Positive Chemotaxis of Rhizobium leguminosarum and other Bacteria towards Root Exudates from Legumes and other Plants

Rhizobium leguminosarum showed positive chemotaxis towards root exudates of its host the edible pea (Pisum sativum L.). Only the fraction of the exudate containing substances with molecular weights less than 1000 showed significant chemotactic activity. Cationic, neutral and anionic fractions were all attractive, the cationic being the most potent and the anionic the least. A range of amino acids, sugars and carboxylic acids were present in the exudate, and many were shown to be attractants. Other Rhizobium species and Escherichia coli were also attracted by pea exudate, and R. leguminosarum and the other bacteria were attracted by exudates from roots of a range of plants including non-legumes. It was concluded that although positive chemotaxis probably facilitates infection of legumes by Rhizobium, it has little or no role in host-symbiont specificity.