Complex reproductive secretions occur in all extant gymnosperm lineages: a proteomic survey of gymnosperm pollination drops

Abstract

Key message Complex 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. Abstract 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. Electronic supplementary material The online version of this article (10.1007/s00497-018-0348-z) contains supplementary material, which is available to authorized users.

Pollination Drop Proteome and Reproductive Organ Transcriptome Comparison in Gnetum Reveals Entomophilous Adaptation

“…The sampling was performed from 1930–2130 h during peak pollination times [14,21], although the pollination drops were sometimes additionally collected from 0830–1030h when the ambient humidity exceeded 90%. Our drop sampling strategy follows the methods applied in previous studies [30,31,33,34,35,63,64]. The female and male drops of G. gnemon/G.…”

“…These proteins, found in the apoplast, are secreted by the nucellus and are referred to as secretome proteins. In contrast, the proteins that are not detected in the apoplast are derived from the breakdown of the nucellus during the formation of a pollen chamber, and are known as degradome proteins [28,34,35]. Degradome proteins in the pollination drops of gymnosperms are probably associated with degenerated megaspores [36] and the nucellus degeneration mediated by programmed cell death (PCD) [37].…”

“…Pollination drops in the Gnetales have a typical function, with fertile reproductive units (FRUs) and sterile reproductive units (SRUs) producing sugar-rich drops as a reward for insect pollinators [7,8,9,11,19]. A recent study [35] has shown that protein numbers in the pollination drops of FRUs (henceforth female drops) in G. gnemon L. and W. mirabilis (17 and one, respectively) are considerably fewer than in those of SRUs (male drops, 25 and 138 in the two species, respectively). The reasons why the protein numbers in female and male drops differ so markedly and their association with insect pollination have yet to be convincingly explained.…”

“…This might be associated with the fact that the total sugar concentration (TSC) in the male drops of G. gnemon was significantly lower than the female drops, although the relative proportion of different sugar types was identical between the female and male drops [27]. In addition, many degradome proteins were reported in the female drops of seven Ephedra species, as well as male drops of G. gnemon and W. mirabilis [34,35]. The degradome proteins are the consequence of cell apoptosis during the formation of the pollen chamber, as reported in Ephedra [34,38] and Gnetum [39].…”

The Evolution of Sexual Fluids in Gymnosperms From Pollination Drops to Nectar

“…Insect-pollinated species in which pollination drops are functioning as nectar have much larger drops than insect-pollinated species in which only pollen is the reward. For example, pollination drops of Gnetum , a group that uses nectar as its primary reward, are in the 150–200 nL range (Kato et al, 1995), whereas pollination drops of cycads, a group that uses pollen as its primary reward, have volumes an order of magnitude less (Prior, 2014). Pollination drops of wind-pollinated species have small volumes (20–100 nL).…”