The seeds of leguminous plants are a source of proteins. nitrogen - fixing bacteria live in the roots of such plants. do you think there is a connection between the two facts?

Yes, Only bacteria can convert molecular nitrogen, into a form which can be utilized by other living beings such as plants. The nitrogen fixed by the bacteria, by this process, become available to the plants, which uptakes it, for their own growth, and is also accumulated in their seeds. Further, leguminous plants have nitrogen-fixing bacteria associated with them in synergy. They provide the plant with additional nitrogen, and in exchange obtain nutrients from the plant, for their growth and division.