I have seen that there is a specific Rhizobium isolate for a specific nodule plant (such as Mesorhizobium for Chickpea; Bradyrhizobium for Soybean; etc.). What is the reason for such specificity? Why does Bradyrhizobium not infect the Chickpea plant & other nodule plants instead of soybean?
These plants and their symbionts most likely co-evolved to obtain this relationship. In this way, the plant produces compounds that selects for its specific symbiont. It is a mutualism. In the case of soybean and Rhizobia, the soybean recruits the Rhizobium species to colonize in the specially evolved nodules. The soybean provides carbon and other nutrients, and the Rhizobia provide fixed nitrogen. Nitrogen can be a limiting resource, so it was evolutionarily advantageous for the soybean to evolve a means to recruit these specific bacteria. The interactions between the plant and microbe have become fine-tuned over thousands of years so that in many cases the relationship is exclusive.
Symbiosis is a matter of evolutionary development, not a matter of spontaneous relationship with everybody
If u look at evolution soo many things matters and at the end what we are observing is natural selection and the survival of thet selectin for many years. Here specifity comes becouse of the host-bacterium communication, the legume root exudates contain chemical compound that brings specificity .
In my simply way, I think this is because they have only key to a plant. Their key was related to 'genetic hand shake' and their's chemical. After they 'accepted' each other, symbiosis happen. This is a good publication related to your question.
http://www.ncbi.nlm.nih.gov/pubmed/17632573
Many times cross innoculation also takes place - you can do it in laboratory conditions -it also should happen in wild.
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