Pollination drops as dynamic apoplastic secretions

Pollination drops are apoplastic secretions that are involved in gymnosperm reproduction. Much of the research on pollination drops has been concerned with morphological and phenological descriptions of the drop. Composition analysis has been limited by the phenology and small volume of the drop. Improved proteomics techniques have enabled a more comprehensive analysis of drop composition. It is emerging that the drop is biochemically complex. Many of the components are found in other apoplastic secretions, such as the cell wall proteome, reproductive secretions in angiosperms, floral and extrafloral nectar. In these liquids, proteins confer a variety of functions. In this review, relevant apoplastic secretions are reviewed with an aim to develop a more comprehensive model of dynamic pollination drop function. Comparing pollination drops to other apoplastic secretions reveals similarities that suggest the drop may facilitate pollen–ovule interactions and ovule defense. An example of enzyme activity in the pollination drop is provided. Chitinase activity was detected in the drop of Pseudotsuga menziesii (Douglas-fir) using both a biochemical and an in-gel assay. This shows that one of the putatively identified proteins is active and probably assists in defending the ovule against fungi.