Cytoplasmic bridge formation in the nodule apex of actinorhizal root nodules

Abstract

High-pressure frozen - freeze-substituted actinorhizal root nodules of several distantly related plant genera were used to document the sequence of structural changes in cortical cells of the nodule apex that happened prior to their infection. The sequence of mobilization of the plant cell cytoplasm requisite to infection by Frankia was (i) penetration of the parenchyma cell vacuole by cytoplasmic strands, which contained microtubules; (ii) movement of the nucleus and other organelles (Golgi stacks, endoplasmic reticulum, mitochondria), involved later in growth of the infection thread, to the cell center on these strands; (iii) thickening of some of these strands generally located at midpoints of the wall, forming cytoplasmic bridges (preinfection threads); and (iv) infection of the cell by initiation of infection threads (containing Frankia) within the cytoplasmic bridges. The infection thread was caged in microtubules that were oriented along its axis, suggesting the cytoskeleton had a major role in the infection process, perhaps guiding the growth of the infection thread across the cell. The coalignment of cytoplasmic bridges, along several cells, towards the advancing microsymbiont suggested Frankia secretes a diffusible signal eliciting this host response.Key words: actinorhiza, cryofixation, development, infection, microtubules, symbiosis.