Pollen Coat Proteomes of Arabidopsis thaliana, Arabidopsis lyrata, and Brassica oleracea Reveal Remarkable Diversity of Small Cysteine-Rich Proteins at the Pollen-Stigma Interface

The pollen coat is the outermost domain of the pollen grain and is largely derived from the anther tapetum, which is a secretory tissue that degenerates late in pollen development. By being localised at the interface of the pollen-stigma interaction, the pollen coat plays a central role in mediating early pollination events, including molecular recognition. Amongst species of the Brassicaceae, a growing body of data has revealed that the pollen coat carries a range of proteins, with a number of small cysteine-rich proteins (CRPs) being identified as important regulators of the pollen-stigma interaction. By utilising a state-of-the-art liquid chromatography/tandem mass spectrometry (LC-MS/MS) approach, rich pollen coat proteomic profiles were obtained for Arabidopsis thaliana, Arabidopsis lyrata, and Brassica oleracea, which greatly extended previous datasets. All three proteomes revealed a strikingly large number of small CRPs that were not previously reported as pollen coat components. The profiling also uncovered a wide range of other protein families, many of which were enriched in the pollen coat proteomes and had functions associated with signal transduction, cell walls, lipid metabolism and defence. These proteomes provide an excellent source of molecular targets for future investigations into the pollen-stigma interaction and its potential evolutionary links to plant-pathogen interactions.