The incorporation of a fusion of green fluorescent protein and tubulin- 6 from Arabidopsis thaliana in root hairs of Lotus japonicus has allowed us to visualize and quantify the dynamic parameters of the cortical microtubules in living root hairs. Analysis of individual microtubule turnover in real time showed that only plus polymer ends contributed to overall microtubule dynamicity, exhibiting dynamic instability as the main type of microtubule behavior in Lotus root hairs. Comparison of the four standard parameters of in vivo dynamic instability--the growth rate, the disassembly rate, and the frequency of transitions from disassembly to growth (rescue) and from growth to disassembly (catastrophe)--revealed that microtubules in young root hairs were more dynamic than those in mature root hairs. Either inoculation with Mesorhizobium loti or purified M. loti lipochitin oligosaccharide signal molecules (Nod factors) significantly affected the growth rate and transition frequencies in emerging and growing root hairs, making microtubules less dynamic at a specific window after symbiotic inoculation. This response of root hair cells to rhizobial Nod factors is discussed in terms of the possible biological significance of microtubule dynamics in the early signaling events leading to the establishment and progression of the globally important Rhizobium/legume symbiosis.