Pollen vegetative cell-specific expression of Bra r 1: useful tool for observation of the vegetative nucleus and identification of transgenic pollen by nuclear-targeted GFP
Bra r 1 encodes a Ca2+-binding protein specifically expressed in anthers of Brassica rapa. In this study, we isolated a genomic clone of Bra r 1 and found sequences similar to Pollen Box core motifs and LAT56/59 box, pollen-specific cis-acting element, in the 5' upstream region of Bra r 1. Reporter gene fusion revealed that the Bra r 1 promoter directs male gametophytic expression in Nicotiana tabacum, Arabidopsis thaliana and B. napus, showing strong expression in mature pollen grains similar to that of endogenous Bra r 1. Genomic DNA of Bra r 1 was introduced into tobacco plants and the highest accumulation of Bra r 1 protein was observed in mature pollen in the same manner as reporter gene expression. Using in vitro-germinated pollen tubes of transgenic tobacco, we firstly demonstrated the subcellular localization of Bra r 1 in pollen tubes. Bra r 1 protein was distributed throughout the pollen tube of transgenic tobacco and slightly intense signals of Bra r 1 were observed in the tip region. In long-germinated pollen tubes, Bra r 1 was detected only in the cytoplasmic compartments while no signals were observed in the empty part of the pollen tube, indicating that cytoplasmic movement toward the tube tip is accompanied by Bra r 1. Hence, we suggest that Bra r 1 is involved in pollen germination and pollen tube growth.