Stigma plasma membrane H + -ATPase is involved in pollen hydration and pollen tube penetration in Brassicaceae self-incompatibility

Abstract During pollination in self-incompatible Brassicaceae plants, the female papilla cells selectively transport water to non-self pollen after self or non-self discrimination through pollen-pistil interaction, and this process is crucial for fertilization. However, the mechanism of the selective water transport has not been elucidated. In this study, we determined that the stigma autoinhibited plasma membrane H⁺-ATPase (AHA) is involved in the selective water transport from papilla cells to pollen. We characterized AHA isoforms that are expressed in the stigma of self-incompatible plant, Brassica rapa by expression analysis. A chemical activator of AHA applied to stigmas suppressed pollen hydration and pollen tube penetration into stigma in cross-pollination of B. rapa. In contrast, an AHA inhibitor allowed pollen hydration and pollen tube penetration in self-pollination. Consistent with these pharmacological effects, stigma AHA was activated through the phosphorylation of penultimate Thr in response to self-pollination compared with cross-pollination. Furthermore, speed of pollen hydration and elongation of pollen tubes were delayed on the stigmas of ost2-2D, a constitutive active mutant of AtAHA1 in the self-compatible Arabidopsis thaliana. These results indicate that regulation of AHA activity in the stigma is involved in selective water transport for pollen hydration, possibly by regulating osmotic pressure in Brassicaceae self-incompatibility.