Pollen-pistil interactions in compatible pollination.

Plant sexual reproduction depends on highly specific interactions between pollen and pistil, the male gametophyte and the female reproductive organ, respectively (1, 2). The pistil is composed of three major structural parts: the stigma, the style, and the ovary. The stigma has a pollen-receptive surface and several underlying secretory cell layers. The style connects the stigma to the basally located ovary containing the ovules. Within each ovule, an egg cell develops inside the embryo sac. In compatible pollination, the pollen grain germinates and extrudes a pollen tube upon landing on the stigmatic surface. Each pollen tube penetrates the stigmatic cell layers and elongates within a specialized tissue in the style called the transmitting tissue, eventually reaching the ovary, where it enters an ovule and penetrates the embryo sac. The pollen tube tip bursts in the embryo sac to release the male germinal cells for fertilization. Incompatible pollen may be arrested at the stigma or anywhere along the pathway of pollen tube elongation. Pollen has a high capacity to support its activity during germination and tube growth (3, 4). Pistil tissues are believed to provide physical and chemical supports and directional guidance to the pollen tube growth process (1, 2, 4-6). The arrest of pollen germination and tube growth in transgenic plants in which the stigmatic or the transmitting tissues are ablated by cytotoxins (7, 8) and the loss of directional pollen tube growth in the ovary of embryo sac-defective mutants (9) indicate that important functions are contributed by the pistil to pollen germination and tube growth. Cytological and biochemical observations (1, 2) suggest that the pollen and pistil extracellular matrix (6) and the pollen cytoskeleton (10) are important for the pollination process. Recent experiments have identified some of these components and many candidate molecules which may participate in compatible pollen-pistil interactions. One of these molecules, a pollen-specific extracellular matrix protein, Pex-1, from maize is described by Rubinstein et aL (11) in this issue of the Proceedings. These findings will be discussed with a view toward exploring how pollen and pistil extracellular components may interact and how these extracellular interactions may be transduced and translated into the activities needed for pollen tube growth.