Pollen Tube Guidance: The Role of Adhesion and Chemotropic Molecules

Abstract

Publisher Summary In many reproductive systems, adhesion and guidance are essential components of fertilization. In animals, sperm cells are motile and when guided to the egg cell, the sperm penetrates through the extracellular matrix (ECM) and adheres to the egg plasma membrane. Sperm cells in marine animals and algae are released into the open sea and must be guided to the egg. Marine invertebrates provide excellent material for the study of fertilization and information about the basic biology of the interaction between cell surfaces, as well as discoveries of the few molecules known to be involved in sperm–egg interaction. Motile sperm cells are also present in the lower divisions of land plants and in some seed plants. In ferns, sperm cells are released from the male gametophyte (gamete producing haploid phase of the life cycle) and swim, usually through rainwater, to the nearby female gametophyte, which bears archegonia with eggs enclosed. In both animals and plants, guidance cues from the female tissues are critical to accomplish fertilization. One of the most extensively studied guidance mechanisms is chemotaxis, defined as the movement of a cell up a molecular gradient; chemotropism refers to the growth of a cell up a molecular gradient. Adhesion molecules have been implicated in guidance in both reproduction and in embryonic cell movements in animal development, including neuron guidance. In plants, adhesion molecules are involved in reproduction, both in guidance and in the fertilization event, of gamete fusion. It is not unusual to find the same molecules involved in both adhesion and chemotropism in animals. However, in plants only a few such molecules have been reported. Recently, molecules involved in both pollen tube adhesion and guidance have been isolated from the flowering plant stigma and style transmitting tract tissues. This chapter discusses adhesion and chemotropism and looks at a chemotropic peptide in the lily stigma.