Some where deep down in the soil a seed is on its way to take off a journey in to the world. It’s waiting eagerly for all the favorable conditions to set in for it to sprout from the ground. The new life, which results from the fusion of the male and female gametes, is actively dividing inside the seed utilizing all the food reserves of endosperm. The single celled zygote divides to form 2-celled stage then 4-celled stage via a continuous division of the active cells. At the 2 celled stage the apical and basal tiers of the embryo are established with a suspensor cell below. Later the embryo becomes globular, it’s before the globular stage i.e early globular stage during which the polarity of the embryo is established. The efflux carriers, carriers of the polar auxin assemble at the basal region of the cell giving the zygote a sense of upper and lower portions of its body.
Scientists have gone further in the research of polarity establishment by evolving mutants, one such mutant of polarity group interest is Gnom. Gnom mutant lacks the senses to establish the polarity of the embryo. It has been found that a protein called efflux carrier protein is absent in the Gnom mutant, which is essential for the alignment of the efflux carriers. These mutant studies have put the threads of the story together to give a hypothesis for the establishment of polarity in the embryo.
With the embryo sensing its upright positions it forms the upper and lower tier of cells in the globular stage. The suspensor divides and forms a thread like structure beneath a ball of cells. The lower region of the ball of cells has specialized cells called the hypophysis. The upper cell of this hypophysis is a progenitor of the quiescent center in the root while the lower cell gives rise to the root cap. Later to this globular stage is the early heart stage during which the root and shoot meristems are formed. Followed by this is the heart stage where the protoderm, ground meristem and procambium are clearly visible. The suspensor which is beneath the site of action of these differentiation's is used by the developing embryo to anchor the tissue to maternal tissue, to provide with the nutrition absorption and in some species it provides proteinaceous substance. Unlike animal’s umblical chord, suspensor in plants have no cytosolic connection with the embryo, but there are species in which invaginations and modifications in the suspensor aid in the transport. The embryo then assumes the torpedo stage with the visible shoot and root apical meristem. The take off stage in embryogenesis is the walking stick stage the embryonic cotyledonary structures bend back to give this stage.
It's amazing to know how the in built and external characteristics of the seed effect the germination of the seed after the zygote had done so many feat's to make itself eligible for germination. Some seeds have a tough seed coat making that impermeable for the water and gaseous exchange while some have the in built mechanisms such as the inhibitors which control there life time in the stage of seed. This tiny embryo, which is safely embedded in the hard seed, in case of the conditions being rough it stays in there waiting patiently for its time to sprout. Since long time seed germination has fascinated many scientists to study what actually triggers the dormancy and quiescence. Findings of the science have unraveled certain mysteries of this puzzle. Dormancy is an inherent character of the species during which the embryo in the seed can either be mechanically or chemically be suppressed from germination. This is for the own good of the seed. Another factor that can actually inhibit the seed germination is the quality of light. Role of phytochrome was discovered in the seed germination of lettuce. Here, the red light is required for the conversion of Pr to Pfr, which is biologically active. Isolation of cop and dete mutants has revealed the existence of a separate receptor proteins to each colour of light and convergence of all the signals from the receptors to a single place, which actually controls various activities in the seedling.
Finally, when the seed germinates in the
presence of water, air and turns its head towards the sunlight, there might be
adverse conditions, which the seed is unaware of during its rest in the soil.
