Abstract
Key messageComplex protein-containing reproductive secretions are a conserved trait amongst all extant gymnosperms; the pollination drops of most groups include carbohydrate-modifying enzymes and defence proteins.Pollination drops are aqueous secretions that receive pollen and transport it to the ovule interior in gymnosperms (Coniferales, Cycadales, Ginkgoales, Gnetales). Proteins are well established as components of pollination drops in conifers (Coniferales) and Ephedra spp. (Gnetales), but it is unknown whether proteins are also present in the pollination drops of cycads (Cycadales), Ginkgo (Ginkgoales), Gnetum (Gnetales), or in the pollination drops produced by sterile ovules occurring on pollen plants in the Gnetales. We used liquid chromatography–tandem mass spectrometry followed by database-derived protein identification to conduct proteomic surveys of pollination drops collected from: Ceratozamia hildae, Zamia furfuracea and Cycas rumphii (Cycadales); Ginkgo biloba (Ginkgoales); Gnetum gnemon and Welwitschia mirabilis, including pollination drops from both microsporangiate and ovulate plants (Gnetales). We identified proteins in all samples: C. hildae (61), Z. furfuracea (40), C. rumphii (9), G. biloba (57), G. gnemon ovulate (17) and sterile ovules from microsporangiate plants (25) and W. mirabilis fertile ovules (1) and sterile ovules from microsporangiate plants (138). Proteins involved in defence and carbohydrate modification occurred in the drops of most groups, indicating conserved functions for proteins in pollination drops. Our study demonstrates that all extant gymnosperm groups produce complex reproductive secretions containing proteins, an ancient trait that likely contributed to the evolutionary success of seed plants.
Highlights
Pollination drops are secreted from the ovules of most gymnosperms near the time of pollination (Fig. 1)
Proteomic analyses of pollination drops from cycads, Ginkgo and female cones of the Gnetales identified: 61 proteins in C. hildae, 40 proteins in Z. furfuracea and 9 proteins in C. rumphii; 57 proteins in G. biloba; 17 proteins in G. gnemon; 1 protein in W. mirabilis (Table 1)
PEAKS scores, TAIR gene models based on BLASTp matches, and their E-values, as well as the corresponding gene ontology (GO) annotations for biological process and cellular component are given for all species in Online Resource 3
Summary
Pollination drops are secreted from the ovules of most gymnosperms near the time of pollination (Fig. 1). These minute drops of liquid, 10–1000 nL, play a critical role in gymnosperm reproduction (Prior et al 2013). Pollination drops receive pollen and move it to the ovule interior where it germinates (Gelbart and von Aderkas 2002). The delivery of pollen directly to the ovule is a characteristic feature of gymnosperms, seed plants whose ovules are exposed around the time of pollination (Tomlinson 2012; Tomlinson and Takaso 2002). The term gymnosperm is traditionally applied to four groups of extant seed plants, i.e. cycads, Ginkgo, Gnetales and conifers, and to many other extinct lineages. Cycads (Singh 1978), Ginkgo
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