· Growth is one of the most fundamental and conspicuous characteristics of living organisms.
· The net result is an irreversible increase in mass, weight, volume, area, size, structure, and development of a cell, tissue, or organism.
· Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm.
· In plants, growth is associated with both anabolic and catabolic activities that occur at the expense of energy.
· Growth is a quantitative phenomenon and can be measured in relation to time.
· Growth in living organisms is intrinsic and differs from extrinsic growth in nonliving objects.
· Growth in plants is open, i.e., it can be indeterminate or determinate.
· Generally, growth involves synthesis of protoplasm (nucleus and cytoplasm) or apoplasmic substances (matrix of the bone marrow, fibers of the connective tissue, or even water).
· Growth occurs when the synthetic activities (anabolism) of cells exceed their destructive activities (catabolism).
· There will be no growth if the synthesis equals destruction. Prolonged starvation may lead to increased catabolism of reserve food (e.g., fat in the adipose tissue). In extreme situation, destruction of the constituent proteins of the protoplasm, leading to decrease in the mass of living matter, may be observed. This is opposed to growth and, thus, may be called degrowth.
Human Growth:
The adult humans continuously peel off and replace more than a gram of skin cells, generate millions of erythrocytes, and renew the digestive tract epithelium from stem cells on a daily basis. Lens cells grow by multiplication. Cardiac and skeletal muscle cells grow by increase in volume. Neurons grow by extension. Cartilage and bone cells grow by secretion of extracellular matrix.
Auxetic growth:
The volume of the individual increases due to growth of the individual cells; division or proliferation of cells does not occur. Such growth is observed among the rotifers, nematodes, and some tunicates.
Multiplicative growth:
Mitotic proliferation of the constituent cells leading to an increase in number is a must. It is typically found during the embryonic growth of animals.
· Growth in plants occurs by cell division and cell enlargement, followed by cell differentiation.
· Growth in plants is generally limited to regions of growing points known as meristems.
· The form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth. One single maize root apical meristem can give rise to more than 17,500 new cells per hour, whereas cells in a watermelon may increase in size by up to 350,000 times.
· During seed germination dry weight decreases whereas fresh weight increases.
· All the changes that occur in an organism starting from its beginning till its death may collectively be termed as development.
· Development is the whole series of changes, such as growth, differentiation, and maturation, which an organism undergoes throughout its life cycle.
· Development is also associated with morphogenesis. Morphogenesis is the process of development of shape and structure of an organism.
· Development occurs even at the subcellular level, e.g., appearance of chloroplasts in cells exposed to light. The last phase of development is senescence. Senescence or old age leads to death.
Sequence of the developmental process in a plant cell
Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity, e.g., heterophylly in cotton, coriander, and larkspur. In such plants, the leaves of the juvenile piant are different in shape from those in mature plants.
The increased growth per unit time is termed as growth rate. Thus, rate of growth can be expressed mathematically.
An organism or a part of the organism can produce more cells in a variety of ways.
The growth rate may be:
· Arithmetic
· Geometrical
In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide whereas the other differentiates and matures. The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate.
On plotting the length of the organ against time, a linear curve is obtained. Mathematically, it is expressed as
Diagrammatic representation of (a) arithmetic, (b) geometric, and (c) stages during embryo development showing geometric and arithmetic phases
In most geometrical growth systems, the initial growth is slow (lag phase), and it increases rapidly thereafter-at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase.
If we plot the parameter of growth against time, we get a typical sigmoid or S-curve.
A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues, and organs of a plant.
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This curve has four phases of growth:
- Lag phase: This is the initial phase of growth when the rate of growth is very slow.
- Log phase: It shows rapid growth and is maximum for the entire life span.
- Diminishing phase: Here the growth slows down.
- Stationary phase: Here the rate of growth starts decreasing and finally it stops.
The total time period during which all the phases occur is called grand period of growth. The rate of growth is also called efficiency index.
Quantitative comparisons between the growth of living system can also be made in two ways:
- Total growth per unit time is called the absolute growth rate (AGR).
- Growth per unit time, e.g., per unit initial parameter is called the relative growth rate (RGR).
Measurment and the comparison of total growth per unit time is called the absolute growth rate.
The growth of the given system per unit tirne expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.
Relative growth rate: It is growth per unit time per unit initial growth.
The cells derived from root-apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed as differentiation.
During differentiation, cells undergo a few major structural and functional changes, as given.
- Loss of nucleus and loss of end wall
- Excretion, secretion, and perforation of end walls
- Loss of protoplasm
- Suberin/tannin deposition
- Secretion of mucilage in root cap
- Differential wall thickening and emptying of tracheids and vessels
- Development of a very strong, elastic, lignocellulosic secondary cell walls
- Secretion of latex in laticifers
The living differentiated cells in plants that have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation.
