A pollen grain germinating on a stigmatic papilla undergoes an SI response if theS-haplotypes match (here, PrsS1with PrpS1). Note that, in contrast to the Brassicaceae, the rejection response takes place in the pollen. PrpS, the pollenS-determinant, is a transmembrane protein that acts as a receptor for the femaleS-determinant, PrsS, which is a ligand secreted by the stigma. Identification of a requirement for the glycosylphosphatidylinositol (GPI)-inositol deacylase HLD1/PGAP1 demonstrates that inositol deacylation, required for maturation of GPI-anchored proteins (GPI-APs), is critical for the SI response. It is proposed that, as well as theS-specific interaction of PrsS with plasma-membrane-localised PrpS, SI induction requires interaction (direct or indirect) of PrpS-PrsS with (as yet unknown and possibly cleaved) GPI-APs, which may act as co- or accessory receptors. After a cognate interaction, the SI pathway is rapidly activated to reject the incompatible pollen, first by inhibiting tip growth and then by initiating programmed cell death (PCD) to ensure prevention of fertilisation. Cognate secreted stigma PrsS interacts with PrpS, located at the pollen plasma membrane, triggering a signalling network involving rapid Ca2+influx (though the nature of the channel involved is not yet known) and increases in cytosolic free calcium ([Ca2+]cyt). Increases in reactive oxygen species (ROS), dramatic depletion of ATP and acidification of cytosolic pH ([pH]cyt) occur within 10 minutes of SI induction. ATP depletion is likely to cause H+-ATPase pump inactivation. Activity of a soluble inorganic pyrophosphatase (sPPase) is inhibited, resulting in inhibition of cellular biosynthesis. Increases in [Ca2+]cytand ROS and a reduction in [pH]cytalso trigger alterations to the actin cytoskeleton, which rapidly undergoes severing and depolymerisation. Clathrin-mediated endocytosis is also rapidly inhibited by SI. These very early events all contribute to the rapid inhibition of pollen tube tip growth within 1-2 minutes of SI initiation. Increases in [Ca2+]cytand ROS and a reduction in [pH]cytsignal to several downstream targets to trigger PCD. An MPK9 homologue is activated by phosphorylation and plays a central role in SI, being required for SI-induced actin alterations and PCD. The actin fragments subsequently aggregate into distinctive, highly stable actin foci that can trigger PCD. The massive (but incomplete) ATP depletion creates a cellular energy crisis that not only rapidly inhibits pollen tube growth, but also triggers a drop in [pH]cyt.This plays a key role in several pivotal SI-induced events, including actin remodelling. Cytosolic acidification is critical for initiation of PCD because the caspase-3-like DEVDase enzyme is inactive at normal [pH]cyt. Activation of this enzyme, several hours after the initiation of the signalling network, seals the fate of the incompatible pollen, with a commitment to cell suicide ensuring that the inhibited SI pollen tubes do not achieve fertilisation.
