Exine and tapetum development in Symphytum officinale (Boraginaceae). Exine substructure and its interpretation

Abstract

After detailing the exine ontogeny, our purpose was to find out whether the sequence of sporoderm developmental events corresponds to self-assembling micellar mesophases, initiated by genomically determined physicochemical parameters and induced by surfactant glycoproteins at increasing concentrations. Indeed, a scaffolding of the future exine, i.e., the glycocalyx, initiates with scattered clots, which then appear as clusters of spherical and worm-like micelles, derived from surface-active glycoproteins. At the middle tetrad stage, a continuous layer of the glycocalyx emerges, consisting of parallel, tightly packed cylinder-like units, which we interpret as a layer of cylindrical micelles, the so-called middle mesophase. These units bear dark-contrasted particles, arranged in strings or columns. These sites of the glycocalyx units–micelles accumulate initial sporopollenin, hence the term “sporopollenin acceptor particles” (SAPs). This process leads to the appearance of procolumellae at the late tetrad stage. The glycocalyx units are rooted into callose and into the microspore cytoplasm. After formation of the tectum and the foot layer, the endexine initiates as a thin layer, and the latter develops into a very thick layer in the post-tetrad period. When callose disintegrates, “bouquets” of SAPs become evident on the tectum, which were evidently hidden inside the callose layer; these structures self-assemble into supratectal gemmae. An unusual, “hybrid” type of tapetum was observed. What is observed in Symphytum exine development allows us to obtain more evidence for the hypothesis of the participation of micellar self-assembly in sporoderm development and to bring together the concepts of micelles and of SAPs.