Example: Formation of interfascicular cambium and cork cambium. Such meristems divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get redifferentiated.
- Direct method: Growth in length can be directly measured by means of an ordinary measuring scale.
- Auxanometer: They are specially designed equipments. They are Pfeffers Auxanometer and Arc Indicator.
- Space marker disc: This was developed by Ganong, and it is used to measure growth in leaf area.
- Crescograph: This was developed by JC Bose and is a highly sensitive instrument. It can measure growth in seconds.
- Horizontal microscope or Travelling microscope
- Dendrograph
From experiments on coleoptile phototropism, Darwin concluded in 1880 that a growth stimulus/signal is produced in the coleoptiles tip, is transmitted to the growth zone, and causes the shaded side to grow faster than the illuminated side.
In 1913, P Boysen-Jensen discovered that the growth stimulus passes through gelatin but not through water-impermeable barriers such as mica.
In 1919, Arpad Paal provided evidence that the growthpromoting stimulus produced in the tip was chemical in nature.
In 1926, FW Went showed that the active growth-promoting substance can diffuse into a gelatin block. Went called the hormone auxin (auxein: to grow). It took 20 years before this auxin was chemically identified as indole-3-acetic acid.
In the mid-1930s, it was determined that auxin is indole-3acetic acid (IAA).
Several other auxins in higher plants were discovered later, but IAA is by far the most abundant and physiologically relevant auxin.
Three types of auxins are found in plants.
- Indole derivative: IPA, IAA, IBA
- Benzene derivative: 2,4-D, 2,4,5 -T
- Naphthalene derivative: NAA, NAAM
Gibberellins Influence Floral Initiation and Sex Determination
- In maize, the staminate flowers (male) are restricted to the tassel, and the pistillate flowers (female) are contained in the ear. Exposure to short days and cool nights increases the endogenous gibberellin levels in the tassels to 100 -fold increase and simultaneously causes feminization of the tassel flowers.
- In dicots such as cucumber, hemp, and spinach, gibberellins seems to have the opposite effect. In these species, application of gibberellin promotes the formation of staminate flowers, and inhibitors of gibberellin biosynthesis promote the formation of pistillate flowers.
Gibberellins Promote Fruit Set
- Applications of gibberellins can cause fruit set and growth of some fruits as in apple (Malus sylvestris).
- Gibberellins, like auxin, can cause the development of parthenocarpic fruits, including apples, black currants, cucumbers, and eggplants.
Malting of Barley
Malting is the first step in the brewing process．During malting， barley seeds（Hordeum vulgare）are allowed to germinate at temperatures that maximize the production of hydrolytic enzymes by the aleurone layer．
Gibberellin is sometimes used to speed up the malting process．The germinated seeds are then dried and pulverized to produce＂malt，＂consisting mainly of a mixture of amylolytic （starch－degrading）enzymes and partly digested starch．
In embryo of cereal grains has specialized absorptive organ－the scutellum－which functions in absorbing the solubilized food reserves from the endosperm and transmitting them to the growing embryo．
CYTOKININS, ETHYLENE, AND ABSCISIC ACID
Cytokinins (Regulators of Cell Division)
Kinetin was discovered as a breakdown product of DNA.
Miller isolated a natural cytokinin from kernels of maize, which he called zeatin; it is the most active of the naturally occurring cytokinins. Letham (1973) isolated this molecule.
Nearly all compounds active as cytokinins are N6-substituted aminopurines, such as benzyladenine (BA): and all the naturally occurring cytokinins are aminopurine derivatives.
A cell division factor was isolated by Miller and Skoog in 1955. They were working on the growth of tobacco pith culture and wanted to grow it indefinitely. Incidentally Miller noticed an old bottle of DNA kept for several years in the laboratory. He added the content of the bottle into the culture medium, and to his surprise, the old stock of nucleic acid could support and hasten the growth of tobacco callus tissue.
Later on Miller and his colleagues isolated and purified the substance in crystalline form, from autoclaved herring sperm DNA. This substance was identified as 6-furfuryl amino purine. They named this compound kinetin because of its property to activate cell division.
Natural Cytokinins are Synthesized in Regions where Rapid Cell Division Occurs
- Root apices
- Developing shoot buds
- Young fruits
- Germinating seeds
- Tumor tissues
- Endosperm
- Embryo
- Apple fruit extract
Cytokinins Regulate Specific Components of the Cell Cycle
- Evidence suggests that both auxin and cytokinins participate in regulation of the cell cycle, and that they do so by controlling the activity of cyclin-dependent kinases.
