Ovule ontogeny of Tabebuia pulcherrima Sandwith (Bignoniaceae): embryo sac development

Abstract

-(Ovule ontogeny of Tabebuia pulcherrima Sandwith (Bignoniaceae): embryo sac development). The chalazal megaspore develops in a Polygonum-type embryo sac. The amyloplast-rich endothelium is partially degraded during the expansion of the micropylar portion of the megagametophyte. Organization of the mature embryo sac is determined by the patterns of vacuolation, nuclear migration, spindle orientation and cellularization. The egg cell is slightly chalazal in relation to the synergids, and its micropylar end does not touch the micropylar channel. At the chalazal pole of the egg apparatus, the common walls between the synergids, the egg and central cells, despite their tenuity, are present in the mature megagametophyte. The polar nuclei do not fuse before fertilization and the antipodals are persistent until the first stages of endosperm formation. The taxonomic significance of some embryological characters for the Bignoniaceae is discussed.RESUMO -(Ontogenia do óvulo de Tabebuia pulcherrima Sandwith (Bignoniaceae): desenvolvimento do saco embrionário). O megásporo calazal origina um saco embrionário tipo Polygonum. O endotélio rico em amiloplastos é parcialmente degradado durante a expansão da porção micropilar do megagametófito. A organização do saco embrionário maduro é determinada pelos padrões de vacuolação, migração nuclear, orientação dos fusos mitóticos e celularização. A oosfera é ligeiramente calazal em relação às sinérgides e sua extremidade micropilar não estabelece contato com o canal micropilar. Na porção calazal do aparato oosférico, as paredes comuns entre as sinérgides, a oosfera e a célula média, embora muito tênues, estão presentes no saco embrionário maduro. Os núcleos polares não se fundem antes da fecundação e as antípodas persistem até o início do desenvolvimento do endosperma. O significado taxonômico de alguns caracteres embriológicos para a família Bignoniaceae é discutido.

Comparative Ovule Ontogeny in Some Members of theTabebuiaAlliance (Bignoniaceae)

“…In all of the plants studied here, a Polygonum-type megagametophyte with eight nuclei and seven cells is formed after three successive mitotic cycles. The events of protoplasm vacuolization, nuclear migration and spindle positioning that were verified in this study during the coenocytic embryo sac development are nearly identical to those described for H. pulcherrimus (Bittencourt and Mariath 2002a). At the time of megagametophyte maturity, the egg apparatus has a hooked configuration, with only the micropylar portion of the synergids and the egg cell walls in direct contact with the embryo sac envelope.…”

“…At the time of megagametophyte maturity, the egg apparatus has a hooked configuration, with only the micropylar portion of the synergids and the egg cell walls in direct contact with the embryo sac envelope. Such features were also reported for other Bignoniaceae and appear to be common for all members of the family (Mauritzon 1935;Swamy 1941;Gonvidu 1950;Ghatak 1956;Davis 1966;Mehra and Kulkarni 1985;Johri et al 1992;Bittencourt and Mariath 2002a). The egg cells of the three species show a polarized cytoplasm with a morphologically evident apical-basal axis, corresponding, respectively, to the future shoot-root axis of the embryo.…”

“…MMC meiosis in Cy. antisyphilitica originates only linear tetrads of megaspores, and the chalazal megaspore gives rise to a monosporic Polygonum-type megagametophyte, as in other previously studied species in the Bignoniaceae (Gonvidu 1950;Ghatak 1956;Davis 1966;Mehra and Kulkarni 1985;Johri et al 1992;Galati and Strittmatter 1999;Souza et al 2005;Renó et al 2007;Sampaio 2007;Souza et al 2008), including H. pulcherrimus (Bittencourt and Mariath 2002a). The same pattern was observed in C. cujete and T. roseoalba, but although linear tetrads predominate, a small percentage of young ovules with T-shaped tetrads were observed in these species.…”

“…This feature appears to be a common trait in Bignoniaceae, with C. cujete, Cy. antisyphilitica and T. roseoalba showing amyloplasts in all cells of the egg apparatus, as described for H. pulcherrimus (Bittencourt and Mariath 2002a).…”

Ovule ontogeny of Tabebuia pulcherrima Sandwith (Bignoniaceae): megasporogenesis and integument development

“…Similar deposition of amyloplasts into the cells of the micropylar channel was found in Jacaranda mimosifolia (Galati & Strittmatter 1999) and other angiosperms (Tilton 1980a). Amyloplast accumulation in the micropylar channel cells seems to be associated to their function in the nourishment of the developing megagametophyte (Tilton 1980a, Bittencourt Jr. 1992, Bittencourt Jr. & Mariath 2002, or more probably are closely involved in embryo nutrition (Tilton 1980a). However, a different interpretation was suggested by Galati & Strittmatter (1999) and other authors (Tilton 1981a), according to whom amyloplast accumulation in the cells that surround the micropylar channel may be related to growth of the pollen tube.…”