Pollen-pistil interactions and self-incompatibility in the Asteraceae: new insights from studies of Senecio squalidus (Oxford ragwort)

Abstract

Background: Pollen-pistil interactions are an essential prelude to fertilization in angiosperms and determine compatibility/incompatibility. Pollen-pistil interactions have been studied at a molecular and cellular level in relatively few families. Self-incompatibility (SI) is the best understood pollen-pistil interaction at a molecular level where three different molecular mechanisms have been identified in just five families. Here we review studies of pollen-pistil interactions and SI in the Asteraceae, an important family that has been relatively understudied in these areas of reproductive biology.

Scope: We begin by describing the historical literature which first identified sporophytic SI (SSI) in species of Asteraceae, the SI system later identified and characterized at a molecular level in the Brassicaceae. Early structural and cytological studies in these two families suggested that pollen-pistil interactions and SSI were similar, if not the same. Recent cellular and molecular studies in Senecio squalidus (Oxford ragwort) have challenged this belief by revealing that despite sharing the same genetic system of SSI, the Brassicaceae and Asteraceae molecular mechanisms are different. Key cellular differences have also been highlighted in pollen-stigma interactions, which may arise as a consequence of the Asteraceae possessing a 'semi-dry' stigma, rather than the 'dry' stigma typical of the Brassicaceae. The review concludes with a summary of recent transcriptomic analyses aimed at identifying proteins regulating pollen-pistil interactions and SI in S. squalidus, and by implication the Asteraceae. The Senecio pistil transcriptome contains many novel pistil-specific genes, but also pistil-specific genes previously shown to play a role in pollen-pistil interactions in other species.

Conclusions: Studies in S. squalidus have shown that stigma structure and the molecular mechanism of SSI in the Asteraceae and Brassicaceae are different. The availability of a pool of pistil-specific genes for S. squalidus offers an opportunity to elucidate the molecular mechanisms of pollen-pistil interactions and SI in the Asteraceae.

Fig. 1.

Stages of floret development in S. squalidus. (A) Developing captiulum; arrows indicate florets of different developmental stages; scale bar = 2 mm. (B) Ray floret; scale bar = 1 mm. (C) Immature disc floret. (D) Disc floret stage 1 (anther mature). (E) Disc floret stage 2 (pistil mature). Scale bars in (C–E) = 0·5 mm.

Fig. 2.

Section through a Senecio squalidus capitulum, showing individual disc florets in different stages of development. Inset pictures show detail of pistils: (A) entire pistil; (B) developing pistil; (C) emerging pistil. Section stained with Toluidine blue. Scale bars = 5 mm (main image), and 1 mm (insets).

Fig. 3.

(A) Illustration of Senecio squalidus pistil. (B) Squash preparation of S. squalidus stigma, stained with aniline blue (section of pistil indicated by hatched line in A). P, pseudo-papillae cells; scale bar = 0·1 mm. (C) Detail of stigma papillae cells with pollen grains attached (indicated by black arrow in A); scale bar = 25 µm. (D) Detail of pollen tube penetrating papillae cells; scale bar = 5 µm.

Fig. 4.

The semi-dry stigma of Senecio squalidus stained for the presence of lipids, peroxidase and reactive oxygen species (ROS). (A) Stigma stained with Sudan black b to visualize the presence of lipids, indicated by black staining. (B) Control stigma stripped of lipids by placing in 10 % SDS prior to staining. (C) Stigma stained with 0·1 m guaiacol, 0·1 m H₂O₂, in 20 mm phosphate buffer, pH 4·5, to visualize peroxidase activity. (D) Control of C. (E) Stigma stained with TMB-HCl (3,39,5,59-tetramethylbenzidine-HCl, 0·1 mg ml⁻¹ in TRIS acetate, pH 5·0) showing the localized activity of ROS in papillae cells, indicated by blue staining. (F) Control of (E). Scale bars = 50 µm.

Fig. 5.

Incompatible and compatible pollinations in Senecio squalidus. Squash preparations of stigmas stained with aniline blue and viewed under UV light. (A,B) Incompatible pollination; pollen tube (arrow) blocked from entering papillae (P). (C) Compatible pollination; pollen tubes penetrating stigma tissue. (D) Compatible pollen tube growing through transmitting tissue (arrow). Scale bars = 0·25 µm.

Fig. 6.

In situ hybridizations on longitudinal sections of Senecio squalidus pistils. Pistils at ovule developmental stages 2 and 3 hybridized with SF21 antisense probe (A,C,E) and hybridized with sense probe (B,D,F). (A) Base of pistil with staining in ovules. (C) Expression is localized to the integument cells surrounding the embryo sac and transmitting tissue immediately above ovule (black arrow). (E) Expression was also detected in mature pollen grains. Scale bars: (A,B) = 50 µm; (C–F) = 25 µm.

Fig. 7.

In situ hybridizations on longitudinal sections of mature Senecio squalidus pistils hybridized with MAP antisense probe (A–C) and hybridized with sense probe (D). (A) Upper section of emerging pistil showing expression in papillar cells of stigma (black arrows) and transmitting tissue (white arrow). (B) Fully mature and reflexed pistil exhibiting expression in papillar cells (black arrows). No expression was detected in pseudopapillae cells at the tips of the stigma. (C) Close-up of papillar cells. (D) Corresponding sense control showing no expression in papillar cells (black arrow) or transmitting tissue (white arrow). Scale bars = 50 µm.

Fig. 8.

In situ hybridizations on longitudinal sections of mature Senecio squalidus pistils hybridized with Nodulin antisense probe (A,B) and hybridized with sense probe (C). (A) Upper section of emerging pistil showing expression in papillar cells of stigma (black arrows). (B) Fully mature and reflexed pistil exhibiting increased expression in papillar cells (black arrow). No expression was detected in pseudopapillae cells (unfilled arrow). Staining of pollen grains occurred due to high levels of intrinsic pollen cytosolic alkaline phosphatase activity (Knox and Heslop-Harrison, 1969); no expression of Nod was detected in pollen via northern blot analysis (Fig. 7B). (C) Corresponding sense control showing no expression in papillar cells. Scale bars = 50 µm.