- Cytokinins inhibit the formation of lateral roots, while auxins promote their formation.
The Auxin: Cytokinin Ratio Regulates Morphogenesis in Cultured Tissues
- It was observed by Skoog and Miller 1965 that the differentiation of cultured callus tissue derived from tobacco pith segments into either roots or shoots depends on the ratio of auxin to cytokinin in the culture medium.
- Whereas high auxin:cytokinin ratios stimulated the formation of roots, low auxin : cytokinin ratios led to the formation of shoots. At intermediate levels, the tissue grew as an undifferentiated callus.
Cytokinins Promote Movement of Nutrients
- Cytokinins influence the movement of nutrients into leaves from other parts of the plant-a phenomenon known as cytokinin-induced nutrient mobilization.
- Nutrients are preferentially transported to and accumulated in the cytokinin-treated tissues. It has been postulated that the hormone causes nutrient mobilization by creating a new source-sink relationship.
Other Physiological Effects
- Cytokinins stimulate cellular division.
- Cytokinins promote cell expansion in leaves and cotyledions.
- Overcome the apical dominance and help in growth of lateral buds.
- Prevent ageing of plant parts; also called anti-ageing hormone.
- Cytokinins are applied to the marketed vegetables, cut shoots, and flowers to keep them fresh for several days.
- Breaks seed dormancy.
- Cytokinin overproduction has been implicated in genetic tumors.
- Cytokinins delay leaf senescence.
- Substitute red light requirement for seed germination.
- Promote sugar transport.
- Cytokinins promote chloroplast development.
- Have anti-ageing effects (Richmond-Lang effect).
- Promote stomatal opening.
- Promote xylem and phloem differentiation.
Bioassays for Cytokinins
- Promotion of cell division in tobacco pith culture
- Chlorophyll preservation test
- Delay in senescence (estimated by retention of chlorophyll)
Ethylene
HH Cousins in 1910 reported that "emanations" from oranges stored in a chamber caused the premature ripening of bananas when these gases were passed through a chamber containing the fruit.
During the nineteenth century, when coal gas was used for street illumination, it was observed that trees in the vicinity of street lamps defoliated more extensively than other trees. Eventually, it became apparent that coal gas and air pollutants affect plant growth and development, and ethylene was identified as the active component of coal gas.
In 1901, Dimitry Neljubovk, in Russia, observed that dark-grown pea seedlings growing in the laboratory exhibited symptoms that were later termed the triple response: reduced stem elongation, increased lateral growth (swelling), and abnormal horizontal growth. Neljubov identified ethylene, which was present in the laboratory air from coal gas, as the molecule causing the response.
Structure, Biosynthesis, and Measurement of Ethylene
- Ethylene can be produced by almost all parts of higher plants.
- In general, meristematic regions and nodal regions are the most active in ethylene biosynthesis.
- However, ethylene production increases during leaf abscission, flower senescence, and during fruit ripening.
- The amino acid methionine is the precursor of ethylene.
- Ethylene biosynthesis is increased by stress conditions such as drought, flooding, chilling, exposure to ozone, or mechanical wounding.
Auxin-Induced Ethylene Production
- In some instances, auxins and ethylene can cause similar plant responses, such as induction of flowering in pineapple and inhibition of stem elongation. These responses might be due to the ability of auxins to promote ethylene synthesis by enhancing ACC synthase activity.
- Some of the inhibitory effects earlier attributed to auxin are now known to be caused by ethylene.
- High concentrations of auxins promote ethylene formation and hence inhibit growth.
Ethylene Promotes the Ripening of Some Fruits
- Ethylene has long been recognized as the hormone that accelerates the ripening of edible fruits.
- During fruit ripening complex carbohydrates are broken down into simple sugars.
- All fruits that ripen in response to ethylene exhibit a characteristic respiratory rise before the ripening phase called a climacteric. Such fruits also show a spike of ethylene production immediately before the respiratory rise.
- Apples, bananas, avocados, and tomatoes are examples of climacteric fruits.
- In contrast, fruits such as citrus fruits and grapes do not exhibit the respiration and ethylene production rise and are called nonclimacteric fruits. E.g., citrus, grape, pineapple.
The Hooks of Dark-Grown Seedlings are Maintained by Ethylene Production
- Etiolated dicot seedlings are usually characterized by a pronounced hook located just behind the shoot apex.
- This hook shape facilitates penetration of the seedling through the soil, protecting the tender apical meristem. The closed shape of the hook is a consequence of the more rapid elongation of the outer side of the stem compared with the inner side.
