Atlas of plant and animal histology

Seeds develop from the ovule of the flower ovary. The development begins after fertilization of the egg cell by the pollen grain.

1. Fertilization

The pollen grain is transported by the wind or some insect to the stigma of the pistil. This is called pollination. Then, the pollen grain germinates, giving rise to a pollen tube that extends through the inner tissues of the stigma, stylus, and ovary until it reaches the female gametophyte of the ovule (Figure 1). During pollen grain germination, the generative nucleus divides into two sperm, or germinative, nuclei. Thus, in the tip of the growing pollen 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 elongation of the pollen tube, which is just an extension of the inner wall (intine) of the pollen grain. The pollen tube gets out through a pore in the external wall (exine) of the pollen grain. Note that there is a very long distance from the stigma to the ovary. If the pollen tube enters the ovule through the chalaza, the fertilization is called chalazogamic, and if it enters through the micropyle, it is porogamic fertilization. When the pollen tube reaches the female gametophyte, one of the synergic cells starts to die to prepare the entrance of the pollen tube tip. Then, the more advanced sperm nucleus fuses with the polar nuclei of the female gametophyte, giving rise to a triploid endosperm. The lagged sperm nucleus fuses with the egg cell and forms the diploid zygote. Thus, the fertilization is actually double, a characteristic feature of angiosperm fertilization. The zygote develops into the embryo, which is the beginning of the sporophyte phase.

At this point, the ovule consists of the embryo sac, surrounded by the nucellus, and one or two integuments, altogether connected to the carpel by the funiculus. At least one primary vascular vessel, formed prior to the endosperm, supplies nutrients to the ovule. The vascular tissue enters through the funiculus and extends to reach the pole of the chalaza through the outer integument.

2. Seed parts

The following components can be distinguished in a typical seed:

Embryo

Embryogenesis is the process that spans from the zygote to the final embryo. During this developmental period, the apical-basal (stem-root) and the radial (inner-superficial) axes are established. The first zygote division leads to two cells, one apical and one basal. The apical one gives rise to the embryo. The basal one becomes very large and vacuolated and develops into the suspensor, which is the structure anchoring the embryo to the micropyle zone and will allow the supply of nutrients to the embryo.

The cells derived from the apical cell proliferate and give a round mass of cells called the proembryo, which becomes enlarged and increases in size. Then, cell differentiation begins. The protoderm is formed in the periphery, and the ground meristem and procambium are differentiated in the inner part. As the embryo develops, the meristem activity is confined to the tips of the embryo, also known as apical meristems.

The mature embryo, before starting dormancy, is composed of an epicotyl (above the cotyledons) bearing an apical meristem and the stem primordium, the cotyledons, and a hypocotyl (under the cotyledons) with a part of the stem primordium and the root primordium. The plumule is the stem region with more advanced development (usually the epicotyl), and the radicule is the embryo root.

Most angiosperms are either monocot or dicot. In dicots, the zygote is divided by a longitudinal wall, separating the future two cotyledons. The cotyledons are leaf-like structures connected to the embryo by vascular tissue. When they are the main store of substances for germination, they look fleshy. In monocots and some dicots, the cotyledons are absorptive, and the nutrients are mainly stored in other parts of the seed. In these cases, the cotyledons are transient suppliers of nutrients to the embryo. Cotyledons do not have meristems if they are going to be the main energy source, but they maintain some meristematic activity if they are going to perform photosynthesis. Cotyledons are connected to the embryo at a point called a node, and they open during germination as a book. Monocot embryonic development is similar to that of dicots but is more elongated due to having only one cotyledon. The embryo of grasses shows an apical meristem with the leaf primordia forming a sheath known as coleoptyl and a root with a sheath called coleorhyza.

Endosperm

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

Coats

Seed coats originate 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 development of the coat. 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, and it is difficult to get them separated, except for some seeds like beans. Both coats together are called the seminal coat or episperm. The tegmen is normally thin and flexible, whereas the testa is hard. On the surface of the testa, a layer of epidermal-like cells develops a thick cuticle, which is a barrier against water loss and external pathogens but also allows gas exchange. In some species, a type of molecule 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. Generally, the cells of the seed coat develop thick cell walls, and some differentiate into sclereids, although clorenchyma and aerenchyma can be found.

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 embryonic 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, and the other two inner layers are fused as the seed develops. The outer ones are highly differentiated and transformed into subepidermal and epidermal layers. The subepidermal layer forms a very thick barrier, whereas the epidermal layer differentiates into a mucilaginous layer. In leguminous species, the seed 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 functions as a seed coat.

