are known to respond differently to a given Rhizobium, especially with regard to the number of nodules produced on them. In studies on the role of hereditary factors in root nodulation, individual non-nodulating plants or those plants resistant to infection have been used to conduct inter-specific hybridization experiments.
The results of experiments in this direction show that resistance in red clover to Rhizobium infection can be attributed to a recessive factor acting in conjunction with a cytoplasmically transmitted component. These resistant plants do not nodulate with any other strain of Rhizobium under bacteriologically controlled conditions or with strains present in unsterile soil. Neither resistance nor susceptibility can be transmitted through grafting. The roots of plants resistant to infection no doubt stimulate the multiplication of Rhizobium in the root region followed by curling of root hairs, although the hairs contain no infection threads in them. This kind of resistance attributable to a single recessive factor is also present in soybean and has been introduced into other soybean varieties. Such resistant lines of plants can serve as controls in judging the symbiotic effectiveness under similar experimental conditions.
Grafting experiments done to obtain nodules on pea roots grafted to non-legumes such as buckwheat and horse bean roots grafted to Nasturtium, show that nodules can be formed only on the roots of legumes. Early or late formation of nodule has been shown to be heritable in a variety of clovers. Early nodulating habit is advantageous in legume establishment particularly in annual species grown in nitrogen-starved soils.
All hybridization and selection work with sparsely and abundantly nodulating lines of subterranean clover and other species of Trifolium have shown that nodule number is inherited in a complex manner. The nuclei of nodule cells are generally tetraploid or of a higher ploidy. Polyploidy affects the number of nodules differently in different species. Autotetraploids of T. subterraneum form fewer nodules than diploids. Polyploids of T. repens, T. ambiguum and soybean have been nodules than diploids of the corresponding species. The influence of ploidy on nodule number may depend on the conditions under which plants grow. Strains of bacteria isolated from tetraploid red clover produce more nodules and are generally more effective on tetraploid than on diploid lines. However, substantial evidence is lacking to show that the genotype of one symbiont directly affects the other.
	As stated earlier, stimulation of nodulation in lucerne and several species of Trifolium by pre-treatment of roots with colchicine has been reported. The mode of action of colchicine is not known. Perhaps, the mitotic poison increases the number of tetraploid cells in the root. Divergent results have come forth on the effects of irradiation of seeds with physical mutagens on the nodulation status of plants raised from such irradiated seeds. Strong irradiation with gamma rays not only decreases nodulation and inhibits nitrogen fixation in pea plants but also results in the mutation of bacteria inside the nodules. On the contrary, seed irradiation of about 800 r over a period of seven days doubles the number of nodules and lateral roots in Trifolium alexandrinum and Trigonella foenum-graecum.

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