Seed is the characteristic reproductive body of flowering plants (angiosperms) and gymnosperms. It consists of a miniature undeveloped plant embryo and stored food reserves enclosed in a protective outer coating called the testa.
Using seeds to grow your own plants allows you to choose from many more varieties than are available at garden centers. However, seeds must first germinate to become seedlings.
In plants, embryonic development can mean anything from the forming egg-cell framed after fertilization in angiosperms or the buds that structure on stems in gymnosperms. It additionally alludes to the underdeveloped state of a plant, which is found in seeds of angiosperms and gymnosperms.
Embryonic development happens as an asymmetric transverse cell division in the zygote. This produces two cells – a small apical cell resting on top of a large basal cell. These two cells give rise to different structures, establishing polarity in the embryo.
During embryogenesis, the concentration of IAA, cytokinin, gibberellins, and abscisic acid (ABA) is high. These hormones encourage cell growth, pattern formation, and polarity establishment in the embryo. However, at the torpedo stage, they trigger embryo maturation, in which a reduction in cell division occurs and H3.1 is replaced by H3.3. H3.3 is critical in embryo maturation because it reprograms the embryo epigenome for the acquisition of post-embryonic developmental potentials.
The endosperm is the nutritive tissue surrounding the embryo. It provides nourishment during the dormant period of the seed and is the primary source of energy for seeds in Angiosperms and Gynosperms. It also controls embryo development and reserves.
Cellular endosperm development involves a series of syncytial cell divisions. The first divisions result in eight endosperm nuclei evenly distributed along a curved region of the micropylar-chalazal (MC) axis. Cellular division continues in this manner until the MC reaches its destination at the chalazal pole, where the syncytial endosperm becomes cellular (Fig. 1).
At the beginning of syncytial cell division, a DNA methylation pattern is established that is largely maintained through subsequent mitosis. A genome-wide profiling study of H3K27me3 in wild-type endosperm revealed that the FIS PcG complex represses a specific set of genes around the site of cellularization, which is consistent with the idea that the FIS complex prevents somatic traits from being acquired during syncytial endosperm development (Weinhofer et al., 2010).
Seed coats are complex structures that control a number of processes including germination, development and nutrient flow. The structure and composition of the seed coat are determined by specialized tissues that differentiate to serve various functions. Nutrients passing from the embryo and endosperm through the seed coat determine the rate of imbibition which in turn controls germination. The seed coat also provides an effective barrier against pathogen penetration.
The heritable trait of seed coat hardness, a major factor determining water permeability, is linked to the lignin content and morphological characteristics of the testa (Agrawal & Menon 1974). Nonetheless, the impermeability of the seed coat does not depend solely on its thickness since seeds that can withstand mechanical damage tend to have thinner testas.
In diploid seeds such as legumes, the seed coat is further protected by a pericarp and the plumule and radicle. Monocots have additional structures known as the coleoptile and coleorhiza which act as sheaths that enclose the plumule, radicle and hypocotyl.
Seeds require the right conditions to germinate and grow into seedlings. These include water, temperature and oxygen. Germination also requires the right triggers. These may be physical (the action of light falling on the seed) or biological (the activity of a plant DNA ligase).
The first step is called imbibition, which is the uptake of water by the seed. This makes the seed swell and split its seed coat, and also breaks down some of its food reserve, which gives it energy.
The swollen embryo then starts growing, and the radicle emerges from the plumule, creating an anchoring root and completing germination. The cotyledons then develop leaves. Oxygen is required for respiration, which will be the seedling’s main source of energy until it grows its own leaves. If the cotyledons can’t access oxygen, they will die. The ability to access oxygen is controlled by the permeability of the seed coating and soil pore space. Seeds that have been buried too deeply in the soil are starved for oxygen.