THE INFECTION PROCESS AND NODULE INITIATION IN THE FRANKIA-CEANOTHUS ROOT NODULE SYMBIOSIS - A STRUCTURAL AND HISTOCHEMICAL-STUDY

Root nodules are induced in actinorhizal plants by the nitrogen-fixing actinomycete Frankia. Nodules may be initiated by root hair infection or by intercellular penetration. In Ceanothus spp. (Rhamnaceae), Frankia colonized the host root tissue by intercellular infection, in spite of the occurrence of root hairs in the infected region. The intercellular infection pathway was characterized by an extensive darkly-staining matrix which filled prominent intercellular spaces of the root cortex, gradually decreasing through a transition zone into the nodule cortex. At the ultrastructural level, most of the matrix was composed of fibrillar electron dense material. Holes or spaces occurred in the electron dense matrix, often in conjunction with apparent loosening of wall layers. Secondary cell division was observed within the root cortical cells embedded in the intercellular matrix. Unusually high levels of pectic compounds and proteins were identified histochemically in the matrix.