Sexual signaling on a cellular level: lessons from plant reproduction.

In this essay, I convey the results of some of our recent studies of reproduction in the model plant, Arabidopsis thaliana. Our work has been influenced by the investigations of a large and interactive community of researchers studying a variety of plant systems, including lily, tobacco, maize, and plants from the mustard family (for recent reviews, see Cheung et al., 2000; Dixit and Nasrallah, 2001; Johnson and Preuss, 2002). This essay does not attempt to review the large body of work from the field; rather, it focuses on the questions of interest to my laboratory. I have long been fascinated by the mechanisms that regulate cell–cell communication. For a higher plant cell, the modes of interaction are quite distinct from what we commonly see in animals. Higher plant cells typically do not migrate; instead, they spend their entire lives next to the same cells, all connected by rigid extracellular walls. Pollen grains are an exception: These cells are, in fact, small organisms. They consist of a large vegetative cell that carries within its cytoplasm two intact sperm cells, complete with their own plasma membranes and walls. To accomplish fertilization, the sperm are delivered through a pollen tube to eggs buried deep within the flower. Pollen tubes expand rapidly, invade female tissues, navigate across multiple cell layers, and finally rupture, delivering the resident sperm (for recent review, see Johnson and Preuss, 2002). Throughout this journey, female cells continuously check the pollen’s identity, inhibiting foreign pollen, while promoting fertilization by pollen from compatible plants. Thus, unlike higher animals, where visual cues, pheromones, and behavior dominate the choice of mating partners, plants choose their mates by relying on cell–cell interactions. Our work has focused on identifying the signaling molecules responsible for this cellular mating dance. Such signals are most likely specific to each plant species; consequently, we anticipate that evolution of these molecules is at least one mechanism that drives speciation. There are several molecular barriers to interspecies pollination, acting at four main checkpoints (Figure 1): 1) receptive stigma cells on the surface of the female reproductive structure (the pistil) regulate adhesion to pollen grains, 2) stigmas cells also control water traffic to the desiccated pollen soon after adhesion has occurred, 3) ovary tissues monitor the growth and invasion of pollen tubes, and 4) unfertilized ovules precisely guide pollen tubes to the eggs. Each of these steps requires the interplay of signaling molecules; pollen grains are recognized by female cells, and in turn, female cells stimulate or inhibit pollen progression. One of the more surprising discoveries of the past few years was the remarkable nature of the adhesive interactions between pollen and the stigma (Zinkl et al., 1999). Once thought to be a passive event, we now know that plants use this step to recognize compatible pollen grains. Despite a dry cell surface, pollen of an appropriate species binds tightly upon contacting a stigma cell. This adhesion reaction is extremely strong—the binding force is 10 7 N per pollen grain, large enough for the stigma to capture pollen as it passes by on a gentle breeze or an insect’s bristles. Thus, from the very first step, cell–cell interactions dominate plant mating. Much of our work is now focused on identifying the adhesive molecules that mediate pollen–stigma binding. Reconstitution experiments demonstrate that on the male side, the adhesives reside in the pollen wall, the exine. Unlike previously characterized adhesion proteins, exine adhesives are extremely resistant to treatments with proteases, strong acids, and heat, and they also resist extraction with aqueous and organic solutions, suggesting nonproteinaceous molecules (Zinkl et al., 1999). Taken together, the chemical analyses performed to date point toward a lipophilic moiety that mediates adhesion, perhaps a lipopolysaccharide. The female receptor for the pollen identification tag is not yet known, and one of the exciting challenges will be to determine if, on the stigma surface, adhesion is mediated by proteins or by other polymers. Exine components alone are not sufficient for pollen recognition—the hydrophobic coat embedded in the surface of the exine also plays a vital role. When the dry surfaces of pollen and stigma cells come into contact, the coat becomes highly motile, with its protein and lipid moieties congealing at the cellular interface (Elleman et al., 1992). This massive migration of coat material creates a patch through which water moves. It is a marvelous mechanism for establishing communication in a nonaqueous setting. The pollen coat is extremely hydrophobic and can be extracted from the exine with cyclohexane or other organic solvents. We used this method to characterize the composition of the Arabidopsis coat, purifying sufficient material to define the resident lipids and proteins. Gas chromatography and mass spectroscopy indicate the lipids often have a chain length of 26 or more carbons, a make-up that suggests a Article published online ahead of print. Mol. Biol. Cell 10.1091/ mbc.ES–01–0001. Article and publication date are at www.molbiolcell.org/cgi/doi/10.1091/mbc.ES–01–0001. * Corresponding author. E-mail address: dpreuss@midway. uchicago.edu.