Flowering is known to be associated with the induction of many cell wall proteins. We report here five members of a tobacco gene family (CELP, Cys-rich extensin-like protein) whose mRNAs are found predominantly in flowers and encode extensin-like Pro-rich proteins. CELP mRNAs accumulate most abundantly in vascular and epidermal tissues of floral organs. In the pistil, CELP mRNAs also accumulate in a thin layer of cells between the transmitting tissue and the cortex of the style and in a surface layer of cells of the placenta in the ovary. This unique accumulation pattern of CELP mRNAs in the pistil suggests a possible role in pollination and fertilization processes. CELP genes encode a class of plant extracellular matrix proteins that have several distinct structural features: a Pro-rich extensin-like domain with Xaa-Pro3-7 motifs and Xaa-Pro doublets, a Cys-rich region, and a highly charged C terminus. The extensin-like domains in these proteins differ significantly in their length and these differences appear to be results of both long and short deletions within the coding regions of their genes. Furthermore, the number of charged amino acid residues in the C-terminal region varies among the CELPs. These structural differences may contribute to functional versatility in the CELPs. On the other hand, the Cys-rich domain is highly conserved among CELPs and the positions of the Cys residues are conserved, suggesting that this region may have a common functional role. The presence of a Pro-rich domain and a Cys-rich domain in these CELPs is reminiscent of a class of hydroxyproline-rich glycoproteins, solanaceous lectins, that are believed to be important in cell-cell recognition. The structure of these CELPs indicates that they may be multifunctional and that their genes may have arisen from recombinational events.

The pistil of flowers is a specialized organ which contains the female gametophytes and provides the structures necessary for pollination and fertilization. Pollen deposited on the stigmatic surface of a compatible plant germinates a pollen tube which penetrates the stigmatic papillae and grows intercellularly through the style towards the ovules in the ovary. Pollen tube growth is largely restricted to the transmitting tissue in the style. Therefore the stylar transmitting tissue is extremely important for the migration of the pollen cell towards the ovary. We have isolated two related cDNAs, transmitting tissue-specific (TTS)-1 and TTS-2, derived from two proline-rich protein (PRP)-encoding mRNAs that accumulate specifically in the transmitting tissue of tobacco. The deduced PRP sequences share similarities with proline-rich cell wall glycoproteins found in a variety of plants. TTS-1 and TTS-2 mRNAs are induced in very young floral buds, accumulate most abundantly during the later stages of flower development when style elongation is the most rapid, and remain at relatively high levels at anthesis. These mRNAs become undetectable in maturing green fruits. In situ hybridization shows that TTS-1 and TTS-2 mRNA accumulation is restricted to the transmitting tissue of the style. The possible roles that these transmitting tissue-specific PRPs may play in maintaining the structural integrity of the style or in the function of this organ is discussed.

We isolated a flower-specific cDNA, FST (flower-specific thionin), which encodes a novel thionin from tobacco. Thionins are basic and cysteine (Cys)-rich, low molecular weight proteins found in many plants. They are believed to play a role in plant defense against pathogens. The central domain of the FST protein shares homology with three gamma-thionins. Like other thionin precursors, the FST protein has an N-terminal domain characteristic of a signal peptide and an acidic C-terminal domain. FST mRNA accumulates specifically in developing flowers and its level drops as flowers mature. Transcripts are present in petals, stamens and pistil but are not detectable in sepals. In situ hybridization revealed that FST mRNA is most abundant in the epidermal cells along the adaxial surface of petals, and in the surface cell layers of the carpel and anther walls. If the FST protein indeed has a protective role in flowers, this pattern of spatial distribution of FST mRNA would appear to maximize this effect on the two internal reproductive whorls. A possible biological role for FST is discussed.