Pistil factors controlling pollination.

The successful establishment of angiosperms on land is in part determined by their floral design. Because plants cannotmove to find the idealmate, they have developed agreat variety of flowers to provide different mechanisms of pollen release, pollen transfer, and deposition of the pollen from the male to the female sexual organs, the anther and the pistil, respectively. Pollination ensures the maintenance of the species, but it is also a means to increase genetic diversity and, with it, the potential to adapt to new environments. The position and morphology of the anthers and the pistil have often coevolved with the mode of pollen dispersal and pollen receipt, aided either by wind or by animals. Nevertheless, pollination can fail at various points during these processes, causing the extinction of rare plants and lower crop yields (Wilcock and Neiland, 2002). The pistil, the pollen-accepting organ that occupies the central position in a flower, is composed of one or more fused carpels that bear the ovules. Pistil development initiates with the formation of the carpel primordia, and the floral identity genes of class C, such as AGAMOUS in Arabidopsis thaliana and PLENA in Antirrhinum majus, dictate carpel identity (Ng and Yanofsky, 2000). Carpel fusion occurs very early in pistil development. Even in species with single pistils, fusion of the carpel margins is required to form a closed carpel. For instance, tobacco (Nicotiana tabacum) and Arabidopsis pistils aremade from the fusion of two carpels, and that of the cultivated tomato is formed from five fused carpels (Gasser and Robinson, 1993). Close to the time of ovary closure, the carpel walls extend vertically to form one or more hollow cylinders, the styles. This process requires cell division at first and cell elongation later. The length, number, and structure of the styles are typical within each species and variable between them. Stylar extension facilitates pollen capture, and the wide variety of pistil morphologies reflects the different pollination mechanisms found among the angiosperms (Barrett et al., 2000). While the style is elongating, the inner tissues differentiate to form the specialized secretory zone of the stigma on the top of the style and the transmitting tissue within the hollow cylinder of the style. In some species, such as lily, the style remains hollow, with only one layer of secretory tissue lining the inner surface of the cylinder (Dickinson et al., 1982). At flower maturity, when pollination takes place, the pistil is fully developed and composed of stigma, style, and ovary. Whether the pollen is transported by the wind or by animal pollinators, after landing on the stigma the pollen grain hydrates and germinates a tube. This tube then penetrates the specialized tissues of the pistil, growing into the stigma and the style to reach the ovules in the ovary. During this process, numerous cell–cell interaction events occur between the cells of the sporophyte (the pistil) and the male gametophyte (the pollen grain and the tube). Most of the existing knowledge regarding pollen–pistil interactionwas gathered from the study of self-incompatibility in several species. Nevertheless, considerable data are now available on the prepollination and postpollination events and molecules that make the pistil ready to interact with the pollen after compatible pollinations (Lord and Russell, 2002). Some of these interactions are discussed in this review.