A Molecular Perspective on Pollination in Flowering Plants

their allotetraploid hybrid, B. napus (Nasrallah and Nas-Plant Cell Biology Research Centre rallah, 1993). Flowers of these plants have dry stigmas School of Botany covered by a waxy cuticle, and successful pollinations University of Melbourne begin when a hydraulic connection is established be-Parkville, VIC 3052 tween a pollen grain and a stigmatic papillar cell, Australia allowing the pollen grain to hydrate and germinate. The pollen tube penetrates the stigmatic cuticle and grows Fertilization in flowering plants is the culmination of a through a specialized region of the papillar cell wall series of events that begins when a pollen grain comes that expands after pollination. After growing within the to rest on the receptive surface (stigma) of a pistil (the papillar cell wall to the base of the cell, the pollen tube female reproductive tissues, including the stigma, style, enters the style where it grows intercellularly toward an and ovary; see Figure 1A). The pollen grain hydrates and ovule. Recognition and rejection of incompatible pollen a pollen tube emerges from the grain and grows through occurs very early in this process; incompatible pollen the style to the ovary. The pollen tube consists of a often does not hydrate fully and either fails to germinate, single large vegetative cell carrying two sperm cells in or produces a pollen tube that is unable to penetrate its tip. When the pollen tube reaches an ovule, one sperm cell fuses with the haploid egg cell to form the embryo, while the second sperm cell fuses with a diploid accessory cell (the central cell) to form the endosperm, a specialized food-storage tissue. Communication between pollen and pistil is an essential factor in many aspects of this process. For example, directional cues enable pollen tubes to locate the ovules (Cheung, 1995), and in plants such as orchids, a signal from pollen induces ovule development after pollination (Zhang and O'Neill, 1993). Recognition systems allow pistils to select from the genetically diverse range of pollen arriving on the stigma so that only compatible pollen effect fertilization. These systems include barriers to cross-species fertilization, although these are not well described at the molecular level, and intraspecific barriers such as self-incompatibility, that prevent self-fertilization. Self-incompatibility has been characterized in the Brassica-ceae (the mustards) and the Solanaceae (potato and tobacco), and in this review we discuss the molecular basis of this remarkable system of cellular recognition in plants. Genetic Control of Self-Incompatibility …