LFIM1 is involved in the growth of lily pollen tube by stabilizing the actin fringe
Hui Su, Jinsheng Zhu, Chao Cai, Weike Pei, Jiaojiao Wang, Huaijian Dong, Haiyun Ren*

Key Laboratory of Cell Proliferation and Regulation Biology of Ministry of Education, and College of Life Science, Beijing Normal University, Beijing 100875, People’s Republic of China

Abstract
An actin fringe structure in the subapex plays an important role in pollen tube tip growth. However, the precise mechanism by which the actin fringe is generated and maintained remains largely unknown. Here, we cloned a 2606-bp full-length cDNA that deduced a 77-KD fimbrin-like protein from Lilium longiflorum, named LFIM1. LFIM1 preferentially expressed in pollens and concentrated at actin fringe in the subapical region, as well as in longitudinal actin-filament bundles in the shank of pollen tubes. Microinjection of LFIM1 antibody into lily pollen tubes inhibited tip growth and disrupted the actin fringe. Furthermore, we verified the function of LFIM1 in its closest relative Arabidopsis thaliana fimbrin5 mutant (afim5). Pollen tubes of afim5 mutants grew with a larger diameter in early stages, but could recover into normal forms in later stages, despite significantly slower growth rates. The actin fringe of the afim5 mutants, however, was impaired during both early and late stages. Impressively, stable expression of afim5pro:GFP:LFIM1 rescued the actin fringe and the growth rate of afim5 pollen tubes. In vitro biochemical analysis showed that LFIM1 could bundle actin filaments. Thus, our study has identified a fimbrin that may stabilize the actin fringe by crosslinking actin filaments into bundles, which is important for proper tip growth of lily pollen tubes.