Root Nodule Initiation and Development in Discaria trinervis Frankia - Symbiosis

Abstract

Root nodule formation in the south American rhamnaceous shrub, Discaria trinervis, was followed by microscopy of clarified entire roots, semithin and ultrathin sections of fixed roots, after seedling inoculation with the local isolate from D. trinervis nodules, Frankia BCU1105 (E. Chaia, unpublished). Nodulation kinetic and nodule distribution around the root tip (RT) at the moment of inoculation were also analyzed in pouch grown seedlings, using different doses of Frankia BCU1105 inoculum. Root infection by BCU1105 hyphae was through intercellular penetration (Miller IM, Baker DD, 1985) of epidermal and cortical cells. Neither root hair deformation with the agargel-slide technique (Wall LG, Huss-Danell K, 1997), nor prenodule formation in outer cortex were observed. Nodule primordia emerging from pericycle were detected 6 days after inoculation and host cell invasion occurred 2–3 days after. Frankia vesicle differentiation inside hypertrophied infected cells was observed 13–16 days after inoculation, at the same time that basal cells of the nodule primordia showed a differential dark pigmentation -not detected in previous stages of nodule development- after NaClO clarifying treatment. We show that root infection in this actinorhizal plant is by intercellular penetration of Frankia, and that nodule induction in pericycle, host cell invasion by Frankia and vesicle differentiation, are time separated events.