Early Events in the Infection of Soybean (Glycine max L. Merr) by Rhizobium japonicum

Abstract

The infectible cells of soybean roots appear to be located at any given time just above the zone of root elongation and just below the position of the smallest emergent root hairs. The location of infectible cells on the primary root at the time of inoculation was inferred from the position of subsequent nodule development, correcting for displacement of epidermal cells due to root elongation. Marks were made on the seedling growth pouches at the time of inoculation to indicate the position of the root tip and the zones of root hair development. Virtually all of the seedlings developed nodules on the primary root above the marks made at the root tips at the time of inoculation. None of the plants formed nodules on the root where mature root hairs were present at the time of inoculation. These results and profiles of nodulation frequency indicate that the location of infectible cells is developmentally restricted. When inoculations were delayed for intervals of I to 4 hours after marking the positions of the root tips. progressively fewer nodules were formed above the root tip marks, and the uppermost of these nodules were formed at progressively shorter distances above the marks. These results indicate that the infectibility of given host cells is a transient property that appears and then is lost within a few hours. The results also indicate that host responses leading to infection and nodulation are triggered or initiated in less than 2 hours after inoculation. The extent of nodulation above the root tip mark increased in proportion to the logarithm of the number of bacteria in the inoculum.The mechanisms by which soybean plants interact with Rhizobiumjaponicum to establish a symbiotic association have not been studied extensively (3). Bieberdorf (2) (9) modified to contain 0.5% mannitol, 0.5% sodium gluconate and 0.5% yeast extract. Every 2 to 3 weeks, single cell colonies from the stock cultures were used to inoculate 10 ml modified B5 medium (1), adjusted to pH 6.8 to 7.0, in 25-ml culture flasks. These starter cultures were maintained on a rotary shaker (100 rpm) at 25 C for 7 days (early stationary growth phase). The A620 nm of the starter culture was determined and a volume of the culture equivalent to 0.5 ml of an 1.0 A620 nm suspension was added to 50 ml fresh medium in a 125-ml culture flask. The remaining starter culture was stored in the refrigerator for use the following week. Inoculum cultures were grown for 3 days in the same manner as the starter cultures, harvested by centrifugation, washed once with sterile phosphate-buffered saline (pH 7.2) (1), and resuspended in onetenth strength phosphate-buffered saline. Suspensions used for inoculation were diluted to 0.08 A620 nm (I x 108 cells/ml) with sterile, half-strength N-free Jensen's plant growth medium (9) prior to use. Growth of Seedlings. Soybean seeds of cultivars Beeson and Williams were obtained from DeWine and Hamma Seed Co., Yellow Springs, OH. Harris seeds were a gift from Dr. K. Nadler, Michigan State University. Seeds (cv. Wil...

Cell Surfaces in Plant-Microorganism Interactions

“…Such molecules are involved in cell-cell interactions, notably in plant-microorganism and plantpest interactions (2,6,17,27). The way elicitors of ethylene control biochemical and molecular events related to plant diseases is under investigation.…”

Different Pathways Act Downstream of the CEP Peptide Receptor CRA2 to Regulate Lateral Root and Nodule Development

“…Similarly to skl, the sunn mutant has a supernumerary nodulation phenotype (Schnabel et al, 2005) but, unlike skl, sunn nodules form adjacent to xylem poles (Penmetsa et al, 2003). The combined negative action of ethylene and AON pathways leads to a tight restriction of the developmental competency to form symbiotic nodules (Bhuvaneswari et al, 1980;Sargent et al, 1987;Kassaw et al, 2015). Indeed, when a root is inoculated, nodule formation results from infections initiated in young, developing root hairs, and competency for further root hair infection and nodule organogenesis diminishes rapidly due to the activation of the local ethylene/EIN2-dependent (Penmetsa and Cook, 1997) and systemic CLE/AON-dependent responses (Kassaw et al, 2015).…”

“…In legumes, the formation of nitrogen (N) fixing root nodules and lateral roots (LRs) predominantly determine root system architecture. Cells that become competent for root nodule and LR formation are both similarly located close to the root apical meristem (RAM; Bhuvaneswari et al, 1980;Sargent et al, 1987;Herrbach et al, 2014). The N 2 -fixing nodules induced by rhizobia provide considerable N to legumes and enable them to grow in N-poor environments.…”

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

“…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.…”

“…In soybean (Glycine max L.), nodulation frequency decreases considerably down in the younger regions of the roots (1,2). 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.…”