Endosperm development in angiosperms & gymnosperms

• Endosperm is nutritive tissue for developing embryos in angiosperms
• Endosperm represents the female gametophyte in gymnosperms
• A zygote is the result of an egg after double fertilization
• The fusion product of polar nuclei and the second male gamete is the Primary endosperm nucleus (PEN)
• Only angiosperms that do not form endosperm are the families Orchidaceae, Podostemaceae and Trapaceae

Types of Endosperm

• Three recognized types of endosperm form based on development mode: Nuclear, Cellular, and Helobial
• Cellular endosperm is largely restricted to dicotyledonous families
• Helobial endosperm is largely restricted to monocotyledonous families

Nuclear Endosperm

• The primary endosperm nucleus (PEN) divides without wall formation
• A condition results where the central cell of the embryo sac forms thousands of nuclei suspended in sap
• The state may persist when the developing embryo consumes it, or it may become cellular at a later stage
• When becoming cellular, wall formation is mostly centripetal
• The degree of cellularization varies
• Endosperm generally becomes completely cellular
• Wall formation often occurs only around the embryo in Phaseolus
• In Crotalaria, wall formation is in the upper region of the embryo sac and the chalazal region remains free-nuclear
• In Lomatia, single-celled, finger-shaped projections are present all over the endosperm in addition to the main chalazal haustorium
• In Grevillea robusta, a chalazal endosperm haustorium remains free-nuclear with a vermiform appendage at the chalazal end
• Liquid endosperm becomes milky in mature coconuts
• Endosperm hardens in Areca catechu
• Stackhousia has endosperm nodules or cytoplasmic vesicles which originate from the peripheral lining of the embryo sac

Cellular Endosperm

• This endosperm characterized by the absence of a free-nuclear stage
• The primary endosperm nucleus divides and follows subsequent nuclear divisions regulated with wall formation
• The occurrence of haustoria is a common feature but is more varied than in nuclear endosperms
• 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
• The division of the primary endosperm nucleus is followed by transverse partitioning of the central cell after fertilization
• A micropylar chamber and a chalazal chamber form
• The endosperm proper derives from the micropylar chamber alone
• A chalazal haustorium is aggressive and forms in lodina rhombifolia before fertilization
• The chalazal chamber functions as an aggressive, uninucleate haustorium, where branching at the free-end gives the haustorium a coralloid appearance
• Endosperm development in Acanthaceae is asymmetric, characteristic micropylar and chalazal haustoria

Helobial Endosperm

• The PEN moves to the chalazal end of the embryo sac, dividing and forming a large micropylar chamber and a small chalazal chamber
• In the micropylar chamber, free-nuclear divisions and cell formation start later
• The nucleus remains undivided in the chalazal chamber, or only divides a few times
• Divisions are typically free-nuclear, and sometimes become cellular in Phylidrum lanuginosum

Functions of Endosperms

• Endosperms serve as an important nutrient source for the successful development of embryos
• Endosperms enrich fat, carbohydrates, and proteins establishment of seedlings during seed germination
• There is limited nutrition in the embryo sac at fertilization
• The endosperm provides sufficient food for the developing embryo with formation
• The division of the zygote begins when the endosperm grows sufficiently
• The endosperm grows more rapidly if the zygote and primary endosperm nucleus divide simultaneously
• Plant immature endosperm juice is used for the developing embryo nutrition
• Coconut milk is used as a nutrient medium in in vitro embryo culture