Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706 Embryology Lectures 27:Endosperm Development

Endosperm Development Endosperm and its types Endosperm Development Endosperm development usually precedes embryo development In most monocots and some eudicots, endosperm stores nutrients that can be used by the seedling In other eudicots, the food reserves of the endosperm are exported to the cotyledons

ENDOSPERM Endosperm is the most common nutritive tissue for the developing embryo in angiosperms. In gymnosperms, it represents the female gametophyte whereas the female gametophyte in angiosperms differentiates before fertilization and is haploid, the endosperm is the product of fertilization and is usually triploid. After double fertilization the egg is called zygote, and the fusion product of polars and the second male gamete is termed Primary endosperm nucleus. The only angiosperms that do not form endosperm are the members of the families Orchidaceae, Podostemaceae and Trapaceae.

Endosperm seed without endosperm exalbuminous (non-endospermous) seed with endosperm albuminous (endospermous)

Monocotyledonous seed (with endosperm) pericarp and fused seed coat endosperm aleurone layer caryopsis coat scutellum (cotyledon) coleoptile first leaves shoot apex radicle coleorhiza Monocotyledonous seed (with endosperm)

Dicotyledonous seed (with endosperm) caruncle raphe radicle shoot apex cotyledons seed coat endosperm Dicotyledonous seed (with endosperm)

Dicotyledonous seed (without endosperm) raphe hilum micropyle cotyledon seed coat radicle plumule Dicotyledonous seed (without endosperm) Bean Seed

Depending upon its mode of development , three types of endosperm have been recognized :- Nuclear Endosperm Cellular Endosperm Helobial Endosperm Cellular endosperm is largely restricted to dicotyledonous families whereas helobial endosperm to monocotyledonous.

Nuclear endosperm Primary endosperm nucleus divides repeatedly to form a large number of free nuclei. No cell plate formation takes place at this stage and a central vacuole appears later. It is followed by cell plate formation which is centripetal. It is the most common type of endosperm e.g., Cotton, Maize, Capsella, Coconut (milk), wheat, etc .

Nuclear Endosperm In this type of endosperm, the division of the primary endosperm nucleus and a few subsequent nuclear divisions are not accompanied by wall formation. This results in a condition where central cell of the embryo sac has formed a few several thousand nuclei freely suspended in its sap. Such a condition of endosperm may persist until it is consumed by the developing embryo (Floerkea, Limnanthes, Oxyspora) or it may become cellular at a later stage. When latter is the case, which is more common, the wall formation is mostly centripetal. The degree of cellularization varies a great deal.

Examples :- Mostly endosperm becomes completely cellular but in Phaseolus cellularization occurs only around the embryo. In Crotalaria, the wall formation is confined to the upper region of the embryo sac, the chalazal region remains free- nuclear, and it often elongates and behaves like an haustorium.

In Lomatia, besides the main chalazal haustorium, numerous single- celled, finger- shaped projections are present all over the endosperm. This increases the absorbing surface of the endosperm.

In Grevillea robusta, presence of vermiform appendage at the chalazal end of the endosperm is reported, in which, chalazal endosperm haustorium remains free- nuclear.

In mature coconut, the liquid endosperm becomes milky. In Areca catechu, the endosperm is hardened.

In Stackhousia, there is formation of endosperm nodules or cytoplasmic vesicles which originate from the peripheral lining of the embryo sac. These are rich in various organelles and its function is uncertain, but they may either merge with the endosperm proper or degenerate.

Cellular endosperm Cell wall formation occurs immediately after division Subsequent divisions also accompanied by cell plate formation. As a result, the endosperm becomes cellular from the beginning. Eg: - Di-cotyledons, Balsam, Petunia, Utricularia,

Cellular Endosperm The cellular endosperm is characterized by the absence of free- nuclear stage. The division of the primary endosperm nucleus and a few subsequent nuclear divisions are followed regularly by wall formation. The occurrence of haustoria is a common feature of this type of endosperm; it is more varied than that is the nuclear endosperm. The haustoria may be micropylar or chalazal and occasionally, present in the same plant. Micropylar haustoria are known to occur in Impatiens roylei and Hydrocera triflora.

A very aggressive chalazal haustorium is formed in Iodina rhombifolia A very aggressive chalazal haustorium is formed in Iodina rhombifolia. The haustorium is actually formed before fertilization. After fertilization, the division of the primary endosperm nucleus is followed by transverse partitioning of the central cell, resulting in the formation of a micropylar chamber and a chalazal chamber. The endosperm proper is derived from the micropylar chamber alone. The chalazal chamber functions as an aggressive, uninucleate haustorium. Profuse branching at the free- end gives the haustorium a coralloid appearance. In the Acanthaceae, the endosperm development is asymmetric and it shows characteristic micropylar and chalazal haustoria.

A very aggressive chalazal haustorium is formed in Iodina rhombifolia.

Helobial endosperm Intermediate above two type e.g., members of order helobiales Monocot) First division is cellular (i.e., wall formation follows the first division) However, inside each of these newly formed cells, free nuclear divisions occur. But finally, the endosperm becomes cellular following the pattern of development of nuclear endosperms.

Helobial Endosperm The primary endosperm nucleus moves to the chalazal end of the embryo sac where it divides forming a large micropylar chamber and a small chalazal chamber. In the micropylar chamber, free- nuclear divisions and cell formation, start at a much later stage. .

In the chalazal chamber, the nucleus either remains undivided or divides only a few times. If latter is the situation, the divisions are usually free- nuclear. Sometimes, in Phylidrum lanuginosum, it may become cellular. Example : Asphodelus tenuifolius

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