Seed coats get very hard and consistent, mostly by having a high amount of sclereids. However, some are fleshy. In both monocot and dicot plants, the testa shows grooves and ridges, sometimes forming wing-like structures. Coats must protect the seed from the environment, but at the same time they 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. On the surface of the coats, there is a scar called hillium, where the ovule is connected through the funiculum, a small column of cells that joins the seed to the coat. There is also a small opening known as the micropyle, which is the entrance for the pollen tube during fertilization and for the water that triggers germination. There are functional chloroplasts in many seeds, although they may work mostly as sensors of external environmental variables rather than producing energy.

3. Vascularization

There are very small seeds, such as the orchid seeds, which are about 200 µm in diameter and do not show vascular bundles, so water and nutrients diffuse using apoplastic pathways (intercellular). In large and midium-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 zones. Other seeds have a vascular network spreading through the entire seed. The vascular bundles are surrounded by parenchyma cells, which are connected to one another by many plasmodesmata to facilitate the diffusion of molecules of transported substances by the vascular tissue.

4. 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.

4. Dormancy

Seed was an outstanding advance for land colonization. One main feature of seeds is their ability to remain in a dormancy state between seed release and seed germination. Dormancy is a seed state where the metabolic activity, or biochemical reactions, is very low, so energy, oxygen, and water consumption are negligible. A combination of environmental variables, such as light, water, temperature, and some chemical substances, may end the dormancy period. These cues inform the seed that germination is going to happen in a favorable environment. The dormancy state is determined by the coats of the seed and by the stalling of the physiological processes in the seed tissues as well as in the embryo.

The end of the dormancy may be started by the embryo, the endosperm, the coats, or a combination of them. Unlike animals, the plant embryo can remain dormant for a long time. It allows that a seed cohort may give rise to new plants over several years. It is remarkable that seeds from 2000-year-old soil strata have been able to germinate and develop into an adult plant. This is an extreme example, but seeds may remain in dormancy for 20 years.

There are growth inhibitors in the seed, mainly abcisic acid. The seed coats prevent the dispersion of these inhibitors. On the other hand, gibberellic acid promotes germination. In Arabidopsis, the abcisic acid/gibberellic acid ratio determines the end of the dormancy state. Other phytohormones, such as auxin, ethylene, and jasmonic acid, are also involved in the begin of germination.

The seeds of the same species do not end dormancy at the same time. Even in the same favorable environments, some seeds remain dormant. It is of note that some apparently unfavorable variables, such as fire, induce the end of dormancy. There is the best bet strategy, that is, some seeds exit dormancy under favorable clues, but not all of them, which will wait for another good opportunity. Another strategy is to produce a seed population with morphological and biochemical differences. It has been proven that seeds with the same genomes show variability in germination time. It has been suggested that stochastic processes generate "transcriptional noise" in gene expression. All these strategies are intended to space out the germination of a population of seeds over time. Thus, a species may produce new plants at different times and in variable environments. As mentioned above, the ratio of abcisic acid/gibberellic acid may be slightly different in the same population of seeds and therefore lead to differential responses. The endosperm and the embryo produce abcisic acid, but the endosperm looks like the key source of the abcisic acid to influence the balance of these two hormones. However, the decision to quit dormancy depends on the embryo radicule.

6. Germinatation

The process of embryo activation, development, and growth until the output of the radicule is called germination. It usually begins with water entering through the seed micropyle, which is a modification of the pollen tube after fertilization. The seed coats are largely waterproof thanks to a cuticle with a high content of waxes. When this barrier is affected by physical or biological agents (environment, pathogens, fungi, animal digestive tract), water and oxygen enter the seed, and the germination inhibitors are removed. Then, embryonic cells relax their cell walls and increase in size. During these first stages, cell proliferation is very low. In Arabidopsis, cellular extension is observed in the hypocotyl region, near the radicule, and in the transition zone. This expansion leads to the breaking of the testa. Then, a metabolism activation happens and a new influx of water, together with the growth of the radicule. At this point, it is said that the germination process ends. During germination, the embryo uses the nutritive material stored in the endosperm or in cotyledons. The start of germination depends on environmental conditions and on the species. The endosperm is softened during germination by hormones, normally gibberelins, and by signals coming from the embryo.

In dicot plants, the radicule is the first structure that grows. It happens at the same time that the provascular tissue develops into functional xylem and phloem. Later on, the epycothyl differentiates into the stem. This process may push the seed out over the ground surface due to the proliferative activity of the hypocothyl. They are known as epigean germinations. Hypogean germination happens when the seed remains in the ground.

• 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.