Plant development: Attractive ovules

How does a pollen tube seek out and find an ovule, its ultimate goal inside the female reproductive organs of a flower? And once an ovule has been found and fertilized, how are other pollen tubes prevented from attempting the same task? These interesting questions have been posed and considered for over 100 years, and direct answers will soon be available. A structural guidance mechanism by itself seems unlikely. It is true that specialized tissues guide and nurture pollen tube growth within the style and the ovary [1]. This defined path leads pollen tubes close to the target ovules, but the final step requires one, and only one, pollen tube to grow along the ovule’s stalk and seek out the small entrance to the ripe embryo sac (Fig. 1). At this point the two haploid sperm cells at the tip of the tube are delivered to one of the two synergids (the specialized ‘helper’ gametophyte cells that surround the egg cell). Then one sperm fuses with the egg and grows into the diploid embryo; the other joins the two haploid nuclei of the adjacent central cell and develops into the nutritive triploid endosperm [2].