Ördög Vince, Molnár Zoltán (2011)
Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem
The development of a mature plant from a single fertilized egg follows a precise and highly ordered succession of events. The fertilized egg cell, or zygote, divides, grows, and differentiates into increasingly complex tissues and organs. In the end, these events give rise to the complex organization of a mature plant that flowers, bears fruit, senesces, and eventually dies. These events, along with their underlying genetic programs and biochemistry, and the many other factors that contribute to an orderly progression through the life cycle, constitute development.
The meaning of the terms growth, differentiation and development
Three terms routinely used to describe various changes that a plant undergoes during its life cycle are growth, differentiation, and development.
Growth is an irreversible increase in volume or size
Growth is a quantitative term, related only to changes in size and mass. For cells, growth is simply an irreversible increase in volume. For tissues and organs, growth normally reflects an increase in both cell number and cell size.
Growth can be assessed by a variety of quantitative measures. Growth of cells such as bacteria or algae in culture, for example, is commonly measured as the fresh weight, cell number or packed cell volume in a centrifuge tube. For higher plants, however, fresh weight is not always a reliable measure. Most plant tissues are approximately 80 percent water, but water content is highly variable and fresh weight will fluctuate widely with changes in ambient moisture and the water status of the plant. Dry weight, determined after drying the material to a constant weight, is a measure of the amount of protoplasm or dry matter (i.e., everything but the water). Dry weight is used more often than fresh weight, but even dry weight can be misleading as a measure of growth in certain situations. Consider the example of a pea seed that is germinated in darkness (Figure 3.4). In darkness, the embryo in the seed will begin to grow and produce a shoot axis that may reach 25 to 30 cm in length. Although we intuitively sense that considerable growth has occurred, the total dry weight of the seedling plus the seed will actually decrease compared with the dry weight of the seed alone prior to germination. The dry weight decreases in this case because some of the carbon stored in the respiring seed is lost as carbon dioxide. In a situation such as this, either fresh weight or the length of the seedling axis would be a better measure of growth. Length, and perhaps width, would also be suitable measures for an expanding leaf. There is not any special universal measure unit to characterize plant growth.
Figure 3.4 Changes in fresh and dry biomass of pea seed as it develops into seedling in darkness (source: Salisbury F.B., Ross C.W., 1992)
It should be obvious that many parameters could be invoked to measure growth, dependent to some extent on the needs of the observer. Whatever the measure, however, all attempts to quantify growth reflect a fundamental understanding that growth is an irreversible increase in volume or size.
Differentiation refers to qualitative changes that normally accompany growth
Differentiation occurs when cells assume different anatomical characteristics and functions, or form patterns. Differentiation begins in the earliest stages of development, such as, when division of the zygote gives rise to cells that are destined to become either root or shoot. Later, unspecialized parenchyma cells may differentiate into more specialized cells such as xylem vessels or phloem sieve tubes, each with a distinct morphology and unique function.
Differentiation is a two-way street and is not determined so much by cell lineage as by cell position with respect to neighboring cells. Thus, even though some plant cells may appear to be highly differentiated or specialized, they may often be stimulated to revert to a more embryonic form. For example, cells isolated from the center of a tobacco stem or a soybean cotyledon and cultured on an artificial medium may be stimulated to reinitiate cell division, to grow as undifferentiated callus tissue, and eventually to give rise to a new plant (Figure 3.5). It is as though the cells have been genetically reprogrammed, allowing them to reverse the differentiation process and to differentiate along a new and different path. This ability of differentiated cells to revert to the embryonic state and form new patterns without an intervening reproductive stage is called totipotency. Most living plant cells are totipotent – something akin to mammalian stem cells – and retain a complete genetic program even though not all of the information is used by the cell at any given time.
