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You are watching: Why are seeds an important structural adaptation

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Oromia Agricultural Research Institute, Holeta Bee Research Center, Ethiopia

Correspondence: Tura Bareke, Oromia Agricultural Research Institute, Holeta Bee Research Center, Ethiopia

Received: September 12, 2017 | Published: August 8, 2018

Citation: Bareke T. Biology of seed development and germination physiology. Adv Plants Agric Res. 2018;8(4):336-346. DOI: 10.15406/apar.2018.08.00335

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The evolution of the seed represents a remarkable transition for photosynthetic organisms. It is the most complex and successful method of sexual reproduction in vascular plants. Seed contain the genetic wisdom of the past and the potential for its perpetuation in the future. Seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Seed evolution eliminates requirement for water during sexual reproduction and allows fertilization event to occur over long distances. The germination of seeds in a particular situation and season is determined by the interaction between the dormancy releasing factors, which influence on the termination of dormancy or initiation of germination and seedling growth in many plant species like phyto-hormones, light, temperature, water, nutrients, moisture or mechanical cues. Seeds of different plants need different pretreatment to get vigor seedling and even for production. Therefore, real attention should be given for the plant propagation, particularly for indigenous tree species and seedling multiplication should be considered as our culture to make suitable environment.

Keywords: ovule, seed, dormancy, physiology, germination


Introduction

All land plants are believed to have had their origin in a unicellular green algal ancestor. The ancestors of early plants were highly dependent on water, not only to maintain their moisture contents but also for structural support. In a terrestrial environment, the surrounding media is air rather than water. Air does not provide any support for upright growth. Because desiccation, or drying out, is a constant danger for organisms exposed to air. However, with the evolution of key morphological innovations, plant-dominated ecosystem has gradually increased leading to major biome formation (Boundless, 2014). The more sophisticated and differentiated multi cellular anatomy, the invention of new reproductive structures and strategies were fundamental for the success of terrestrial colonization by plants.1 There are several reproductive adaptations in plants. The first is the ability to alternate life cycle stages between a diploid (sporophyte) and multi-cellular haploid stage (gametophyte). Related to this adaptation is the evolutionary trend to increase the size and independence of the sporophyte. Evolution of pollen is also an adaptation that allows reproduction to occur over great distances and in the absence of free water. Seeds allow for a dormancy stage and provide food for the developing embryo. Flowers promote efficient pollination, and fruits aid in seed protection and dispersal.2 The seed habit is the most complex and successful method of sexual reproduction in vascular plants. The seed plants comprise two major groups: the Acrogymnospermae (also referred to as gymnosperms; c. 800 living species) and the Angiosperm (also referred to as angiosperms; c. 250000 living species).3 These groups are by far the most diverse lineages within the vascular plants. Gymnosperms have no flowers or fruits, and have unenclosed or “naked” seeds on the surface of scales or leaves. Gymnosperm seeds are often configured as cones, while angiosperms, also called flowering plants, have seeds that are enclosed within an ovary (usually a fruit). The characteristics that differentiate angiosperms from gymnosperms include flowers, fruits, and endosperm in the seeds.4 Seeds are the connection between the past and the future. They contain the genetic wisdom of the past and the potential for its perpetuation in the future. The natural packaging of genetic information in a seed is remarkable in itself as a package to protect the genotype for long periods of time when properly stored in germplasm collections. Therefore, this review gives an overview of the seed evolution, Biology of seed development and germination physiology of seed.

Adaptive mechanism of plants on land

The major challenge for early plants first migrating onto land was the lack of water. To overcome such problem, plants have been developed the new structures that help them to colonizing the new and dry environments.2 (Table 1)

Seed evolution

The seed plants are a monophyletic lineage within the lignophytes. The major evolutionary novelty that unites this group is the seed. A seed is defined as an embryo, which is an immature diploid sporophyte developing from the zygote, surrounded by nutritive tissue and enveloped by a seed coat. The embryo generally consists of an immature root called the radicle, a shoot apical meristem called the epicotyls, and one or more young seed leaves, the cotyledons; the transition region between root and stem is called the hypocotyls. An immature seed, prior to fertilization, is known as an ovule.5 (Figure 1) Evidence from the fossil plant record indicates that plants were producing sporangia yielding two kinds of spores from the early Devonian (~140 million years ago). These include megaspores and microspores.6 The transition from plants that were homo sporous (one spore size) to heterosporous (two spores’ sizes) is considered one of the most important evolutionary trends in the development of seed- bearing plants. It is postulated that the larger spores of heterosporous plants were the precursor of ovules, and the small spores, the precursor to pollen. Retention of the megaspore in the megaspore in the mega sporangium was the first in the direction towards evolution of the ovule. The evolution of the seed represents a remarkable life-history transition for photosynthetic organisms. The evolution of the seed involved several steps. The exact sequence of these is not certain, and two or more steps in seed evolution may have occurred concomitantly.5

The probable steps in seed evolution are as follows:

Heterospory: Heterospory is the formation of two types of haploid spores within two types of sporangia: large, fewer numbered megaspores, which develop via meiosis in the mega sporangium, and small, more numerous microspores, the products of meiosis in the The ancestral condition, in which a single spore type forms, is called homospory. Each megaspore develops into a female gametophyte that bears only archegonia; a microspore develops into a male gametophyte, bearing only antheridia. Although Heterospory was prerequisite to seed evolution, there are fossil plants that were heterosporous but had not evolved seeds, among these being species of Archeopteris. Note that heterospory has evolved independently in other, non-seed plants, e.g., in the extant lycophytes Selaginella and Isoetes and in the water ferns.Endospory: Endospory is the complete development of the female gametophyte within the original spore wall. The ancestral condition, in which the spore germinates and grows as an external gametophyte, is called exospory.Reduction of megaspore number to one: Reduction of megaspore number occurred in two ways. First, there evolved a reduction in the number of cells within the mega sporangium that undergo meiosis (each termed a megasporocyteor megaspore mother cell) was reduced to one. After meiosis, the single diploid megasporocyte gives rise to four haploid megaspores. Second, of the four haploid megaspores produced by meiosis, three consistently abort, leaving only one functional megaspore. This single megaspore also undergoes a great increase in size, correlated with the increased availability of space and resources in the mega sporangium.Retention of the megaspore: This was accompanied by a reduction in thickness of the megaspore wall.

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Evolution of the integument: Most likely, the final event in seed evolution was the envelopment of the mega sporangium by tissue, called the The integument grows from the base of the mega sporangium (which is often called a nucellus when surrounded by an integument) and surrounds it, except at the distal end.5

Significance of seed evolution

Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes), seeds contain a diploid embryo that will germinate into a sporophyte. It has storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, freeing reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.2 From an ecological perspective, seeds are at the intersection of a species with its environment. The fundamental reason for the existence of seeds is to assure the survival of plant species. Beyond this important biological function, they represent the basis to high agricultural productivity and play an important role as a direct and indirect source of food for humans and animals, materials for industry and other essential products for the maintenance and improvement of the quality of human life. In this way, as plant breeding becomes more diversified and provides surprising results, there are increases in demand for seeds with enhanced attributes and efficient performance in field.