Ethylene Induces Flowering
- Although ethylene inhibits flowering in many species, it induces flowering in pineapple and its relatives, and it is used commercially in pineapple for synchronization of fruit set.
- Flowering of other species, such as mango, is also initiated by ethylene.
Ethylene Enhances the Rate of Leaf Senescence
- Senescence is regulated by the balance of ethylene and cytokinin.
- In addition, abscisic acid (ABA) has been implicated in the control of leaf senescence.
- Generally, it is accepted that ethylene hastens abscission and auxin delays it.
- Also, abscisic acid (ABA) enhances the abscission of some plants as a third abscission regulator.
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Ethylene has important commercial uses
Because of its high diffusion rate, ethylene is very difficult to apply in the field as a gas, but this limitation can be overcome if an ethylene-releasing compound is used.
The most widely used such compound is ethephon or 2chloroethylphosphonic acid (CEPA), which was discovered in the 1960s and is known by various trade names, such as Ethrel. Ethephon releases ethylene slowly.
Ethephon hastens fruit ripening of apple and tomato and degreening of citrus, synchronizes flowering and fruit set in pineapple, and accelerates abscission of flowers and fruits. It can be used to induce fruit thinning or fruit drop in cotton, cherry, blackberries, grapes, blueberries, and walnut, thus making mechanical harvesting possible. It is also used to promote female sex expression in cucumber.
Other Physiological Effects
- Ethylene is also used to hasten the ripening of walnuts and grapes.
- It is used to synchronize flowering in pineapple to get that perfect shape.
- Leaf epinasty results when ethylene from the root is transported to the shoot.
- Ethylene has the ability to break dormancy and initiate germination in certain seeds, such as cereals.
- In peanuts, ethylene production and seed germination are closely correlated.
- Ethylene can also break bud dormancy, and ethylene treatment is sometimes used to promote bud sprouting in potato.
- Ethylene promotes the elongation growth of submerged aquatic species.
- Ethylene induces the formation of roots and root hairs.
- Ethylene seems to play a role in sex expression in cucurbits.
- Ethylene inhibits polar transport of auxin and decreases sensitivity of gravity.
Bioassay
- Triple response test
- Pea stem swelling test
Abscisic Acid: A Seed Maturation and Stress Hormone
In 1949, Paul F Wareing discovered that the dormant buds of ash and potatoes contain large amounts of a growth inhibitor, which he called dormin. During the 1960s, Frederick T Addicott reported the discovery in leaves and fruits of a substance capable of accelerating abscission of cotton balls, which he called abscisin II.
Abscisin and dormin were soon found to be chemically identical. The compound is now known as abscisic acid (ABA).
Synthesis and Occurrence
- Abscisic acid has been found to be a ubiquitous plant hormone in vascular plants.
- Abscisic acid (ABA) is also called inhibitor-B. It is found in leaves (where it is partially synthesized), stems, and green fruits, roots, buds, fruits, and seeds. It has also been found in phloem and xylem sap and in nectar.
- Its biosynthesis is related to the process of cartenoid production.
The Chemical Structure of ABA
ABA is a 15-carbon compound that resembles the terminal portion of some carotenoid molecules.
ABA biosynthesis takes place in chloroplasts and other plastids from violaxanthin.
ABA is Translocated in Vascular Tissue
- ABA is transported by both the xylem and the phloem, but it is normally much more abundant in the phloem sap.
- ABA synthesized in the roots can also be transported to the shoot via the xylem.
ABA Inhibits GA-Induced Enzyme Production
ABA inhibits the synthesis of hydrolytic enzymes that are essential for the breakdown of storage reserves in seeds. For example, GA stimulates the aleurone layer of cereal grains to produce $\alpha$-amylase and other hydrolytic enzymes.
ABA inhibits this GA-dependent enzyme synthesis by inhibiting the transcription of $\alpha$-amylase mRNA.
Other Physiological Effects
- ABA inhibits precocious germination and vivipary.
- Abscisic acid prevents seed germination.
- ABA accumulates in dormant buds.
- ABA closes stomata in response to water stress.
- ABA promotes root growth and inhibits shoot growth at low water potentials.
- Inhibits cell division in the vascular cambium.
- Enables seeds to withstand desiccation and to become dormant.
- Acts as an antagonist to gibberellins.
- It is also called stress hormone.
- ABA is clearly involved in leaf senescence, and through, its promotion of senescence, it might indirectly increase ethylene formation and stimulate abscission.
- ABA promotes desiccation tolerance in the embryo.
- ABA promotes the accumulation of seed storage protein during embryogenesis.
- Seed dormancy is controlled by the ratio of ABA to GA.
Biology Faculty
M.Sc. Jiwaji University
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