Figure 3.5 (a) Regeneration of shoots on leaf explants of carnation as a sign of totipotency. (b) Regenerated shoots can be isolated for elongation and rooting (source: Jain S.M., Ochatt S.J., 2010)
Development is the sum of growth and differentiation
Development is an umbrella term, referring to the sum of all of the changes that a cell, tissue, organ, or organism goes through in its life cycle. Development is most visibly manifested as changes in the form of an organ or organism, such as the transition from embryo to seedling, from a leaf primordium to a fully expanded leaf, or from the production of vegetative organs to the production of floral structures. Embryogenesis, vegetative, and reproductive development are the stages of sporophytic development of higher plants.
During embryogenesis, the single-celled zygote elaborates a rudimentary but polar organization that features groups of undetermined cells contained in the shoot and root apical meristems. During vegetative growth, indeterminate patterns of growth, which reflect inputs from both intrinsic programs and environmental factors, yield a variable shoot and root architecture. During reproductive development, vegetative shoot apical meristems are reprogrammed to produce a characteristic series of floral organs, including carpels and stamens, in which the haploid gametophytic generation begins.
The nature of plant meristems
Unlike animals, which are characterized by a generalized growth pattern, plant growth is limited to discrete regions where the cells retain the capacity for continued cell division. These regions are called meristems. Two such regions are the apical meristems located at the tips of roots and stems. These regions of active cell division are responsible for primary growth, or the increase in the length of roots and stems.
Meristems are centers of plant growth
The tip of the root is covered by a root cap, which provides mechanical protection for the meristem as the root grows through the abrasive soil medium. The root cap also secretes polysaccharides, which form a mucilaginous matrix called mucigel. Mucigel lubricates the root tip as it moves through the soil. The root cap along with its coating of mucigel is also involved in perception of gravity by roots. The root apical meristem (RAM) is a cluster of dividing cells located at the tip of the root just behind the root cap. Each time a cell in the meristem divides, one of the two daughter cells will be retained to continue cell division while the second daughter cell proceeds to elongate, thus increasing the length of the root and pushing the root tip through the soil. In the center of the meristem is a region of slowly dividing cells called the quiescent zone. Cell divisions responsible for new tissues in the elongation root and regeneration of the root cap take place around the periphery of the quiescent zone.
The shoot apical meristem (SAM) is structurally more complex than the root apical meristem (Figure 3.6). This is understandable because in addition to producing new cells that elongate and extend the length of the axis of the shoot, the shoot apical meristern must also form primordia that give rise to lateral organs such as leaves, branches, and floral parts. Similar to the root apical meristem, each time a cell divides in the SAM, one daughter cell is left behind to elongate and move the shoot apex forward while the other daughter cell remains within the meristem to continue dividing.
Figure 3.6 The shoot apical meristem generates the aerial organs of the plant. (A) the layered appearance of the shoot apical meristem. (B) the shoot apical meristem also has cytohistological zones (source: Taiz L., Zeiger E., 2002)
Tissues that are derived directly from the root and shoot apical rneristems are called primary tissues. The stems and roots of woody plants, however, grow in diameter as well. An increase in diameter results from the activity of a meristem called the vascular cambium. Tissues laid down by the vascular cambium are called secondary tissues, so the vascular cambium is responsible for secondary growth. The primary tissue of roots and shoots contains a central core of vascular, or conducting, elements. The vascular cambium develops between the xylem and phloem and produces new xylem toward the inside and new phloem toward the outside. Because of its heavy cell walls and eventual lignification, xylem is a rigid and long-lasting tissue that eventually occupies the bulk of most woody stems or trunks. Phloem is a more fragile tissue and with each year's new growth the previous year's cells tend to be pushed outward and crushed.
The root and shoot apical meristems use similar strategies to enable indeterminate growth
Although it might seem difficult to imagine two parts of a plant more different than a shoot and a root, certain features of the RAM and SAM and the roles they play in enabling indeterminate patterns of growth invite comparisons. Each of these structures features a spatially defined cluster of cells, termed initials, that are distinguished by their slow rate of division and undetermined fate. As the descendants of initials are displaced away by polarized patterns of cell division, they take on various differentiated fates that contribute to the radial and longitudinal organization of the root or shoot and to the development of lateral organs.
