Heteroauxin Production by Efficient and Inefficient Strains of Rhizobia

IT has been suggested by Thimann1, Link, Wilcox and Link2, and Link3 that β-indole-acetic acid (heteroauxin) is the agent partially or entirely responsible for the formation of nodules on the roots of leguminous plants. Thimann1 advances the hypothesis that the root nodule bacteria convert tryptophane into heteroauxin, (a) by stimulating elongation of a lateral root and (b) by arresting root elongation through continued auxin production, resulting in the formation of a nodule. If this condition exists, there may be differences in the rate of formation and in total amounts of heteroauxin derived from tryptophane, between efficient and inefficient strains of Rhizobia. (Efficient: Rhizobia which infect the host plant and benefit the latter by the symbiosis; inefficient: Rhizobia which infect the host plant but fail to aid in fixing atmospheric nitrogen.) Nodules produced by efficient strains are generally characterized as being large and usually located on the crown of the tap-root, indicating early invasion, while inefficient strains produce very small nodules which are distributed over the secondary root system, indicative of relatively late invasion.