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Flowers' Attractive Features
A team of Japanese researchers has identified a key component of the attraction between the sexes--in flowers--in the 24 August 2001 issue of the international journal Science.
Gradual attraction of the pollen tube towards the embryo sac, guided by the synergid cells (not shown). The arrowhead points to the end of the embryo sac, and the arrow indicates the tip of the pollen tube.
In flowering plants, fertilization is facilitated by the pollen tube, which grows out from the pollen grain to deliver the male gametes to the female egg. But what guides the pollen tube towards its eventual target? Tetsuya Higashiyama of the University of Tokyo, Japan, and colleagues now identify the two synergid cells, which lie adjacent to the egg cell in an embryo sac, as the signalers that lure the pollen tube towards its final destination.
Using laser ablation, the scientists determined that only one of the synergid cells is necessary to attract the pollen tube, while the second cell appears to boost the signal. After fertilization, one synergid cell degenerates and the other loses its powers of attraction, possibly providing a defense against multiple fertilization attempts.
In a related Perspective, Alice Y. Cheung and Hen-ming Wu of the University of Massachusetts, Amherst note that attraction of pollen tubes through synergid cells is likely to be common, if not universal, among flowering plants.
-- Becky Ham