This tender seedling can then set on the quiescence to arrest its further
growth into the world of tough luck. Quiescent period is that window of period
to give the seedling a second chance to its life. During this period all the
metabolic activities are cut down. Studies have helped in understanding the
action of
In the young seedling the roots are developing beneath the ground silently. Roots play a pivotal role in the plant’s life by supplying the water and nutrients. Apart from this roots have many major roles to play in some plants in getting adapted to the environmental conditions. With one major exception, the general pattern of root growth and development is essentially the same for all species of plant. The resulting overall root system morphology is, however, highly diverse with species, cultivars and even family specific characteristics. The exception to the underlying similarity is the development of secondary vascular tissues in dicotyledonous but not monocotyledonous plants. Root growth begins with the first stages of activity in the embryo, and continues until well after the death of the shoot. Radicle elongation frequently signals outward signs of the beginning of germination. In species lacking a radicle, a seminal root usually acts as a replacement radicle. As this first root (referred to as the tap root) elongates, it develops root hairs and in two to three days the first lateral branches begin to develop at its base and in an acropetal direction (relative to the tap root tip). Depending on species or cultivar characteristics, seminal roots begin to emerge from the embryo or seedling with in the first day or two in an acropetal (relative to the shoot) direction. As these seminal roots elongate, they also develop root hairs and eventually lateral roots.
As the cells are formed in the zone of cell division in the roots they enter the zone of elongation and further the zone of differentiation. Mature cells above the newly formed cells send the signals for differentiation. It has been found that highest concentration of auxin in roots is found in the columella initials of the root. An experiment where this concentration is disturbed has resulted in the increase of progenitor cells instead of giving the cells their respective fates. This shows the role of auxin in determing the fate of the new cells.
Molecular program for the development of lateral roots is same as that of the primary roots except for the cells from which they arise. Primary roots arise from the undifferentiated meristematic cells whereas the lateral roots arise from the differentiated pericycle cells. This reprogramming of the already differentiated cells is a complex and less understood feature in the lateral root development. Isolation of alf (aberrant lateral root formation) mutants in Arabidopsis has made the scientists to propose a pathway for the lateral root formation in plants. This model proposed a three-gene concept in lateral root development. ALF 1-1 controls the concentration gradient of auxin running from shoot to root. A mutant to this gene puts forward lots of lateral roots, showing that this gene is actually involved in the negative control of the lateral root formation. ALF 4-1 is the gene believed to be responsible for the detection of the auxin by the pericycle cells in the root. A mutant of this gene cannot form the lateral roots and it cannot be saved by the exogenous application of the auxin. Hence, it’s believed to be a gene involved in the manufacture of the auxin receptor protein in the pericycle cells of roots. ALF 3-1 is the gene involved in the auxin control at the tip of the lateral root primordial. A mutant for this gene produces no lateral roots but has small outgrowths of lateral root primordia, which die after 3-4 days, these can be rescued by the exogenous application of auxin.
Root hairs are the epidermal structures of the root formed from a set of cells called the trichoblast cells. These trichomes are specific in their arrangement pattern. In other words the development of the trichoblasts and atrichoblasts in the epidermal layer of the root is determined during the torpedo stage of embryogensis, is highly specific.
The internal anatomy of a plant will change as the plant matures. In plants, unlike in animals, organ initiation and development continues throughout the life cycle. Moreover, there is plasticity in plant development that allows it to respond to environmental conditions. The changes in meristem identity or function result from a genetically regulated developmental program that is influenced by different environmental factors such as light and day length. The internodes in the terminal bud are very short so that the developing leaves grow above the apical meristem that produced them and thus protect it. New meristems, the lateral buds, develop at the nodes, each just above the point where a leaf is attached. When the lateral buds develop, they produce new stem tissue, and thus branches are formed. Under special circumstances (such as changes in photoperiod), the apical meristem is converted into a flower bud. This develops into a flower. The conversion of the apical meristem to a flower bud is a permanent differentiation so that no further growth of the stem occurs. However, lateral buds behind the flower can develop into branches.
Since all plant organs develop from meristems, these structures are where plant development and plant forms are ultimately regulated. Meristems are formed during embryogenesis and consist of groups of undifferentiated cells that will initiate organ primordia during plant life. The apical shoot meristem give rises to the complete shoot of the plant. Different zones can be distinguished within the apical shoot meristem. Cells in the central zone are large and divide infrequently. In vegetative and inflorescence meristems, the central cells play a role comparable to animal stem cells in that they are characterized by their undifferentiated state and their ability to give rise to daughter cells that differentiate into specialized cell types. There is a regulated intercellular communication between the cells of central zone and the peripheral zone. To know what a cell becomes, fluorescence dye is used to mark one of the cells in the central zone of the shoot apical meristem. The dye is localized to the central zone except for a transition zone. This suggests the existence of selective symplastic couple between these two zones. This prevents the spread of distinct signals from one zone to other.
