Sporoderm and tapetum ontogeny in Juniperus communis (Cupressaceae). Connective structures between tapetum and microspores

• Outer granular ectexine is formed on the base of spherical micelles.
• Inner ectexine consists of strings and appears on the base of cylindrical micelles.
• Lamellate endexine emerges on the base of laminate micelles.
• Connective filaments between microspore and tapetum are long cylindrical micelles.
• Genomic regulation of the exine pattern includes self-assembly mechanisms.

The purpose of this work was to study in detail the successive stages of pollen wall development in Juniperus communis and intimate connection between the microspores and nutritive tapetum with TEM. Another goal was to clear up the mechanisms underlying the developmental processes. The key stages observed are: the appearance of the initial outer glycocalyx layer as a row of spherical units at the early tetrad stage, followed by the emergence of the underlying radially oriented string-like units, resulting in the formation of a reticulate layer. After sporopollenin accumulation, spherical units turn into granules. String-like units start from the microspore plasma membrane, pierce the glycocalyx layer, pass between the primexine granules and through callose jacket and come out to the anther loculus, reaching the tapetal cells. Their substructure and size are similar to viscin threads, but not their function and chemical composition. All the structures, observed in sporoderm development, correspond to subsequent mesophases of a micellar system. The latter develops by self-assembly, under genomic control, regulating increasing concentration of glycoprotein surfactants in the periplasmic space. A thick network of filaments, connecting microspores and tapetum, supplies microspores with necessary nutrients and testifies the opinion that, in the process of development, exine is a living, dynamic system, arranged from plasmodesmata-like units.