Sporoderm development in Liriodendron chinense (Magnoliaceae): a probable role of the endoplasmic reticulum

Abstract

The developmental events in the sporoderm and the cytoplasm of Liriodendron chinense microspore from the early tetrad stage until late free microspore stage were observed. Various forms of the endoplasmic reticulum (ER) and surprising unusual aggregates of ER seen during microspore development attract special attention. Being scanty at early and middle tetrad stage, while primexine matrix (glycocalyx) acquires well‐defined form, the ER becomes distinct in unusual forms at the late tetrad stage. Thin long tubules with an osmiophilic contents, which cannot be compared with the tubular smooth endoplasmic reticulum (SER), undulate through the cytoplasm. Towards the end of the tetrad period when callose begins to disintegrate, and a distinct tectate‐columellate pattern of the ectexine becomes evident, two new forms of the SER occur in the cytoplasm. Instead of single tubules observed previously, 3–tubuled aggregates meander through the cytoplasm, the middle tubule contains an osmiophilic substance. The second form of the SER looks like an ordinary tubular SER, but has ampoule‐like dilations with dark granular contents. Later on the tubules undergo major changes: multi‐tubuled aggregates of parallel tubules overcrowd the cytoplasm, the outer tubules of each aggregate carrying ribosomes. These aggregates undulate through the cytoplasm, branch, and are associated with lipid globules. The tips of many aggregates are pressed to the plasmalemma. The ontogenetic period of time of the presence of these ER aggregates, their structure and localization in the microspore cytoplasm allow me to assume that these ER aggregates synthesize sporopollenin precursors.

Sporoderm development and substructure inMagnolia sieboldiiand other Magnoliaceae: an interpretation

“…-The first layer appearing after endexine establishment consists of granular units that gradually accumulate in the periplasmic space ( Figure 12). This layer was attributed to the first layer of intine in all investigated Magnoliaceae (Gabarayeva, 1986b(Gabarayeva, , 1987c(Gabarayeva, , 1991a(Gabarayeva, , 1996. To corroborate this, mature pollen grains were acetolised and in fact, this is the case (Figure 13D, E).…”

“…Evidence exists that sporopollenin can be produced both from the tapetum and the micropore. We believe that at least a partial source of the sporopollenin monomers in the tetrad period is the microspore itself (Echlin, 1971;Heslop-Harrison, 1976;Dickinson, 1976b;Dickinson & Potter, 1976;Audran, 1981;Gabarayeva, 1991aGabarayeva, , 1991bGabarayeva, , 1993bGabarayeva, , 1996Gabarayeva & Grigorjeva, 2002;Hemsley & Gabarayeva, 2007). Oxidation and other degradation experiments (Southworth, 1974(Southworth, , 1986Rowley & Dahl, 1977;Rowley & Prijanto, 1977;Blackmore & Claugher, 1987;Blackmore, 1990;Rowley & Claugher, 1991;Gabarayeva et al, 2003a) have shown that some differences in the chemical composition of sporopollenin really exist and may also be indicative of the microspore sporopollenin monomer source in the tetrad period.…”

Early evolution of angiosperm pollen as inferred from molecular and morphological phylogenetic analyses

“…See text for discussion and taxon notes for references. References and notes for taxa: Magnoliaceae: Praglowski (1974a), Walker (1976b), Gabarayeva (1996). See text for discussion of variation in infratectal structure.…”

“…still scored Magnoliaceae as intermediate. However, TEM studies show that infratectal elements in Magnolioideae (Magnolia s. l.) vary from very short columellae (Aromadendron, Magnolia sieboldii) to tall columellae (Alcimandra, Elmerrillia, Pachylarnax, Talauma) to irregular, elongated granules and/or lumpy columellae (Magnolia denudata, M. fraseri, Manglietia, Michelia), while Liriodendron, their sister group, has relatively regular columellae (Praglowski 1974a, Walker 1976b, Gabarayeva 1996. If Magnolioideae are scored as uncertain (1/2), because there are not yet sufficient data to reconstruct their ancestral state, but Liriodendron is scored as columellar (2), the most parsimonious state for Magnoliaceae as a whole is columellar (2).…”

“…The position of Magnoliaceae as the second branch in Magnoliales has been corroborated by the analysis of . Liriodendron has fairly regular columellae, but Magnolioideae vary between columellar and intermediate (Praglowski 1974a, Walker 1976b, Gabarayeva 1996; because these two clades are sister groups, it is most parsimonious to assume the family was originally columellar (see Materials and Methods). Typical granular exine structure arose in the remaining Magnoliales; it is an important morphological synapomorphy of Annonaceae, Eupomatia, Degeneria, and Galbulimima (~Himan-tandraceae), along with a single prophyll and seeds with testal ruminations .…”