THE NODULATION OF ISOLATED LEGUMINOUS ROOTS

Abstract

THE PROCESS whereby bacteria of the genus Rhizobium produce nodules on the roots of leguminous plants involves a complex physiological system which is poorly understood. One obvious way to simplify the system is to study the nodulation of isolated roots. Several advantages come from the use of root culture methods: the nutrition of excised roots grown in vitro can be controlled, substances normally contributed by the shoot can be excluded, and substances whose effects are to be tested can be added to the medium in known amounts. To date only three reports have appeared describing attempts to obtain nodulation of isolated leguminous roots. Lewis and McCoy (1933) reported the first and only successful attempt. They observed the formation of 4 nodules on one root of Phaseolus vulgaris L. out of 60 roots tested. The poor nodulation obtained by -these workers and the complete failure of others (Seppilli et al., 1941; McGonagle, 1944) made it seem particularly worth while to reinvestigate nodule formation on isolated roots. The following facts, pertinent to the problem of the nodulation of isolated roots, had already been established for light-grown and etiolated black wax bean and soybean seedlings bearing only the cotyledons (Raggio and Raggio, 1956a). Such seedlings will nodulate when grown in a nitrate-free inorganic medium. The presence in the medium of nitrate or sucrose, alone or in combination and at the levels usually used in excised root culture, inhibits nodule formation. In all nutrient media containing sucrose, and especially in those containing nitrate in addition, great bacterial multiplication occurs. On the basis of these conclusions the following inferences were made regarding nodulation of isolated roots: (1) Critical substances for growth and development of the primary root are known to migrate from the cotyledons into the root during the first few days of germination (McAlister and Krober, 1951). Since it has been found that the materials contained in the seeds of both black wax bean and soybean are necessary but are also sufficient for nodulation, primary roots excised from seeds germinated for several days should nodulate