Post-pollination prefertilization drops affect germination rates of heterospecific pollen in larch and Douglas-fir.

Pollen of larch (Larix × marschlinsii) and Douglas-fir (Pseudotsuga menziesii) was used in homospecific and heterospecific crosses. Germination of heterospecific pollen in ovulo was reduced in post-pollination prefertilization drops. This provides evidence of selection against foreign pollen by open-pollinated exposed ovules in these two sister taxa, which share the same type of pollination mechanism. Of the other prezygotic stages in pollen-ovule interactions, uptake of pollen by stigmatic hairs did not show any selection. Pollen tube penetration of the nucellus was similar for hetero- and homospecific pollen tubes, but heterospecific tubes only delivered gametes in one cross. To test for differences in the post-pollination prefertilization drops of each species, drops were gathered and analysed. Glucose and fructose were present in similar amounts in Douglas-fir and larch, while sucrose was found in larch only. Other carbohydrates such as xylose and melezitose were species-specific. In P. menziesii, sucrose is absent due to its conversion to glucose and fructose by apoplastic invertases. In contrast, Larix × marschlinsii drops have sucrose because they lack apoplastic invertases. The presence of invertase activity shows that the composition of gymnosperm post-pollination prefertilization drops is not static but dynamic. Drops of these two species also differed in their calcium concentrations.

Nectar and pollination drops: how different are they?

BACKGROUND: Pollination drops and nectars (floral nectars) are secretions related to plant reproduction. The pollination drop is the landing site for the majority of gymnosperm pollen, whereas nectar of angiosperm flowers represents a common nutritional resource for a large variety of pollinators. Extrafloral nectars also are known from all vascular plants, although among the gymnosperms they are restricted to the Gnetales. Extrafloral nectars are not generally involved in reproduction but serve as 'reward' for ants defending plants against herbivores (indirect defence). SCOPE: Although very different in their task, nectars and pollination drops share some features, e.g. basic chemical composition and eventual consumption by animals. This has led some authors to call these secretions collectively nectar. Modern techniques that permit chemical analysis and protein characterization have very recently added important information about these sugary secretions that appear to be much more than a 'reward' for pollinating (floral nectar) and defending animals (extrafloral nectar) or a landing site for pollen (pollination drop). CONCLUSIONS: Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

Application of proteomics to the study of pollination drops.

PREMISE OF THE STUDY: Pollination drops are a formative component in gymnosperm pollen-ovule interactions. Proteomics offers a direct method for the discovery of proteins associated with this early stage of sexual reproduction. • METHODS: Pollination drops were sampled from eight gymnosperm species: Chamaecyparis lawsoniana (Port Orford cedar), Ephedra monosperma, Ginkgo biloba, Juniperus oxycedrus (prickly juniper), Larix ×marschlinsii, Pseudotsuga menziesii (Douglas-fir), Taxus ×media, and Welwitschia mirabilis. Drops were collected by micropipette using techniques focused on preventing sample contamination. Drop proteins were separated using both gel and gel-free methods. Tandem mass spectrometric methods were used including a triple quadrupole and an Orbitrap. • RESULTS: Proteins are present in all pollination drops. Consistency in the protein complement over time was shown in L. ×marschlinsii. Representative mass spectra from W. mirabilis chitinase peptide and E. monosperma serine carboxypeptidase peptide demonstrated high quality results. We provide a summary of gymnosperm pollination drop proteins that have been discovered to date via proteomics. • DISCUSSION: Using proteomic methods, a dozen classes of proteins have been identified to date. Proteomics presents a way forward in deepening our understanding of the biological function of pollination drops.