The peripheral zone, the site of organogenesis, consists of small cells that divide rapidly and differentiate. The third zone is the rib zone, which gives rise to cells of the stem and forms the boundary against fully differentiated cells. Like cells in the peripheral zone, the rib cells originate from undifferentiated cells in the central zone. The rib zone has also been suggested to act as an organizing center for the shoot. In stems, mitosis in the apical meristem of the shoot apex (also called the terminal bud) produces cells that enable the stem to grow longer and, periodically, cells that give rise to leaves. The point on the stem where leaves develop is called a node. The region between a pair of adjacent nodes is called the internode. The measurement of time between the production of successive leaf primordia is called plastochron. The various factors effecting the branching and branch factors are competence of axillary buds to grow (age of the bud), nutritional status of the bud and distance of that bud from both shoot apical meristem and other buds (apical dominance).
The shoot tissues arise from the shoot apical meristem as dermal tissue, ground tissue, and vascular tissue. Secondary meristems (vascular cambium and cork cambium) then add girth to the plant by adding secondary xylem, phloem, and cork. The apical meristem includes the a group of dividing cells that give rise to three primary meristematic tissues, protoderm, ground meristem, and procambium. The outermost layer of cells is the protoderm, which forms a single sheet of meristematic cells that give rise to the epidermis. This epidermis eventually covers all of the newly formed organs of the stem. The ground meristem forms the pith and cortex tissues of the stem and the mesophyll (middle leaf) tissue of the leaf. The procambium forms the primary xylem and phloem in stems, leaves as well as floral appendages.
The sites of leaf primordia show the characters of change in rate of cell division and change in orientation of division, which is signaled by change in cytoskeleton orientation. Through cell lineage analysis cells destined to become primordia can be identified 3 days before emergence. There are three tiers with 35-40 cells per tier whose fate is not yet determined. The epidermis arises from layer-1, bulk of the leaf and palisade from layer-2 and vascular tissue from layer-3.
There are two theories to explain the position of the emergence of the leaf primordia, the Field theory and the Biophysical model. According to the field theory the primordia produce an inhibitor to position the next leaf primordia but not the one immediately next. Surgical separation of one leaf primordia will not affect the emergence immediately next leaf primordia but will affect the next one. Biophysical model states that there are some biophysical factors, which actually determine the position of emergence of the leaf primordia.
As primordia elongates it establishes the apical basal axis. Dorso ventral polarity of the leaves is under the control of a single gene. The stage at which the primordia are determined as leaf depends on the species. Leaf determination is partially effected by central cells of the shoot apical meristem. It is a positional effect but not a cell lineage effect.
Determination of primordia into leaf occurs after the emergence of the primordia. There might be environmental factors, which can determine its fate to become other structures. Once the fate of the primordia is determined to be a leaf then it proceeds independently. There are two steps that follow soon after the determination of a primordia into leaf, they are the determination of the determinate verses the indeterminate growth and establishment of the dorsoventral polarity.
Trichomes are the appendages, which in aid the leaves in various ways, found on the dorsal side of the leaf. Trichome development involves various stages and isolation of the mutants for these various stages has helped the scientists understand the developmental pattern of these trichomes. Trichome distribution is nonuniform and there is a sort of a inhibitory signal in spacing of these appendages. Another special structure found only on the leaves is the stomata, these are found on the ventral side of the leaf. In monocot leaves the cells divide with the small guard mother cell always towards the tip of the leaf. This guard mother cell forms the two dumbbell shaped guard cells surrounding the stomata. In dicots the primary meristemoids forms two unequal cells and the smaller one becomes the guard mother cell. The guard mother cell undergoes symmetric division to form the guard cells. The positioning of these stomata is crucial for the gaseous exchange in plants. In monocots its the cell lineage, which determines the spacing whereas in dicots the positional lineage determines the position of the stomata.
