I. Primexine development in Passiflora racemosa Brot.: overlooked aspects of development

Abstract

Developmental stages during the tetrad period were examined in detail by transmission electron microscopy with an emphasis on substructure. Our purpose was to find out whether the sequence of sporoderm developmental events provides additional evidence for our recent hypothesis on the underlying cause of exine ontogeny as a sequence of self-assembling micellar mesophases initiated by genomically given physicochemical parameters. Osmiophilic globules encrusting the surface of postmeiotic microspores and tapetal cells are temporary prepattern units which come first. The second prepattern structures are highly ordered bundles of microfilaments and microtubules which determine the position of microspore surface invaginations and clusters of the glycocalyx inside them. The first glycocalyx units are microgranules which during the middle tetrad stage rearrange into radially oriented rodlike units. The latter form lens-like clusters of the glycocalyx-1, located inside the invaginations. These clusters predestine the position of the future luminae in the exine reticulum. The second glycocalyx layer is laid down as a continuous layer over the whole microspore surface and has similar substructure, that is radial rods. Glycocalyx-2 is a framework for procolumellae which appear at the late tetrad stage. Therefore, the sequence of substructural units in the primexine is: globules, microgranules, rod-like units, and layers of radially oriented rods tightly packed in the periplasmic space. This sequence corresponds to the first three mesophases of self-assembling micelles: spherical micelles, cylindrical micelles, and layers of hexagonally packed cylindrical micelles (middle mesophase). We observed the same sequence in other species during primexine development, and the findings of this study provide new evidence for our hypothesis.

Experimental modelling of exine-like structures

“…The more species from different plant groups we study for sporoderm development, the more evidence we accumulate for the idea that the sequence of sporoderm developmental events represents, in essence, the sequence of self-assembling micellar mesophases, initiated by genomically given physicochemical parameters and induced by surfactant glycoproteins of the glycocalyx at increasing concentrations (Gabarayeva, 2000: basal angiosperms;Gabarayeva et al, 2009aGabarayeva et al, , 2009bGabarayeva et al, , 2010aGabarayeva et al, , 2010bGabarayeva et al, , 2011aGabarayeva et al, , 2011bGabarayeva et al, , 2013aGabarayeva et al, , 2013b: Trevesia burckii Boerl., Persea americana Mill., Acer tataricum L., Alsophila setosa Kaulf., Symphytum officinale L., Passiflora racemosa Brot. ; Moore et al, 2009: Osmunda regalis L.; Blackmore et al, 2010: Compositae;Gabarayeva & Grigorjeva, 2010: Chamaedorea microspadix Burret, Swida alba (L.) Opiz in Bercht.…”

Artificial pollen walls simulated by the tandem processes of phase separation and self‐assemblyin vitro

“…The reiteration of the other physical process (self‐assembly) is also typical. For instance, micellar mesophases, observed earlier in Passiflora (and other species) (Gabarayeva et al , 2013a,b), occur twice in exine development. In general, self‐assembling mechanisms and their nonlinear nature presuppose the effect of reiteration of similar patterns in many spore and pollen taxa – the manifestation of homoplasy.…”

“…We think that self‐assembly and phase separation work ‘hand in hand’. It is clear that this phenomenon is especially demonstrated in species with ‘two‐storied’ exines, such as Passiflora (Gabarayeva et al , 2013a,b; Radja et al , ), Tanacetum (Gabarayeva et al , ), and Echinops (Gabarayeva et al , ). Reconsideration of the developmental events in, for example, Echinops exaltatus shows that phase separation comes into force first at the early tetrad stage, resulting in the establishment of a thick, elaborated glycocalyx up to the mid‐tetrad stage, and then the second wave of phase separation results in the appearance of typical gaps at the transition of the mid‐tetrad stage to the late tetrad stage.…”