Changes in actin microfilament arrays in developing pea root nodule cells

Various microorganisms that form symbiotic associations with plant roots al ter the cytoskeleton of host cells. The objective of this study was to dete rmine the organization of actin microfilaments in developing Pisum sativum L. (pea) root nodule cells at various stages after infection by Rhizobium l eguminosarum bv. viciae. Fluorescently labelled microfilaments in uninfecte d pea root nodule cells occur in association with the nucleus, along cytopl asmic strands, and as long microfilament bundles randomly organized in the cortex of the cell. These actin arrays are also present in recently infecte d cells that have been invaded by an infection thread and contain a small n umber of bacteroids. In addition, the recently infected cells contain diffu se cytoplasmic actin, long actin microfilament bundles near the vacuole, an d a nuclear-associated network of microfilament bundles. In older infected cells, the predominant array is a network of cytoplasmic microfilaments tha t are wavy and extend in multiple directions within the cell; the network i s equally abundant in all regions of the cytoplasm and may interact with th e bacteroids and organelles. Thus, actin microfilaments reorganize during t he pea root nodule infection process to form distinct arrays whose organiza tion depends on the stage of infection.