Plants, and all other living things, require nitrogen for growth; it is an essential component ofnucleic acids and proteins. Although air is mostly nitrogen, this gaseous form is inaccessible toplants and must be fixed into ammonium to render it biologically relevant. Soil bacteria calledrhizobia fix nitrogen, but to do this they must first take up residence inside the roots oflegumes like pea, alfalfa, clover, and soybean.
Soon after a legume begins to grow, rhizobia invade its root hairs and multiply, causing theplant to form specialized organs—nodules—that contain the proliferating bacteria. Thissymbiotic arrangement benefits both parties: legumes can thrive without nitrogen fertilizersonly if they have functional nitrogen-fixing nodules, while the bacteria receive the energyneeded to multiply and fix nitrogen from the plant. When the plant dies, the fixed nitrogen isreleased into the soil so other plants can use it. This process has significant implications foragriculture, as nitrogen is the most common nutrient deficient in the earth’s soil and, thus,the one most commonly supplied by chemical fertilizers.
Rhizobia are a diverse group taxonomically, genetically, and metabolically. They can be foundin distant genera. Their symbiotic trait appears to have arisen independently multiple times byhorizontal transfer of genes. However, it is not thought that this horizontal gene transferis sufficient to confer symbiosis, or to explain the biodiversity of rhizobia. There must beselective pressures preventing or permitting the expression of the acquired symbiosis trait andadaptive mechanisms to deal with these pressures. But neither the pressures nor themeasures taken to circumvent them are known.
What does the passage indicate about the expression of the symbiosis trait in rhizobia?
A.It occurs only in certain strains of rhizobia with metabolic similarities.
B.It is not determined by environmental pressures.
C.It is responsible for the biodiversity of rhizobia.
D.It does not result from horizontal gene transfer alone.
Choice D is the best answer. When discussing the expression of the symbiosis trait in rhizobia,the author states that "it is not thought that ... horizontal gene transfer is sufficient toconfer symbiosis." In other words, horizontal gene transfer alone is not enough to cause theexpression of the symbiosis trait.