A Rapid Regulatory Response Governing Nodulation in Soybean

Abstract

The number of nodules which develop on the primary root of soybean seedlings (Glycine max L. Meff) after inoculation with Rhizobium japonicum is substantially diminished in the region of the root developmentally 10 to 15 hours younger than the region maximally susceptible to nodulation at the time of inoculation. This rapid inhibition ofnodulation has been investipted by inoculating soybean seedlings with rhizobia at two different times, 15 (4). Inocula were prepared from mid-exponential phase cultures by centrifugation, washing with sterile phosphate buffered saline, resuspension in one-tenth-strength phosphate-buffered saline, and dilution with half-strength N-free Jensen's plant growth medium to the desired cell density (4). Approximate cell densities were determined from the absorbance of diluted suspensions at 620 nm (0.03 A620 nm = 1 x 108 cells/ml). To test the effects of dead R. japonicum cells, suspensions of bacteria were diluted to 1 x I05 cells/ml and exposed in a Petri dish to sufficient UV irradiation to kill more than 99.9% of the cells. Aliquots from such suspensions and from unexposed suspensions of the same bacteria were used to inoculate test and control seedlings, respectively.Cultures of the heterologous rhizobia R. leguminosarum (strain 603)

Shoot versus Root Signal Involvement in Nodulation and Vegetative Growth in Wild-Type and Hypernodulating Soybean Genotypes

“…The extent of nodulation upon successful inoculation is generally restricted by a plantmediated, feedback-regulated process termed autoregulation. This process involves suppression of nodule emergente from ontogenetically younger root tissues by previously formed nodules on older parts of the root system (Bhuvaneswari et al, 1981;Pierce and Bauer, 1983;Kosslak and Bohlool, 1984). It has been proposed that once a critical number of SCDs in the root cortex are initiated, a precursor molecule from the root is transported to the This paper is dedicated to the memory of Chuxing "Chuck" Sheng, who unexpectedly passed away on July 15, 1996, while in the prime of his career.…”

When Does the Self-Regulatory Response Elicited in Soybean Root after Inoculation Occur?

“…Figure 1 shows the relative frequency of nodule formation at various positions along the primary root, relative to RTl and RT2. The nodulation profile shown in Figure IA, where seedlings were inoculated at RTl with phages only and at RT2 with PRI 1, is typical ofthe previously published profiles (2,7). It is, however, interesting to note in Figure lB that, even when the seedlings were first treated with rhizobia mixed with phages, there was no inhibition of nodulation at RT2 (compare nodulation profiles at RT2 of Fig.…”

“…The suppressive response was not elicited by heterologous or by dead homologous bacteria (7). Based on the short distance between the regions of maximum nodulation and suppressed nodulation, it was inferred that the suppressive response was elicited very rapidly, within an hour or so, by the first inoculum (2,7). More recent studies (3), however, revealed that the initiation of infection in the younger regions was not suppressed.…”

“…Previous studies (7), involving double inoculation of the roots, showed that the nodulation in the younger regions of soybean root was suppressed by prior inoculation of more mature regions of the root. The suppressive response was not elicited by heterologous or by dead homologous bacteria (7).…”

“…Previous studies (7), involving double inoculation of the roots, showed that the nodulation in the younger regions of soybean root was suppressed by prior inoculation of more mature regions of the root. The suppressive response was not elicited by heterologous or by dead homologous bacteria (7). Based on the short distance between the regions of maximum nodulation and suppressed nodulation, it was inferred that the suppressive response was elicited very rapidly, within an hour or so, by the first inoculum (2,7).…”