A Pollen Coat–Inducible Autoinhibited Ca2+-ATPase Expressed in Stigmatic Papilla Cells Is Required for Compatible Pollination in the Brassicaceae

Abstract

In the Brassicaceae, intraspecific non-self pollen (compatible pollen) can germinate and grow into stigmatic papilla cells, while self-pollen or interspecific pollen is rejected at this stage. However, the mechanisms underlying this selective acceptance of compatible pollen remain unclear. Here, using a cell-impermeant calcium indicator, we showed that the compatible pollen coat contains signaling molecules that stimulate Ca 2+ export from the papilla cells. Transcriptome analyses of stigmas suggested that autoinhibited Ca 2+ -ATPase13 (ACA13) was induced after both compatible pollination and compatible pollen coat treatment. A complementation test using a yeast Saccharomyces cerevisiae strain lacking major Ca 2+ transport systems suggested that ACA13 indeed functions as an autoinhibited Ca 2+ transporter. ACA13 transcription increased in papilla cells and in transmitting tracts after pollination. ACA13 protein localized to the plasma membrane and to vesicles near the Golgi body and accumulated at the pollen tube penetration site after pollination. The stigma of a T-DNA insertion line of ACA13 exhibited reduced Ca 2+ export, as well as defects in compatible pollen germination and seed production. These findings suggest that stigmatic ACA13 functions in the export of Ca 2+ to the compatible pollen tube, which promotes successful fertilization.

RNA silencing of exocyst genes in the stigma impairs the acceptance of compatible pollen in Arabidopsis

“…As well, one very strong cargo candidate is the Autoinhibited Ca 2+ Adenosine Triphosphatase13 (ACA13) that is required in the stigma for compatible pollinations and proposed to pump extracellular Ca 2+ to the growing pollen tube (see model in Fig. 7; Iwano et al, 2014). ACA13 has Ca 2+ transporter activity and was found to rapidly localize from Golgi-associated vesicles to the papillar plasma membrane under the compatible pollen grain .…”

“…Proposed model of exocyst function during compatible pollenstigma interactions in Arabidopsis. A, Early stages of the Arabidopsis compatible pollen response: (1) the pollen exine and pollen coat mediate pollen adhesion to stigmatic papilla; (2) the pollen grain hydrates at 5 to 10 min postpollination; and (3) the pollen tube penetrates and grows through papillar cell wall at 15 to 20 min postpollination (Preuss et al, 1993;Zinkl et al, 1999;Iwano et al, 2004Iwano et al, , 2014Safavian and Goring, 2013). B, Following compatible pollination, an unknown signaling event (black arrow) is proposed to initiate exocyst complex assembly and docking of secretory vesicles to the papillar plasma membrane for delivery of cargo to the compatible pollen grain.…”

The Full-Size ABCG Transporters Nb-ABCG1 and Nb-ABCG2 Function in Pre- and Postinvasion Defense against Phytophthora infestans in Nicotiana benthamiana

“…pollen coat layers but keep pollen viable (Doughty et al, 1993;Iwano et al, 2014) and capsidiol is soluble in cyclohexane (Matsukawa et al, 2013), we used cyclohexane to wash secreted capsidiol from INF1-treated leaves. Leakage of chlorophyll was not detected by washing with cyclohexane for 30 s, indicating that there was no major damage to leaf cells (Supplemental Figure 9).…”

The ARC1 E3 Ligase Promotes a Strong and Stable Self-Incompatibility Response in Arabidopsis Species: Response to the Nasrallah and Nasrallah Commentary

“…Exo70A1 is proposed to assemble with the remaining subunits of the exocyst complex to dock secretory vesicles (reviewed in Zá rský et al, 2013). SNARE proteins mediate vesicle fusion to the plasma membrane, and unknown cargo (ACA13 as one candidate; Iwano et al, 2014) are released to enable pollen hydration pollen tube entry through the stigmatic papillar cell wall (compatible pollen is accepted). (D) Model for the reconstituted self-incompatibility signaling pathway in the Sha ecotype.…”