The role of plant villin in the organization of the actin cytoskeleton, cytoplasmic streaming and the architecture of the transvacuolar strand in root hair cells of Hydrocharis

In many types of plant cell, bundles of actin filaments (AFs) are generally involved in cytoplasmic streaming and the organization of transvacuolar st rands. Actin cross-linking proteins are believed to arrange AFs into the bu ndles. In root hair cells of Hydrocharis dubia (Blume) Baker, a 135-kDa pol ypeptide cross-reacted with an antiserum against a 135-kDa actin-bundling p rotein (135-ABP), a villin homologue, isolated from lily pollen tubes. Immu nofluorescence microscopy revealed that the 135-kDa polypeptide co-localize d with AF bundles in the transvacuolar strand and in the subcortical region of the cells. Microinjection of antiserum against 135-ABP into living root hair cells induced the disappearance of the transvacuolar strand. Concomit antly, thick AF bundles in the transvacuolar strand dispersed into thin bun dles. In the root hair cells, AFs showed uniform polarity in the bundles, w hich is consistent with the in-vitro activity of 135-ABP. These results sug gest that villin is a factor responsible for bundling AFs in root hair cell s as well as in pollen tubes, and that it plays a key role in determining t he direction of cytoplasmic streaming in these cells.