The development, maturation, and germination of seeds
The life of an individual plant begins when an egg nucleus in the maternal organs of a flower is fertilized by a sperm nucleus to form a zygote. Growth and differentiation of the zygote produces an embryo contained within a protective structure called a seed. Under appropriate conditions, the embryo within the seed will renew its growth and will continue to develop into a mature plant.
Seeds bearing embryos are formed in the flowers
Flowers appear to vary enormously in structure, yet all flowers follow the same basic plan. A generic flower consists of four whorls or circles. The two outermost whorls – the sepals and petals – are vegetative structures; and the two innermost – the stamens and pistil – are the male and female reproductive structures, respectively. At the base of the pistil, or female structure, is the ovary, which contains one or more ovules.
Within each ovule, a single large diploid cell, called the megaspore mother cell, undergoes mitosis to produce four megaspore cells. Only one megaspore cell survives and that cell undergoes meiotic division to produce an embryo sac with eight haploid nuclei. Subsequent cell division produces a mature embryo sac in which the eight nuclei are segregated into seven cells. One of those cells is the egg. Another is the large central cell containing two polar nuclei.
The male structures, or stamens, surround the pistil and consist of an anther perched on a stalk, or filament.
In some flowers, the sepals and petals may both be colored. Pollen, containing the sperm nucleus, is produced in the anthers of the stamens. The female egg cells are produced in the ovary at the base of the pistil. Pollen is transferred to the stigma or stigmatic surface of the pistil, where it sends out a pollen tube that grows down the style and delivers the sperm nucleus to the egg.
The anther contains a large number of microspore mother cells, each of which undergoes meiotic division to form uninucleate, single-celled microspores. The microspores subsequently become encased in heavy, resistant outer walls and the nucleus divides mitotically, forming two cells – a tube cell and a generative cell – within the original spore wall. This is the mature pollen grain.
Mature pollen grains are shed from the anthers and carried to the stigmatic surface of the pistil by insects, wind, or some other vector. Once the pollen grain lands on the stigmatic surface – an event called pollination – the pollen grain takes up water and sends out a pollen tube that grows down the style of the pistil toward the ovule. The tube nucleus migrates down the pollen tube and appears to direct its growth. The cell wall of the generative cell breaks down and the generative nucleus divides once to form two sperm nuclei that follow the tube nucleus down the tube as it elongates.
In the final stage, the elongating pollen tube enters the ovule by growing through the micropyle (the space between the ends of the surrounding integuments) and releases the two sperm nuclei into the embryo sac. Ultimately, one of the two sperm nuclei enters the egg cell and fertilizes the egg cell nucleus to form the zygote. The second sperm nucleus enters the large central cell and fuses with the two polar nuclei to form a triploid endosperm nucleus. The endosperm nucleus will go on to form the primary nutritive tissue, or endosperm, for the developing embryo. The involvement of two sperm nuclei in this way is called double fertilization, a characteristic unique to the flowering plants or angiosperms.
Seed development is characterized by extensive cell divisions
The development of a seed begins with the fertilized ovule, or zygote. The early stage of seed development is characterized by extensive cell divisions that form the embryo and, in endospermic seeds, the tissues that store nutrients that will support the eventual germination of the seed and seedling development.
The first division of the zygote is usually transverse and immediately establishes polarity of the embryo. The upper cell is destined to become the embryo itself while the lower cell produces a stalk-like suspensor that anchors the embryo at the base of the embryo sac. The typical dicot seed will then pass through several recognizable stages (Figure 3.7). During the early stages of embryo development, cell division occurs throughout the entire cell mass but at the heart-shape stage both the shoot and root apical meristems begin to organize as centers of cell division.
