Abstract
Early events occurring at the surface of the female organ are critical for plant reproduction, especially in species with a dry stigma. After landing on the stigmatic papilla cells, the pollen hydrates and germinates a tube, which penetrates the cell wall and grows towards the ovules to convey the male gametes to the embryo sac. In self-incompatible species within the Brassicaceae, these processes are blocked when the stigma encounters an incompatible pollen. Based on the generation of self-incompatible Arabidopsis lines and by setting up a live imaging system, we showed that control of pollen hydration has a central role in pollen selectivity. The faster the pollen pumps water from the papilla during an initial period of 10 min, the faster it germinates. Furthermore, we found that the self-incompatibility barriers act to block the proper hydration of incompatible pollen and, when hydration is promoted by high humidity, an additional control prevents pollen tube penetration into the stigmatic wall. In papilla cells, actin bundles focalize at the contact site with the compatible pollen but not with the incompatible pollen, raising the possibility that stigmatic cells react to the mechanical pressure applied by the invading growing tube.
Highlights
Flowers of Brassicaceae species have a dry stigma, highly discriminatory with early control of pollen capture following pollination (Dickinson, 1995)
Stage 13–14E stigmas from these four lines rejected pollen from the original A. lyrata S14 plant, whereas A. lyrata pollen produced numerous pollen tubes on Col-0 stigmas (Supplementary Fig. S1A–C).We quantified the expression level of the AlSRK14 transgene in stage 13–14E stigmas of two of the strongest transgenic plants and found that the relative expression was twice as low as that detected in A. lyrata S14 stigmas but still sufficient to induce the self-incompatibility response (Supplementary Fig. S1D)
In spite of several attempts to introduce AlSCR14 into Col-0, we did not succeed in generating transgenic pollen that could be rejected by stigmas of Col-0/AlSRK14
Summary
Flowers of Brassicaceae species have a dry stigma, highly discriminatory with early control of pollen capture following pollination (Dickinson, 1995). The stigma consists of a dome of flask-shaped epidermal cells (papillae). Pollen grains need to hydrate on the stigma so as to permit the emergence of a pollen tube that penetrates the papilla cell wall and grows within the pistil tissues (reviewed in Chapman and Goring, 2010). In vitro assays showed that growing through the stigma is required for pollen tubes to efficiently target the ovules (Palanivelu and Preuss, 2006). Genetic ablation of the stigmatic papillae prevents normal pollen hydration or germination (Tung et al, 2005).
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