New studies conducted on plant mating have concluded that humans may not be the only species rejected by the opposite sex. According to researchers from the University of Missouri, pollen proteins may contribute to a signaling process that determines if a female plant accepts or rejects certain pollen grains.
This process in plant reproduction can be compared to the human “mating game.” Since plants reproduce by means of wind or animals, and do not actually choose their mates, the female reproductive component of the flower, called the pistil, decides if the male pollen grain delivered is acceptable. Through a molecular process, the pollen grain communicates with pistil and discloses information that will allow the pistil to know that the grain delivered is of the correct species.
Scientists used two pistil proteins called NaTTS and 120K, which contribute to the growth of pollen down the pistil, to test the binding of pollen proteins to complementary pistil proteins. The team of researchers compared the experiment to Velcro. The two pistil proteins would act as the “hook” portion of the Velcro, screening for the pollen grain with complementary “hooks.” If the Velcro matches, then plant fertilization and reproduction is a success.
With knowledge of this “molecular conversation” taking place in plants, cross-fertilization between cross species can be reduced and fruit trees can be grown more efficiently.

Link to article: http://www.sciencedaily.com/releases/2008/10/081023113107.htm

Link to abstract: http://www.jbc.org/cgi/content/full/283/40/26965