Background Legumes are able to enter into a symbiotic relationship with nitrogen-fixing soil bacteria, collectively called rhi- zobia. The symbiosis culminates in the formation of the root nodule, which provides an optimal environment for the bacteria to fix atmospheric nitrogen for use by the plant. It was estimated that the legume-rhizobial symbi- osis can fix more than half of the amount of nitrogen produced by the chemical fertilizer industry [1].