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Endosperm is the tissue produced in the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet. For example, wheat endosperm is ground into flour for bread (the rest of the grain is included as well in whole wheat flour), while barley endosperm is the main source for beer production. Other examples of endosperm that forms the bulk of the edible portion are coconut "meat" and "milk", and corn, including popcorn.
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Origin of endosperm
Primitive plants, have seeds that have small embryos and abundant endosperm, and the evolutionary development of seed plants tends to show a trend to plants with mature seeds with little or no endosperm. In more modern plants the embryo occupies most of the seed and the endosperm is non developed or consumed before the seed is mature.12
Double fertilization
Endosperm is formed when the two sperm nuclei inside a pollen grain reach the interior of an embryo sac or female gametophyte. One sperm nucleus fertilizes the egg, forming a zygote, while the other sperm nucleus usually fuses with the two polar nuclei at the center of the embryo sac, forming a primary endosperm cell (its nucleus is often called the triple fusion nucleus). This cell created in the process of (double fertilization) develops into the endosperm. Because it is formed by a separate fertilization, the endosperm constitutes an organism separate from the embryo.
About 70% of angiosperm species have endosperm cells that are polyploids,3 typically triploid (containing three sets of chromosomes) but can vary widely from diploid (2n) to 15n. [1]
Endosperm formation
There are two different types of endosperm formation, the nuclear (or liquid endosperm) type, where formation of cell wall is delayed for a number of cell divisions, and the cellular, where cell wall formation is initiated as the nuclei divide. The nuclear type is the most common one in angiosperms. Sweet corn is picked for eating at the tender liquid endosperm stage, before cell walls have formed and the sugars have been converted to starch. The 'milk' of the coconut is a liquid endosperm.
The role of endosperm in seed development
In some groups (e.g. grains of the family Poaceae) the endosperm persists to the mature seed stage as a storage tissue, in which case the seeds are called "albuminous" or "endospermous", and in others it is absorbed during embryo development (e.g., most members of the family Fabaceae, including the common bean, Phaseolus vulgaris), in which case the seeds are called "exalbuminous" or "cotyledonous" and the function of storage tissue is performed by enlarged cotyledons ("seed leaves"). In certain species (e.g. corn, Zea mays); the storage function is distributed between both endosperm and the embryo. Some mature endosperm tissues store fats (e.g. castor bean, Ricinis communis) and others (including grains, such as wheat and corn) store mainly starches. The dust-like seeds of orchids have no endosperm. Orchid seedlings are mycoheterotrophic in their early development. In some other species, such as coffee, the endosperm also does not develop.[2] Instead the nucellus produces a nutritive tissue termed "perisperm". The endosperm of some species is responsible for seed dormancy.4 Endosperm also mediates the transfer of nutrients from the mother plant to the embryo, it acts as a location for gene imprinting, and is responsible for aborting seeds produced from genetically mismatched parents.3 In angiosperms, the endosperm contain hormones such as cytokinins, which regulate cellular differentiation and embryonic organ formation.5
Cereal grains
Cereal crops are grown for their palatable fruit (grains or caryopses), which are primarily endosperm. In the caryopsis, the thin fruit wall is fused to the seed coat. Therefore, the nutritious part of the grain is the seed and its endosperm. In some cases (e.g. wheat, rice) the endosperm is selectively retained in food processing (as in white flour), and the embryo and seed coat removed. Endosperm thus has an important role within the human diet, worldwide.
The aleurone is a maternal tissue that is retained as part of the seed in many small grains. The aleurone functions for both storage and digestion. During germination it secretes the amylase enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.6
References
- ^ http://www.seedbiology.de/dormancy.asp#evolution
- ^ Friedman, William E. (1998), "The evolution of double fertilization and endosperm: an ”historical" perspective", Sexual Plant Reproduction 11: 6, doi:
- ^ a b Olsen, By Odd-Arne (2007), "Endosperm: Developmental and Molecular Biology", ISBN 3540712348: 2
- ^ Mechanisms of plant growth and improved productivity: modern approaches edited by Amarjit S. Basra 1994. ISBN: 0824791924 p.
- ^ Pearson, Lorentz C. (1995). The diversity and evolution of plants. Boca Raton: CRC Press, 547. ISBN 0-8493-2483-1.
- ^ Endosperm Development. URL accessed on April 29, 2006.
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