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 R hizobium, it has little or no role in host-symbiont specificity.
The ability of a range of carbohydrates to support the growth of plasmodia of the myxomycete Physarum polycephalum was investigated and a method for the objective study of the chemotaxis of plasmodia was developed. Plasmodia showed positive chemotaxis to solutions of glucose, maltose, mannose and peptone, and to galactose after a delay. They did not respond to sucrose, fructose or ribose. The chemotactic effectiveness of sugars paralleled their ability to support growth.
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