Why are most rhizobia beneficial to their plant hosts, rather than parasitic?

Multiple strains per plant and root-to-root (not seed-borne) transmission should favor rhizobia that invest in their own reproduction, rather than symbiotic N2 fixation, as analogous factors may favor pathogen virulence. But legumes can select for greater mutualism, controlling nodule O2 supply and reducing reproduction of rhizobia that fix less N2. More.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.