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 fro m sunflower and tobacco. In pistils excised from these species, transcripts were detected in the stigma and in the transmitting tissue where they accu mulated in an ovary-oriented increasing concentration gradient. We studied the cellular localization of the SF21 protein during various stages of pist il development as well as in pollen grains of tobacco and sunflower. Here w e demonstrate that in young tobacco pistils (from stage 2 onwards) this pro tein is expressed exclusively in the papillae and secretory cells of the st igma 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 trans mitting tissue cells of the style where it exhibits a similar, but temporal ly-delayed, dual nuclear and cytoplasmic localization pattern. SF21 is no l onger present in either the stigma or style at the time of pollination, ind icating that it is not directly involved in the pollination process. In tob acco pollen grains, SF21 appears just prior to pollen germination and local izes to the apical region of growing pollen tubes, suggesting a possible ro le in pollen tube growth. This temporal and spatial expression pattern as w ell as the dual nuclear/cytoplasmic localization suggest that SF21 could be involved in several molecular functions during pistil development and poll en tube growth. (C) 2001 Annals of Botany Company.