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In the process of pollination, haploid pollen germinates on the stigma surface and a pollen tube grows through the diploid tissues of the pistil toward the ovary. The pistil has two basic functions: to prevent unwanted pollen from gaining access to the ovary and to support the growth of desirable pollen. Pollen-pistil signaling allows these different types of pollen to be distinguished. Self-incompatibility (SI) systems, controlled by the S locus, are the best-understood pollen-pistil signaling systems. Other SI systems have been investigated at the molecular level, but the physiology of pollen tube rejection is best understood in the field poppy, Papaver rhoeas. This species has a gametophytic SI system: Pollen is rejected when its S haplotype is the same as either of the two S haplotypes expressed in the diploid pistil. Recent advances reveal new ways that SI controls pollen tube metabolism. A soluble pyrophosphatase is down-regulated as part of the rapid SI response, and, over the long term, perturbations of the actin cytoskeleton lead to programmed cell death in incompatible pollen tubes. Manipulating incompatible pollen tube metabolism in this way may leave more resources available for supporting the growth of compatible pollen tubes, the complementary function of the pistil.