Plant Sex: Pollen Tubes on the Move

Mae West said that anything worth doing is worth doing slowly. Plants would probably agree, if they could. Courtship in flowering plants begins when pollen lands on the receptive surface (stigma) of a carpel, the female part of the flower. The ensuing dance of biochemical signals and responses culminates in the flower's decision to accept or reject the suitor. A variety of molecules have been implicated in the recognition process, including flavanols, lipids, and receptor kinases.1,2 If the interaction is compatible, the pollen grain extends a tube that by squeezing through exudates and a matrix of glycoproteins and polysaccharides covering female cells, grows inside and down the connecting style portion of the carpel until it reaches the ovary at the bottom. Each pollen tube carries two nonmotile sperm thar are bound for egg-bearing structures called ovules inside the ovary. But how do pollen tubes so reliably end up where they belong? Wayward pollen tubes don't usually poke through the sides of the carpel because they respond to directional cues all along the way. Some of those cues are glycoproteins that also feed the growing tubes.3,4 Now a Japanese team from the University of Tokyo and Kyoritsu Women's Junior College confirms that ovules themselves beckon pollen tubes on the last leg of their journey.5 Using a technique called semi-in vitro culture, the group germinated pollen of Torenia (a common horticultural plant) on the stigmas of intact carpels. After tubes started growing, they removed the top 15 mm of the styles and placed them in semisolid nutrient medium with isolated ovules. Around six hours after pollination, pollen tubes emerged from the cut ends and meandered through the medium. Torenia is ideal for these experiments because its egg and a pair of associated cells called synergids protrude through a pore in the ovule, notes team member Tetsuya Higashiyama, a graduate student in the Department of Biological Sciences at the University of Tokyo. Since the egg apparatus is hidden in most other plants, Torenia's is more accessible to pollen tubes in the medium. Equally important, scientist-voyeurs can easily spy on plant foreplay through the "keyhole" of their microscope. What the Tokyo scientists saw was that some of the pollen tubes grew directly toward the protruding eggs and synergids. While the number of tubes that reached ovules was low (1-13 percent), probably due to the in vitro conditions, none approached heat-inactivated ovules or those previously fertilized. They also saw a lot of coitus interruptus: Pollen tubes failed to penetrate the egg apparatus, repeatedly coiling in front instead. The team concluded that virgin ovules attract pollen tubes with a chemical signal. Says Higashiyama, "if the directional signal is generated by the gradient in the concentration of chemoattractant, the gradient might be steep so that the narrow tip of the pollen tube detects [it]. This is consistent [with] the fact that the guidance appeared to be effective in a relatively short distance." Since "pollen tubes were not guided [when] synergids had been broken," Higashiyama thinks they are the source of the signal. This isn't the first time synergids have been implicated in pollen tube guidance. Synergids can contain lots of calcium ions,6 which influence the direction of pollen tube growth in vitro.7,8 Recognition and guidance mechanisms undoubtedly differ between species, and Higashiyama doesn't know very much about the Torenia signal, but he is now "doing basic characterization of the guidance activity to reveal its nature and precise origin." Barry A. Palevitz (palevitz@dogwood.botany.uga.edu) is a contributing editor for The Scientist and a professor of botany at the University of Georgia. L.K. Wilhelmi and D. Preuss, "Blazing new trails: pollen tube guidance in flowering plants," Plant Physiology, 113:307-12, 1997. M. Wolters-Arts et al., "Lipids are required for directional pollen-tube growth," Nature, 392:818-21, April 1998. A.Y. Cheung et al., "A floral transmitting tissue-specific glycoprotein attracts pollen tubes and stimulates their growth," Cell, 82:383-93, 1995. H.-m. Wu et al., "A pollen tube growth stimulatory glycoprotein is deglycosylated by pollen tubes and displays a glycosylation gradient in the flower," Cell, 82:395-403, 1995. T. Higashiyama et al., "Guidance in vitro of the pollen tube to the naked embryo sac of Torenia fournei," The Plant Cell, 10:2019-31, Dec. 1998. R. Chaubal and B.J. Reger, "Calcium in the synergid cells and other regions of pearl millet ovaries," Sexual Plant Reproduction, 5:34-46, 1992. J.P. Mascarenhas and L. Machlis, "Chemotropic response of Antirrhinum majus pollen to calcium." Nature, 196:292-3, 1962. B.J. Reger et al., "Chemotropic responses by pearl millet pollen tubes," Sexual Plant Reproduction, 5:47-56, 1992.

Plant Sex: Pollen Tubes on the Move

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