Evolution 2014: Is thyme a facultative mutualist with leguminous plants?

Mary McKenna of Howard University presented work that suggested that thyme plants may be facultative mutualists when associated with various legume species. In work done at the Blandy Experimental Farm, her students have demonstrated that legumes growing in the presence of thyme plants form more root nodules in association with their nitrogen-fixing bacterial symbionts. This effect occurs in three legumes that are associated with different strains of nitrogen-fixing bacteria. Why is tantalizing… could it be that the thymol produced by the thyme plants attract nitrogen fixing bacteria (act as a “signal”), or is thymol killing off competitors to these bacteria, or is thymol nutritive to these bacteria? Or a combination of these factors?