Roots of young soybean (Glycine max (L.) Merr.) seedlings inoculated with Rhizobium japonicum Kirchner USDA 110 ARS were examined in serial sections by light microscopy to ascertain the extent of infection. The location of each infection site was established in relation to the zones of root and root hair development at the time of inoculation. Each infection locus was classified as to its relative state of differentiation using a developmental scale encompassing the first 10 days of nodule development. Both the initiation and maturation of Rhizobium infections were found to be governed by the acropetal development of host root hairs. Regions of the root where mature root hairs were present at the time of inoculation were not susceptible to Rhizobium infection. Infections developed most frequently in root hairs which emerged shortly after inoculation. Many infections formed on the root but relatively few developed into nodules. Most infection loci which formed infection threads stopped developing at stages prior to meristem formation. A high proportion of the infection loci were pseudoinfections, i.e., localized areas of cortical cell division without infection thread formation. The maturation of infections in younger regions of the root was suppressed by prior exposure of older regions of the root to rhizobia. Development was suppressed at stages after meristem formation but before nodule emergence.

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ABSTRACrThe initiation of Rhizobium infections and the development of nodules on the primary root of soybean Glycine max L. Meff cv Williams seedlings are strongly affected by exposure of the cotyledons/hypocotyls to light. Seedlings in plastic growth pouches were inoculated with R. japonicum in dim light and the position of the root tip of each seedling was marked on the face of the pouch. The pouches were covered and kept in the dark for various times before exposing the upper portions of the plants (cotyledons and hypocotyls) to light. Maximum nodulation occurred if the plants were kept in the dark until 1 day after inoculation. The exposure of plants to light 2 days before inoculation reduced the number of nodules by 50% while the number of nodules was reduced by 70% if the plants were kept in the dark until 7 days after. inoculation. Anatomical studies revealed that exposure to light prior to inoculation reduced both the number of infection centers with visible infection threads and the number of infections which developed nodule meristems. Plants kept in the dark for 7 days after inoculation formed a normal number of infection threads above the root tip mark, but very few of these infections developed a nodule meristem. It appears that light stimulates soybean to produce substances which can both inhibit the formation of infection threads and enhance the development of nodules from established infection threads. The effects of light on nodulation appear to be expressed independently of the Rhizobium-induced suppression of nodule formation in younger regions of the root.The establishment of a symbiotic association between Rhizobium species and legumes involves infection of the root and the development of organized nodular growths containing bacteria. Nodule formation is a complex process requiring the completion of many steps between the initial contact of rhizobia with the root surface and the final differentiation of host and bacterial cells to form a mature nodule (1,10,12 exposed to light developed 30% fewer nodules on the primary root than similar seedlings kept in the dark until 3 d after imbibition. These observations have led us to investigate more systematically the effects of light on the initiation of infections and the development of nodules in soybean. MATERIALS AND METHODSBacterial culture. Rhizobium japonicum strain USDA-I-110-ARS (resistant to azide, rifampicin, and streptomycin) was obtained from Dr. D. Kuykendall, USDA Beltsville, and was used in all of the studies described here. Freeze-dried ampules of the stock culture of this strain were used to initiate starter cultures in a yeast extract-mannitol-gluconate medium as described previously (2, 3). Subcultures ofthe bacterium in this medium were used for the inoculation of seedlings after dilution of a culture in early exponential growth phase (A at 620 nm = 0.1-0.3) with sterile distilled H20 to obtain 105 cells/ml (108 cells/ml = 0.05 A at 620 nm). Viable cell counts ofdiluted inoculum suspensions were recorded for each experiment to d...

A cyanobacterium, Anabaena azollae, lives symbiotically with the water-fern Azolla. It occupies cavities formed in the AzoIIa leaves and can provide the association with its total N requirement by fixing N2. Multicellular trichomes, present on the walls of the cavities, are intimately associated with the microsymbiont. The discovery of transfer cell ultrastructure in some of these trichomes has led to suggestions that they may be involved in metabolic exchange between the partners. To gain insight into their role in the symbiosis, the ontogeny of the cavity trichomes has been examined in detail and its stages correlated with microsymbiont development and N2 fixation activity.

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