Figure 3.7 Pattern formation during Arabidopsis embryogenesis (source: Taiz L., Zeiger E., 2010)
Nutrients are stored in endosperm that will support germination and seedling
Throughout the development of the embryo, there is a continuous flow of nutrients from the parent plant into the endosperm or the cotyledons. In some cases, such as the cereal grains and most other monocots, he endosperm is retained until maturity and may comprise the bulk of the seed. These are called endospermic seeds. The endosperm of mature endospermic seeds consists of cells filled with starch along with protein and some small amounts of lipid. In some monocot seeds, the endosperm is surrounded by one or more distinctive layers of cells, called the aleurone. Aleurone cells are distinguished by the presence of numerous protein bodies and are the source of enzymes needed to mobilize nutrients during germination. Endospermic dicot seeds have retained a significant amount of endosperm and at maturity the cotyledons are thin, leaf like structures. In nonendospermic dicot seeds the cotyledons enlarge at the expense of the endosperm and may occupy as much as 90% of the seed volume at maturity. Both endosperm and cotyledons contain large quantities of stored carbon (in the form of carbohydrates, lipids, and protein), mineral elements, and hormones that support the growth and development of the seedling until it can establish itself as a photosynthetically competent plant.
Maturation is characterized by cessation of embryo growth and development of desiccation resistance
Maturation is terminated by a dramatic desiccation in which the water content of the seed is reduced from 80% or 90% to approximately 5%. Surrounding the mature seed is a hard coat derived from maternal tissues (the integuments) which surrounded the seed during its development in the ovary. Comprised of heavy-walled cells and covered with a thick, waxy cuticule, the seed coat often presents a significant barrier to the uptake of both water and oxygen by the seed.
Germination is resumption of embryo growth
Because seeds are severely dehydrated, any metabolic reactions take place so slowly they are scarcely detectable. Seeds are thus quiescent, or resting, organs that represent a normal hiatus in the life cycle of a plant. The embryo appears to be in a state of suspended animation, capable in some cases of surviving adverse conditions for long periods of time. Resumption of embryo growth, called germination, is dependent upon a number of factors, but three are especially important: adequate water to re-hydrate the tissues, the presence of oxygen to support aerobic respiration, and a “physiological” temperature. Although many seeds will germinate over a wide range of temperatures, the optimum range for most seeds is 25°C to 45°C.
The initial step in germination of seeds is the uptake of water and rehydration of the seed tissues by the process of imbibition. Like osmosis, imbibition involves the movement of water down a water potential gradient. Imbibition differs from osmosis, however, in that it does not require the presence of a differentially permeable membrane and is driven primarily by surface-acting or matric forces. In other words, imbibition involves the chemical and electrostatic attraction of water to cell walls, proteins, and other hydrophilic cellular materials. Matric potential, like osmotic potential, is always negative.
Imbibition of water is followed by a general activation of seed metabolism within minutes of water entering the cells, initially utilizing a few mitochondria and respiratory enzymes that had been conserved in the dehydrated state. Renewed protein synthesis is also an early event, utilizing preexisting RNA transcripts and ribosomes, as existing organelles are repaired and new organelles are formed. This is followed closely by (1) the release of hydrolytic enzymes that digest and mobilize the stored reserves, and (2) renewed cell division and cell enlargement in the embryonic axis. Seeds that store carbon reserves principally in the form of fats and oils will carry out the synthesis of hexose sugars via gluconeogenesis.
In most species, germination is considered complete when the radicle emerges from the seed coat. Radicle emergence occurs through a combination of cell enlargement within the radicle itself and imbibition pressures developed within the seed. Rupture of the seed coat and protrusion of the radicle allows it to make direct contact with water and nutrient salts required to support further growth of the young seedling.
Many seeds will not germinate even though the minimal environmental conditions have been met. These seeds are said to be dormant and will not germinate until additional conditions have been met. The most common causes of seed dormancy are the impermeability of the seed coat to water or oxygen or physiological immaturity of the embryo at the time the seed is shed from the mother plant. Immature seeds must undergo complex biochemical changes, collectively known as after-ripening, before they will germinate. After-ripening is usually triggered by low temperature, a mechanism that appears to ensure that the seed will not germinate precociously in the fall but will germinate when favorable weather returns in the spring.
