What is the significance of the wisteria’s relationship to rhizobia?

Taylor and Turtle spend time in the library, where they find the Horticultural Encyclopedia and look up wisteria. Taylor reads to her about wisteria: they thrive in poor soil, for their secret is rhizobia, microscopic bugs that live underground in little knots on the roots and turn the soil into fertilizer. Taylor compares the wisteria and the rhizobia to relationships such as Edna and Virgie.