Mechanisms of pollen wall development in Lysimachia vulgaris.

Exine, this complex sporopollenin-containing and highly variable among taxa envelope of the male gametophyte, consists of two layers, ectexine and endexine. We traced in detail the pollen wall development in Lysimachia vulgaris (Primulaceae), with emphasis on driving forces and critical ontogenetic time. By observation on the sequence of the emergent patterns and by analysis of their substructure with TEM, we intended to clarify the obvious and not-obvious ways of exine construction and to find out the common features in pattern development in other representatives in living nature. The ectexine and endexine ontogeny follows the main stages observed in many other species: first, the appearance of microspore plasma membrane invaginations with isotropic contents within, changed later to anisotropic state; then successive appearance of spherical, rod-like, and lamellate units in the periplasmic space. The lamellate endexine appears unusually early in the exine development. All these elements and their aggregations are manifestation of well-known physical phenomena: phase separation and micellar self-assembly. A consideration of similar surface patterns in very remote taxa suggests the participation in their development of some general nature phenomena as the lows of space-filling operations.

Pollen wall development in Impatiens glandulifera: exine substructure and underlying mechanisms.

The aim of this study was to investigate in detail the pollen wall ontogeny in Impatiens glandulifera, with emphasis on the substructure and the underlying mechanisms of development. Sporopollenin-containing pollen wall, the exine, consists of two parts, ectexine and endexine. By determining the sequence of developing substructures with TEM, we have in mind to understand in which way the exine substructure is connected with function. We have shown earlier that physical processes of self-assembly and phase separation are universally involved in ectexine development; currently, we try to clear up whether these processes participate in endexine development. The data received were compared with those on other species. The ectexine ontogeny of I. glandulifera followed the main stages observed in many other species, including the late tetrad stage named "Golden gates". It turned out that the same physico-chemical processes act in endexine development, especially expressed in aperture sites. Another peculiar phenomenon observed in exine development was the recurrency of micellar sequence at near-aperture and aperture sites where the periplasmic space is widened. It should be noted that, in the whole, the developmental substructures observed during the tetrad and early post-tetrad period are similar in species with columellate exines. Evidently, these basic physical processes proceed, reiterating again and again in different species, resulting in an enormous variety of exine structures on the base of a relatively modest number of genes. Granular and alveolar exines emerge on the base of the same basic processes but are arrested at spherical and cylindrical micelle mesophases correspondingly.

Ontogenesis in miniature. Pollen wall development in Campanula rapunculoides.

MAIN CONCLUSION: Our findings suggest a reconsideration of pollen wall ontogeny process, entailing examination of physical factors, which enable a new understanding of exine developmental processes as self-formation. The pollen wall, the most complex cell wall in plants, is especially interesting as a model of ontogeny in miniature. By a detailed study of each developmental stage of Campanula rapunculoides pollen wall, we aimed to understand the establishment of complex pollen walls and the underlying developmental mechanisms. Other aim was to compare our current observations with studies in other species to reveal the common principles. We also tried to analyse the reasons for commonalities in ontogenies of exines in remote species. TEM, SEM, comparative methods were used in this study. The sequence of events leading to exine emergence from early tetrad stage to maturity is as follows: the appearance of spherical micelles in the periplasmic space and de-mixing of the mixture in periplasm (condensed and depleted layers); appearance of plasma membrane invaginations and columns of spherical micelles inside condensed layer; appearance of rod-like units, pro-tectum and thin foot layer; the appearance of spiral substructure of procolumellae and of dendritic outgrowths on the tops of procolumellae, of vast depleted zone in aperture sites; formation of the endexine lamellae on the base of laminate micelles; gradual twisting of dendritic outgrowths (macromolecule chains) into clubs on the tops of columellae and into spines; final sporopollenin accumulation. Our observations are consistent with the sequence of self-assembling micellar mesophases. Complex organisation of the exine is established through processes of self-assembly operating together with another physical process-phase separation. After genomic determination of the exine building substances, purely physical processes which are not under direct genomic control play an important role after genomic control of constructive substances. The comparison of the underlying mechanisms of exine development in remote species occurred to be general and similar to crystallisation. Our ontogenetic experience has shown the commonality of pollen wall ontogenies in remote species.