The pattern of development from embryo to adult
The first structure to emerge when a seed germinates is the radicle. The radicle, which is the nascent primary root, anchors the seed in the soil and begins the process of mining the soil for water and nutrients. As the primary roots elongates, it gives rise to branch, or lateral, roots. Unlike the situation in the shoot apical meristem, lateral roots do not originate in the root apical meristem. Lateral root primordia originate in the pericycle, a ring of meristematic cells that surround the central vascular core, or stele, of the primary root. The growing lateral root works its way through the surrounding cortex, either by mechanically forcing its way through or by secreting enzymes that digest the cortical cell walls. Lateral root primordia arise in close proximity to the newly differentiated xylem tissue, which allows vascular elements developing behind the growing tip of the lateral root to maintain connections with the xylem and phloem of the primary root.
Emergence of the radicle is followed by elongation of the shoot axis. It proceeds through a combination of cell division and enlargement of the cells laid down by the meristem. The rate and extent of elongation is subject to a variety of controls, including nutrition, hormones, and environmental factors such as light and temperature. The final height of a shoot is determined by the rate and extent to which internodes – the sections of stem between leaf nodes – elongate. In some plants, such as pea (Pisum sativum), elongation occurs primarily near the apical end of the youngest internode. The older internodes effectively complete their elongation before the next internode begins. In other plants, elongation may be spread through several internodes, which elongate and mature more or less simultaneously. Still others exhibit changing rates of elongation with successive internodes, usually increasing. In some plants, internodes fail to elongate, thus giving rise to the rosette habit in which all the leaves appear to originate from more or less the same point on the stem. This rosette habit is common in biennial plants (those that flower in the second year) such as cabbage and root crops such as carrot (Daucus carota) before they reach the flowering stage. Failure of internode elongation is commonly related to low levels of the plant hormone, gibberellin, since application of the hormone usually stimulates internode elongation in rosette plants.
Senescence and programmed cell death
The final stage in the development of cells, tissues, and organs is senescence, an aging process characterized by increased respiration, declining photosynthesis, and an orderly disassembly of macromolecules. Senescing cells and tissues are metabolically very active – a number of metabolic pathways are turned off and new pathways, principally catabolic in character, are activated. Catabolism of proteins, for example, releases organic nitrogen and sulfur in the form of soluble amines, while nucleic acids release inorganic phosphate. Chlorophyll is broken down and lipids are converted to soluble sugars via gluconeogenesis. The products of these pathways are all small, soluble molecules that are readily exported from the senescing tissue. Senescence thus enables the plant to recover nutrients from cells or tissue that have reached the end of their useful life and reallocate them to other parts of the plant that survive or for storage in the roots.
Programmed cell death (PCD) is a specialized type of senescence
Programmed cell death (PCD) is broadly defined as a process in which the organism exerts a measure of genetic control over the death of cells. PCD requires energy and is normally regulated by a distinct set of genes.
PCD is essential for normal vegetative and reproductive development. One example is the development of xylem tracheary elements. In order to function efficiently as a conduit for water transport, the protoplast of the developing tracheary element must die and be removed at maturity. PCD also operates in the formation of aerenchyma, a loose parenchymal tissue with large air spaces. Aerenchyma normally forms in the stems and roots of water lilies and other aquatic plants. These air spaces, created by a cell death program, provide channels for oxygen transport to the submerged portions of the plant. Even corn (Zea mays) and other terrestrial plants can be induced to form aerenchyma when subject to flooding.
PCD is also an important factor in plant responses to invading pathogens and abiotic stress. When a plant recognizes a pathogen, for example, host cells in the immediate area of the infection undergo PCD. This deprives the invading pathogen of living tissue and either slows or prevents it spread.