Phloem is the conducting tissue, which has the companion cells and the cytoplasmic strands connecting through out the length of the plant. Here the conducting cells of the phloem are living without the nucleus and it is believed that the companion cells control the activity of these cells by producing the proteins needed by the sieve elements. One such protein ‘P’ produced by the companion cell helps to clog the sieve element if the cell is disrupted in its cytoplasmic strands. In the conducting cells there is a sieve plate with callose depositions around the sieves. Xylogenesis involves four steps in the formation of the xylem vessels. They are A. Microtubule reorientation, which gives rise to cell wall formation. Cellulase synthase will follow microtubules and lays down the cell wall. Golgi vesicles will follow help in depositing the other cell wall polymers. B. Cellulose and other secondary cell wall depositions like xylan get increased followed by the deposition of lignin at these secondary thickenings. C. Lignification, this is dependent up on the cell wall thickenings. It allows strengthening and waterproofing of the xylem elements. Lignification occurs by polymerization of aromatic compounds called monolignols. D. The last step is the autolysis during which the cell commits suicide. Autolysis is dependent on the secondary thickening but not dependent on the cellulose deposition. Cell death begins by rapid destruction of tonoplast. This is followed by swelling and disruption of other organelles with in hours.
Phloem formation is favored by a high sucrose/auxin concentration ratio, while xylem formation is favored by a lower ratio. W. P. Jacobs studied regeneration of vascular tissues around wounds and found that when a vascular bundle is severed in Coleus, a new bundle is formed which bypasses the wound and reconnects the cut strands. This and much other work support the view that polar movement of auxin from leaves to roots induces continuous vascular tissue differentiation along the flow of auxin. Thus auxin is the limiting and controlling factor for both phloem and xylem differentiation. At low levels of auxin only phloem is induced, suggesting that xylem does not form without phloem. The non-polar transport of auxin in phloem promotes vascular differentiation in wounding below mature leaves. Polar transport of auxin also controls the size and density of vascular elements along the plant axis. High auxin levels near young leaves induce numerous vessels that remain small and differentiate rapidly, low auxin further down results in slower differentiation and fewer but larger vessels.
The formation of the leaf vascular pattern has developmental consequences beyond the simple siting of veins. The developing veins of the leaf primordium appear to serve as morphogenetic centers that organize spatially much of the differentiating leaf cell types, through an yet undefined intercellular signaling system. This makes functional sense, since many of the cell types must function in cooperation with the veins, requiring a physical proximity. Provascular cells constitute a vascular meristematic tissue that exists transiently at sites that foreshadow the venation network as the leaf primordium develops. The process of vascular pattern formation begins with the siting of provascular cells among apparently equivalent ground cells in the leaf primordium. The polar transport of auxin is required for the progressive formation of aligned provascular cells in a pattern, and auxin is required for subsequent vascular cell differentiation. A variety of mutants with defects in auxin transport or response also exhibit defects in vascularization, and auxin transport inhibitors interfere with vein pattern formation.
Now the plant enters the final delopmental stage in its life cycle, the flowering stage. In contrast to the indeterminate shoot meristem, which essentially gives rise to flower primordia indefinitely, flowers are determinate structures that produce a defined number of organ primordia. The vegetative meristem now reprograms itself to form a completely new structure. Scientists have isolated mutants in Arabidopsis, which lack the vegetative growth but directly form the inflorescence meristem. These mutants have helped to understand the role of the emf1 repressor in the plants. This repressor keeps the plants vegetative meristem from converting to inflorescence meristem until and unless acted by the long day or GA signals on this repressor. Reproductive development occurs in two stages, one is vegetative meristem to floral meristem and other inflorescence meristem, which is a phase change to flowering and the other inflorescence meristem to floral meristem.
