Development of cycad ovules and seeds. 1. Implication of the ER in primary cellularisation of the megagametophyte in Encephalartos natalensis Dyer and Verdoorn

Abstract

Very little is known about the pre-and postshedding megagametophyte development and utilisation of accumulated reserves, respectively, in cycads (Zamiaceae). In the present study on developing ovules of the recalcitrant-seeded species, Encephalartos natalensis, cells of the megagametophyte were found to become progressively packed with starch and protein as the two main storage reserves, a limited number of discrete lipid bodies, and occasional mitochondria all of which appeared to be embedded in a homogeneous matrix. ER-derived vesicles (and not Golgi-derived vesicles) appeared to be the principal contributors of the primary cell wall components, pectin and xylan, during megagametophyte cellularisation. This was confirmed by the use of enzyme-gold localisation. High-pressure freezing (HPF) and freeze substitution (FS) of samples the following season showed that while the apparently featureless cytomatrix of the megaspore was an artefact of conventional fixation, there was still an insignificant occurrence of Golgi bodies during primary wall formation. When enzyme-gold localisation was employed on the HPF-FS material, label for pectin and xylan was found only in the regions of ER and vesicles and not in any of the few Golgi bodies or their associated vesicles. Immunocytochemistry revealed that pectin and xylan were restricted to the ER and not to any vesicles or to the occasional Golgi body that was found. This suggests that the ER exclusively, is involved in the deposition of these primary cell wall components during the cellularisation of the E. natalensis megagametophyte. While cellularisation took place over approximately 1-2 weeks, subsequent development of the megagametophyte cells involved the accumulation of storage reserves, this phase lasting approximately 8 months-after which the seeds were shed whether pollination/fertilisation had recently occurred, or not.

Cycads in vitro

“…It is, therefore essential that male and female plants of the same species cone at the same time for fertilized seed to be produced in nature, which may present a problem in populations with a few individuals. Although a few studies have been conducted, cycad seeds also appear to be recalcitrant (Dehgan and Schutzman 1989, Forsyth and van Staden 1983, Woodenberg et al 2010, i.e. they are 'wet' and desiccation-sensitive, and may succumb to dehydration in the field during the protracted development of the embryo after seed-shed (Woodenberg et al 2014).…”

Status and conservation needs of Cycas pectinata Buch.-Ham. in its natural habitat at Baroiyadhala National Park, Bangladesh

“…Moreover, Cycads are dioecious plant and only very rarely the timing of cone development in male and female plants match (Cheek 2000;Lindstrom & Hill 2007). Woodenberg et al (2010Woodenberg et al ( , 2014 mentioned rare pollination and sensitivity of seeds to desiccation as two more reasons for poor natural regeneration of Cycads.…”

Host-Parasite Interactions from the Inside: Plant Reproductive Ontogeny Drives Specialization in Parasitic Insects

“…In some other gymnosperms (e.g. cycads and Pinaceae), storage may begin in megagametophytes prior to fertilization [ 14 ], sometimes even in the absence of pollination (e.g. Pseudotsuga ).…”