FIMBRIN1 is involved in lily pollen tube growth by stabilizing the action fringe

Abstract

An action fringe structure in the subapex plays an important role in pollen tube tip growth. However, the precise mechanism by which the action fringe is generated and maintained remains largely unknown. Here, we cloned a 2606-bp full-length cDNA encoding a deduced 77-kD fimbrin-like protein from lily (Lilium longiflorum), named FIMBRIN1 (FIM1). Ll-FIM1 was preferentially expressed in pollen and concentrated at action fringe in the subapical region, as well as in longitudinal action-filament bundles in the shank of pollen tubes. Microinjection of Ll-FIM1 antibody into lily pollen tubes inhibited tip growth and disrupted the action fringe. Furthermore, we verified the function of Ll-FIM1 in the fim5 mutant of its closest relative, Arabidopsis thaliana. Pollen tubes of fim5 mutants grew with a larger diameter in early stages but could recover into normal forms in later stages, despite significantly slower growth rates. The action fringe of the fim5 mutants, however, was impaired during both early and late stages. Impressively, stable expression of fim5pro:GFP:Ll-FIM1 rescued the action fringe and the growth rate of Arabidopsis fim5 pollen tubes. In vitro biochemical analysis showed that Ll-FIM1 could bundle action filaments. Thus, our study has identified a fimbrin that may stabilize the action fringe by cross-linking action filaments into bundles, which is important for proper tip growth of lily pollen tubes. © 2012 American Society of Plant Biologists.