SF21 is a Protein which Exhibits a Dual Nuclear and Cytoplasmic Localization in Developing Pistils of Sunflower and Tobacco.

SF21 was originally described as a pollen- and pistil-expressed protein from sunflower and tobacco. In pistils excised from these species, transcripts were detected in the stigma and in the transmitting tissue where they accumulated in an ovary-oriented increasing concentration gradient. We studied the cellular localization of the SF21 protein during various stages of pistil development as well as in pollen grains of tobacco and sunflower. Here we demonstrate that in young tobacco pistils (from stage 2 onwards) this protein is expressed exclusively in the papillae and secretory cells of the stigma where it is located first in the nucleus and subsequently also in the cytoplasm. Only several stages later (stage 10) does it appear in the transmitting tissue cells of the style where it exhibits a similar, but temporally-delayed, dual nuclear and cytoplasmic localization pattern. SF21 is no longer present in either the stigma or style at the time of pollination, indicating that it is not directly involved in the pollination process. In tobacco pollen grains, SF21 appears just prior to pollen germination and localizes to the apical region of growing pollen tubes, suggesting a possible role in pollen tube growth. This temporal and spatial expression pattern as well as the dual nuclear/cytoplasmic localization suggest that SF21 could be involved in several molecular functions during pistil development and pollen tube growth.