The organ primordial of floral meristem reside in four concentric rings called whorls, with four sepals in the outermost first whorl, followed by petals, stamens and carpels in the 2nd, 3rd, and 4th whorls, respectively. There are numerous genes required for the initiation and development of flowers, and for simplicity they can be divided into distinct classes. The first class is Flowering Time genes, mutations in which cause early or late flowering. Flowering Time genes can themselves be divided into distinct classes, based on their differential responses to a number of environmental conditions, such as day length and vernalization. The second class specifies Meristem Identity, and includes genes such as LEAFY, APETALA1, and CAULIFLOWER which specify flower meristem identity, as well as TERMINAL FLOWER, which maintains inflorescence meristem identity. Third class includes the Flower Organ Identity genes, which determine the fate of organ primordia and are incorporated into the "ABC" model of flower development. Examples of organ identity genes include APETALA1 (which is involved in both meristem and organ identity), APETALA2, APETALA 3, PISTILLATA and AGAMOUS. A fourth class includes late-acting genes that control ovule development and extensive genetic and recent molecular studies have begun to uncover the complex array of interactions among genes in this class.
With all the floral parts forming, the inside of the flower is also undergoes lot of changes. The pollen sacs are formed in the anthers, which contain the pollen mother cells. Each pollen mother cell is diploid and undergoes meiosis to produce four haploid microspores. One of the haploid microspore undergoes mitosis to form four pollen grains with a sperm cell and tube cell. Pollen grain is a two-celled gametophyte containing a generative cell and a vegetative cell. The generative cell undergoes one round of mitosis to form two sperm cells. Inside the pistil are the ovary and embryosac developing. Ovule primordium emerges, which later forms the megasporocyte. There is the funiculus at the base followed by the chalazal zone, which forms the integuments. The megasporocyte becomes nucellus at the distal region. The ovule now has the diploid megaspore mother cell, which after meiosis forms four haploid megaspores and three cells degenerate. The remaining megaspore undergoes mitosis to produce eight identical megaspore nuclei. Three of these will form the distal nuclei, while two remain at the center to form the polar nuclei. One of the nuclei forms the egg cell, which is surrounded by two such nuclei.
Once the anthesis occurs and the pollen fall on the stigma, its the chemical signals between the pollen grain and the stigma, which determine the compatibility and the formation of the pollen tube. The pollen tube formed travels down the style and into the ovary. Both the sperm enter the ovule where as one fertilizes the egg to form the diploid zygote. The other sperm fuses with the polar nuclei to form the triploid endosperm. The endosperm serves as the reserve food to the developing embryo. This endosperm ratio is what matters for the successful development of the seed. If the ratio is improper it leads to the development of improper transfer cell layer between the endosperm and the embryo leading to the abortion. Transfer cell layer is the layer through which the nutrients are supplied to the embryo. Normally, this layer is present eight days after the pollination.
Fruit is an axillary structure developed by the plant for effective seed dispersal. Fruit development occurs in three steps, ovary development, cell division and cell expansion. The stimulus of fertilization brings about the hormonal changes and results in fruit development. Fruit is initially green in color with lots of photosynthetic activity and as the embryo develops it changes its color and the texture. The texture change and softening are associated with the hormone, ethylene.
Inside the developing fruit is the developing seed. Seed development occurs in four stages: 1. Morphogenesis:here the embryo and extra embryonic tissue is formed. Endosperm supplies the food to the developing embryo. Early embryonic genes are active and the protein synthesis is high. 2. Maturation: here there is accumulation of the storage proteins required for the growth of the embryo. 3. Dessication: Ovary abscisses from the ovary wall. Late embryonic abundant proteins are synthesized. 4. Germination: Here there is accumulation of the GA and decline in the ABA.
Once the seed is ready and fruit matures, it starts its travel for ambient conditions to sprout. The journey of the next generation seed begins again where its parent plant had started. This cycle of seed to plant and plant to seed hides many more developmental secrets from the modern science, whose unraveling might help the world in better understanding of Mother Nature.
(Note: Author, Vijayanand
Nadella claims no responsibility for the validity of the information in this article
as it is a compilation from various peer reviewed journal articles)