Pollen wall development in Hydrangea bretschneiderii Dippel. (Hydrangeaceae): advanced interpretation through physical input, with in vitro experimental verification.

We aimed to unravel the underlying mechanisms of pollen wall development in Hydrangea bretschneiderii. For this, we tested our hypothesis that distinct physical processes, phase separation and micellar self-assembly, underpinned exine development by taking the substances, determined by the genome, through several phase transitions. We traced each developmental stage with TEM; then, we obtained in vitro simulations corresponding to those stages. The main steps of exine ontogeny observed in the microspore periplasmic space were initiated with phase separation, resulting in the conversion of homogeneous contents to heterogeneous two-layered state of the material. After each step of phase, separation self-assembly picked up the initiative and took the substances through the sequence of micellar mesophases which were the base for all the exine structures. These mesophases are as follows: spherical micelles, transforming first into columns, and then to cylindrical micelles which turn to columellae after initial sporopollenin accumulation. The tectum appeared along the interface of the phase separated material. After the tetrad disintegration and the next phase separation, laminate mesophase appeared being the base for the endexine lamellae. Then, a new step of phase separation at aperture sites brought the appearance of a granular endexine layer; the latter became intermixed finally with lamellae. This gives, together with experimental simulation, strong evidence that the genome "shifts a part of work" on exine formation onto physical processes, and the latter are an inherent mechanism of evolution.