The specific name is derived from the specific name of its plant host.

The need for further evolution may have been lost due to the readily available food source provided by the plant host.

All symbionts within a plant host interact, often in unpredictable ways.

Rhizobia require a plant host; they cannot independently fix nitrogen.

P. poarum is found on at least seventy plant hosts.

They cannot be cultured in the laboratory in the absence of a plant host.

Each species is usually specific to a different plant host, or closely related group of hosts.

At the moment reports say that it has around 401 plant hosts.

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

Hopefully, they have managed to reach the plant host by this time.