Atlas of plant and animal histology

Seed develops from the ovule of the flower ovary. The developing begins after fertilization of the egg cell by the microspore of the grain pollen.

1. Fertilization

The pollen grain is transported to the stigma of the pistil by the wind or some insect. Then, a pollinic tube is produced and extended through the inner tissues of the stigma, stylus and ovary, until it reaches the female gametophyte of the ovule (Figure 1). In the tip of the growing pollinic tube, there are three nuclei: one vegetative nucleus and two generative or sperm nuclei. It is thought that the vegetative nucleus is responsible for the enlargement of the pollinic tube, which is just an extension of the inner wall (intine) of the pollen grain. The pollinic tube gets out through a pore of the external wall (exine) of the pollen grain. Note that there is a very long distance from the stigma to the ovary. If the pollinic tube enters the ovule through the chalaza, the fertilization is called chalazogamic, and if it enters through the micropile, it is a porogamic fertilization. The fertilization is actually double, a characteristic feature of angiosperm fertilization. One of the generative nucleus fuses with the egg cell and the other with the central cell of the gametophyte. The fusion of a generative nucleus with the egg cell results in the zygote that develops into the embryo and starts the sporophyte stage. The other generative nucleus and the central cell develop into the endosperm of the seed. Since the central cell contains two polar nuclei, the endosperm is a triploid tissue, which is a storing tissue needed during germination. The endosperm development can be divided in different stages: formation, cellularization, differentiation, maturation and cell death. The endosperm is a mayor player during the expansion and turgor of the seed. The auxin hormone is needed for a proper endosperm development.

2. Seed parts

The following components can be distinguished in a typical seed:

Embryo

The embryo is composed of an embryonary axis (tigellum) with the radicle in one end and the plumule in the other end, and one or two cotyledons. The fusion of a generative nuclei with the egg cell gives the zygote that develops into the embryo. The first mitosis of the zygote gives two cells, the inner one develops into the embryo and the outer one divides many times and differentiates into the supensor, which connects the embryo to the other surrounding tissues of the ovule. In dicot plants, the cell that develops into the embryo divides in two cells separated by a longitudinal wall that separates the two cotyledons. Cotyledons are structures that may store nutritive substances used during germination, so they have fleshy consistence. They are connected to the embryo by at a point called node. During germination, cotyledons open out like a book. The portion of the embryo apical to the insertion point of the cotyledons (toward the plumule) is known as epicotyl, whereas the basal part (toward the radicle) is called hypocotyl.

Endosperm

The endosperm is a nurturing tissue that wraps the embryo or it is found at a side of the embryo. In angiosperms, it develops after the fusion of a generative nucleus with the two polar nuclei, leading to a triploid tissue referred as secondary endosperm. In gymnosperms, the endosperm is haploid and it is called primary endosperm. The endosperm provides nutritive substances to the embryo and during the first stages of the plant development. The endosperm cells store starch or proteins. Proteins can form amorphous granules called gluten or crystal protein complexes known as aleurone. In some angiosperm species, the nucellus is another storing tissue that is a component of the ovule and forms the perisperm, although it is not present in most seeds.

Seeds having endosperm during the mature stages are called endospermic or albuminous seeds. Non-endospermic or exalbuminous seeds are those that consume the endosperm during the early stages of maturation. These last seeds store the nutritive substances in the cotyledons, as in pea, beans and mustard.

Coats

Seed coats are originated in the ovule, from tissues surrounding the egg cell. The formation of the coats remains inhibited before fertilization. However, fertilization removes the restriction and favors the coat development. Seed coats are differentiated from the inner and outer integuments of the ovule. The inner integument becomes the tegmen and the outer integument develops into the testa. Tegmen and testa are usually strongly attached to each other so that it is difficult to get them separated, excepting in some seeds like beans. Both coats together are called seminal coat or episperm. Tegmen is normally thin and flexible, whereas testa is hard. In the surface of the testa, a layer of epidermal-like cell layer develops a thick cuticle, which is a barrier against water loss and external pathogens, but it allows the gas exchange. In some species, a type of molecules known as defensins can be found in the seed coats. These proteins are repellent or toxic to herbivores. In some seeds there is an additional protective element composed of toxic substances.

Seed coats show a broad variety of histological organizations depending on the species. For example, coats may partially develop from the nucellus or even from the embryo sac. In Arabidopsis, seed coats are divided into 3 inner layers and 2 outer layers that form the so-called integument. The endothelium is the inner layer, the other two layers are fused as seed develops. The outer ones are highly differentiated and transformed into subepidermal and epidermal layer. The subepidermal layer is a very thick wall, whereas the epidermal layer differentiates in a mucilaginous layer. In leguminous species, the sea coat is made up of many layers that include macrosclereids and osteosclereids in the outer layers, and parenchyma in the inner layers. In cereals, the endothelium and the outer integument form two cell layers, whereas the pericarp partially function as a seed coat.

Seed coats get very hard and consistent, mostly by a high amount of sclereids. However, some are fleshy. In both monocot and dicot plants, testa shows grooves and ridges, sometimes forming wing-like structures. Coats must protect the seed from the environment, but at the same time sense the exterior to begin germination when conditions are favorable. In many seed coats, there is a superficial epidermal layer with a thick cuticle that repels pathogens, but allows the exchange of gases. In the surface of the coats there is a scar called hillium where the ovule is connected with the funiculum, a small column of cells that join the seed to the coat. There is also a small opening known as micropil, which is the entrance for the pollinic tube during fertilization and for the water that triggers germination. There are functional chloroplasts in many seeds, although they may work mostly as sensors the external enviromental variables instead of produce energy.

3. Vascularization

There are small seeds without vascular bundles because they are small. For example, orchid seeds are about 200 µm in diameter, so that water and nutrients diffuse using apoplastic pathways (intercellularly). In large and middle size seeds, vascular bundles are developed, showing more complexity in larger seeds. In some seeds, the vascular network ends at the funiculus-ovule or in placenta-chalaza joining places. Other seeds have a vascular network spreading through the entire seed. The vascular bundles are surrounded by parenchyma and connected by many plasmodesmata to facilitate the diffusion of molecules between the seed tissues and the transported substances.

4. Dormancy

Dormancy is a physiological stage where seeds show a very low metabolic activity (chemical reactions inside the seed). Thus, seed do not need much energy, oxygen and water. Seeds may remain in this stage for long periods, sometimes for years. A combination of external factors, such as light, water, temperature and chemical substances may finish the dormancy period. The exit from dormancy may controlled by the embryo, the endosperm, the seed coats or by some of them together.

5. Seed dispersal

Some seed coats develop structures like spines, wings or "parachutes" to be dispersed by the wind. Another dispersion mechanism takes advantage of animal movement after they eat the fruits, but seeds are not digested. Seeds are then released with the stools.

6. Germintation

The process of embryo activation, development and grow, until the output of the radicle, is called germination. It usually begins with water entrance through the seed micropyle, which is a modification of the pollinic tube after fertilization. Then, cells increase in size, and relax their cell walls. There is a metabolism activation and a new incorporation of water, together with the growing of the radicle. Thus, the germination process ends. During germination, the embryo uses the nutritive material stored in the endosperm or in cotyledons. The starting of germination depends on environment conditions and on the species. The endosperm is softened during germination by hormones, normally giberelins, and by signals coming from the embryo.

Bibliography

Finch-Savage WE, Leubner-Metzeger, G. 2006. Seed dormancy and the control of germination. New phytologist. 171: 501-5023.

Radchuk V, Borisjuk L. 2014. Physical, metabolic and developmental functions of the seed coat. Frontiers in plant science. 5: 510.