Nodulation factors trigger an increase of fine bundles of subapical actin filaments in vicia root hairs: Implications for root hair curling around bacteria

Legume plants and Rhizobium bacteria can grow in isolation, but when they meet, they can profit from each other in a symbiotic partnership. This interplay starts with root hair cell growth and gene expression, and leads to root hair curling around bacteria, growth of plant-created infection threads containing the bacteria, and cell divisions in the root cortex, giving rise to nodules. These nodules become a comfy home for the bacteria, well supplied with food for them to live and reproduce. The bacteria pay with useful nitrogen-compounds that the plant needs, but can not manufacture, which the bacteria make by fixing nitrogen from the air. The molecules excreted by the bacteria and to which the roots respond are lipochito-oligosaccharides, LCOs, also called Nod (nodulation) factors. We report one of the first reactions of root hairs to LCOs, the increase in the density of subapical on average axially aligned fine bundles of actin filaments, FB-actin, and discuss a model for the curling of the hairs around the bacteria.