Fine structural studies of embryogenesis in a callus ofRanunculus sceleratus

Abstract

IDRésumé. -Les cultures de cals de /lanunculus sceleratus donnent naissance it un grand nombre d'embryoïdes qui gernwnt in situ ou lorsqu'ils sont st'•parés el transférés sur un milieu frais. Les pl'tites plantt•s t~n résultant, forment ulléricurcmt•nt des cmbryoïdes à partir de ecllult•s individuelles situées dans ]'{•pidermc de l'hypocotylc.Des études au microscope électronique ont montr.:, des similitudes entre les t•<•llult•s cmbryogènes des cals et celles des tiges de petites plantes pron•nant de l'(,piderme en matière de taille des cellules, taille du noyau, degré de vacuolisation, abondance des ribosomes et des mitnchondrit•s, présence d'arnyloplastes et prt:,dominance de sphérosomes. Elles ont rnontr(-ullt-ricuremt•rlt que la discontinuité protoplasmique entre les cellules emb1•yogènes et les n•llules non impliquét•s directt•mt•nt dans l'embryogt>nèse, m• pré,cé•dait pas l'initiation du développement de l'embryoïde.Des embryoïdes semblent provenir dans quelques cas, de cellules individuellt•s à la surface d'aggrt-gats des cals, mais il n'est pas certain que tous ont pour origine une cellule individuelle. Les cl'llules intt•rnes des aggr<;gats dt> cals t•ontiennent des structures mitochondriales extrêmement allongées, suivant le contour du noyau, et l'on pense que cela suggère, soit que ces structures sont formées ti la suite de la fusion de mitochondries, soit que ce sont ct•s structures qui donnent naissance à des mitochondries.

In vitro somatic embryogenesis and adventitious root initiation have a common origin in eggplant (Solanum melongena L.)

“…In both cases the cells of this region exhibited starch grains which were proposed to be somatic embryogenesis markers (Buffard-Morel et al 1992) while in normal roots they are known to act as statholiths (Mauseth 1995). The presence of starch grains in root primordia has also been reported in several other somatic embryogenesis systems (Thomas et al 1972, Street & Withers 1974, Ozias-Akins & Vasil 1982, Schwendiman et al 1988.…”

“…Such embryos were also described for Helianthus smithii Heiser (Laparra et al 1997), Cicer arietinum L. (Shri & Davis 1992), Sorghum (Thomas et al 1972. Laparra (1997) and Thomas (1972) interpreted them as an aborted somatic embryo, probably due to the low levels of auxin, insufficient for the establishment of the root pole.…”

“…In this system, proembryos were observed as early as two days after culture initiation. Studies on somatic embryo development of other species reported significantly longer periods for embryo or embryogenic callus formation, most of them longer than ten days (Ozias-Akins & Vasil 1982, van Hengel et al 1998, Bespalhok Filho & Hattori 1997, Hutchinson et al 1996, Ho & Vasil 1983, Sagare et al 1995, Laparra et al 1997, Guerra & Handro 1998 and some of them reaching up to 40-60 days (Schwendiman et al 1988, Thomas et al 1972.…”

Origin and Early Growth of Celery Embryoids

“…The route where the embryoids are formed directly from single cells and small cell groups in the medium corresponds to the origin of carrot embryoids described by Steward et al (1964) and Steward, Ammirato and Mapes (1970). The alternative route, where the embryoid remains attached to the aggregate and then is released at a late stage in development, is similar to the route described for embryoid production in Ranunculus sceleratus (Konar and Nataraja, 1965;Konar, Thomas and Street, 1972a;Thomas, Konar and Street, 1972), Atropa belladonna (Konar, Thomas and Street, 1972b), sorghum (Dunstan, Short and Thomas, 1978) and in other studies of carrot (Halperin and Wetherel, 1964;Halperin, 1967;Halperin and Jensen, 1967;McWilliam, Smith and Street, 1974). The celery embryoid appears to have the same varied path of development as the carrot.…”

Cell Differentiation in Embryoids and Plantlets of Celery Tissue Cultures

“…The problem was whether the embryoid developed from a single isolated cell, in the same way as the zygote, or was derived from cells within the large cell mass of an aggregate. Most of the evidence suggested that the embryoids were initiated from surface cells of the cell aggregates and, where viable single cells were present, these first divided to form an aggregate from which embryoids were initiated from the surface as before (Halperin and Wetherel, 1964;Halperin, 1967;Halperin and Jensen, 1967;Thomas, Konar and Street, 1972;Konar, Thomas and Street, 1972;McWilliams, Smith and Street, 1974). At this early stage of its development the tissue culture embryoid is not strictly comparable to the plant embryo since the latter originates from a single isolated cell, the zygote which divides directly to form the embryo, whereas the embryoid arises from a relatively large mass of un differentia ted cells.…”