An integral insight into pollen wall development: involvement of physical processes in exine ontogeny in Calycanthus...

We aimed to understand the underlying mechanisms of development in the sporopollenin-containing part of the pollen wall, the exine, one of the most complex cell walls in plants. Our hypothesis is that distinct physical processes, phase separation and micellar self-assembly, underpinexine development by taking the molecular building blocks, determined and synthesised by the genome, through several phase transitions. To test this hypothesis, we traced each stage of microspore development in Calycanthus floridus with transmission electron microscopy and then generated in vitro experimental simulations corresponding to every developmental stage. The sequence of structures observed within the periplasmic space around developing microspores starts with spherical units, which are rearranged into columns to then form rod-like units (the young columellae) and, finally, white line centred endexine lamellae. Phase separation precedes each developmental stage. The set of experimental simulations, obtained as self-assembled micellar mesophases formed at the interface between lipid and water compartments, was the same: spherical micelles; columns of spherical micelles; cylindrical micelles; and laminate micelles, separated by gaps, resembling white-lined lamellae. Thus, patterns simulating structures observed at the main stages of exine development in C. floridus were obtained from in vitro experiments, and hence purely physicochemical processes can construct exine-like patterns. This highlights the important part played by physical processes that are not under direct genomic control and share influence on the emerging ultrastructure with the genome during exine development. These findings suggest that a new approach to ontogenetic studies, including a consideration of physical factors, is required for a better understanding of developmental processes.