REARRANGEMENT OF ACTIN MICROFILAMENTS IN PLANT-ROOT HAIRS RESPONDING TO RHIZOBIUM-ETLI NODULATION SIGNALS

The response of the actin cytoskeleton to nodulation (Nod) factors sec reted by Rhizobium etli has been studied in living root hairs of bean (Phaseolus vulgaris) that were microinjected with fluorescein isothioc yanate-phalloidin. In untreated control cells or cells treated with th e inactive chitin oligomer, the actin cytoskeleton was organized into long bundles that were oriented parallel to the long axis of the root hair and extended into the apical zone. Upon exposure to R. etli Nod f actors, the filamentous actin became fragmented, as indicated by the a ppearance of prominent masses of diffuse fluorescence in the apical re gion of the root hair. These changes in the actin cytoskeleton were ra pid, observed as soon as 5 to 10 min after application of the Nod fact ors. It was interesting that the filamentous actin partially recovered in the continued presence of the Nod factor: by 1 h, long bundles had reformed. However, these cells still contained a significant amount o f diffuse fluorescence in the apical zone and in the nuclear area, pre sumably indicating the presence of short actin filaments. These result s indicate that Nod factors alter the organization of actin microfilam ents in root hair cells, and this could be a prelude for the formation of infection threads.