Growth

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. For example, expansion of a leaf is growth

Indeterminate growth of plants The plants show a unique type of growth known as indeterminate or open type of growth. This growth type, where meristems continuously add new cells to the plant structure, is termed the “open” form of growth. This distinctive attribute is attributed to the existence of meristems in specific body areas. Cells within these meristems have the ability to divide and perpetuate themselves. However, the resulting product eventually loses this division capacity, contributing to the plant’s overall structure Types of meristems

Primary growth: Shoot apical and root apical meristems principally contribute to the elongation of the plants along their axis. Secondary growth: Lateral meristems, vascular cambium and cork-cambium increase the girth of the organs. Generally found in dicotyledonous plants and gymnosperms.

Measurement of growth Growth can be measured by a variety of parameters such as: increase in fresh weight, dry weight, length, area, volume and cell number. For example,

a. Expression of growth as an increase in cell number: one single maize root apical meristem can give rise to more than 17,500 new cells per hour.

b. Expression of growth as an increase in size of the cell: cells in a watermelon may increase in size by upto 3,50,000 times.

c. Expression of growth as an increase in the length: growth of the pollen tube.

d. Expression of growth as an increase in surface area: growth in a dorsiventral leaf.

AUXIN

Auxin is a class of plant hormones that play a crucial role in regulating various aspects of plant growth and development. These hormones are essential for plant survival and are involved in processes such as cell elongation, root and shoot development, tropisms (responses to environmental stimuli), and the formation of lateral roots. Auxins are primarily synthesized in the apical meristems of the plant, which are regions of active cell division and growth.

1. Function:

   • Cell Elongation: One of the primary functions of auxin is to promote cell elongation, particularly in the growing tips of stems and roots. This elongation results from the activation of proton pumps in the cell membrane, leading to the influx of protons and water into the cell, which causes turgor pressure and cell expansion.

   • Apical Dominance: Auxin helps regulate the dominance of the apical bud (the growing tip) over lateral buds in the plant. High auxin concentrations in the apical bud inhibit the growth of lateral buds, ensuring that the main shoot continues to grow.

   • Root Development: Auxin is crucial for root development. It stimulates the formation of lateral roots and is responsible for gravitropism, which helps the roots grow downward in response to gravity.

   • Tropisms: Auxin mediates various tropisms, including phototropism (growth towards light) and gravitropism (growth in response to gravity). It does so by redistributing auxin to different sides of the stem or root, causing differential growth.

2. Synthesis:

   • Auxins are synthesized primarily in the shoot apex (the growing tip of the plant) and young leaves.
   • The main auxin in plants is called indole-3-acetic acid (IAA). IAA is synthesized from the amino acid tryptophan via several enzymatic reactions.

3. Transport:

   • Auxins can move within the plant in a polar manner, meaning they predominantly move from the apical meristem towards the base (basipetal) in the stem and from the tip of the root towards the shoot (acropetal) in the root.
   • Two main transport mechanisms are responsible for auxin movement: polar auxin transport (PAT) and basipetal transport.

4. Mechanisms of Action:

   • Auxin acts as a signaling molecule by binding to specific receptor proteins located in the plant cells. These receptors trigger a signal transduction cascade that ultimately affects gene expression.
   • One of the well-known responses to auxin is the activation of AUX/IAA genes, which leads to changes in gene expression and, consequently, changes in cell growth and development.
   • Auxin also influences the pH of the cell wall, making it more acidic, which promotes cell elongation.

5. Role in Commercial Agriculture:

   • Synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthaleneacetic acid (NAA), are commonly used as herbicides, rooting hormones, and plant growth regulators in agriculture.

GIBBERELLIN

Gibberellins are a class of plant hormones that play a significant role in regulating various aspects of plant growth and development. These hormones are involved in processes such as stem elongation, seed germination, flowering, and fruit development. Gibberellins are essential for coordinating and controlling the growth and development of plants.

1. Function:

   • Stem Elongation: One of the primary functions of gibberellins is to promote stem elongation. They stimulate cell division and elongation in the internodes (the segments between leaves) of the stem, leading to increased plant height.

   • Seed Germination: Gibberellins break seed dormancy and promote germination by initiating the synthesis of enzymes like α-amylase, which hydrolyze starch in the seed into sugars that nourish the developing embryo.

   • Flowering: Gibberellins are involved in the regulation of flowering, particularly in plants that require a period of vernalization (exposure to cold) or a certain day length before they can flower. They also affect the development of floral organs.

   • Fruit Development: These hormones play a role in fruit development, including fruit size and seed development within the fruit.

   • Dwarfism: Some dwarf varieties of plants are genetically deficient in gibberellin production or response. Exogenous application of gibberellins can restore normal growth in these plants.

2. Synthesis:

   • Gibberellins are synthesized in various plant tissues, including young leaves, shoot tips, and root tips.
   • The biosynthesis of gibberellins involves a complex pathway that includes several enzymatic steps, with ent-kaurene as an intermediate compound.

3. Transport:

   • Gibberellins are transported in a non-polar manner, meaning they can move both upward (acropetal) and downward (basipetal) within the plant.
   • The transport of gibberellins involves both active and passive processes, and they can move via the xylem and phloem.

4. Mechanisms of Action:

   • Gibberellins exert their effects by binding to specific receptor proteins in the nucleus of plant cells. The gibberellin-receptor complex then initiates changes in gene expression.
   • One of the key mechanisms involves the degradation of a family of transcriptional repressor proteins known as DELLA proteins. In the presence of gibberellins, DELLA proteins are targeted for degradation, allowing for the expression of genes associated with growth and development.

5. Role in Commercial Agriculture:

   • Gibberellins are used in agriculture and horticulture to manipulate plant growth and development. For example, they are applied to increase the height of certain crops, such as wheat and rice, to improve yields.
   • Gibberellin application is also used to promote fruit development and seed germination in some crops.

Abscisic acid

Abscisic acid (ABA) is a plant hormone that plays a crucial role in regulating various physiological processes, primarily related to stress responses and growth regulation. ABA helps plants adapt to unfavorable environmental conditions and control developmental processes.

1. Function:

   • Stress Response: ABA is best known for its role in responding to environmental stressors. It helps plants cope with various stress conditions, including drought, salinity, extreme temperatures, and pathogen attacks. ABA triggers a series of responses that help the plant conserve water and protect itself from damage.

   • Stomatal Regulation: ABA plays a critical role in regulating stomatal aperture. When ABA levels increase in response to water deficiency, it signals the closure of stomata (tiny openings on the leaf surface), reducing water loss through transpiration.

   • Seed Dormancy: ABA is involved in the induction and maintenance of seed dormancy. It inhibits germination until conditions are favorable for seedling growth, such as the presence of sufficient moisture and the absence of stress factors.

   • Root Growth Inhibition: ABA can inhibit root growth, particularly in response to soil drying. This helps plants prioritize water uptake by reducing unnecessary root elongation.

   • Senescence: ABA can promote leaf senescence, a process where older leaves deteriorate and are eventually shed. This is a strategy to conserve resources during times of stress.

2. Synthesis:

   • ABA is synthesized in various plant tissues, including roots, leaves, and seeds.
   • The biosynthesis of ABA involves several enzymatic steps, including the conversion of carotenoids to xanthoxin, which is then converted into ABA.

3. Transport:

   • ABA can be transported within the plant through the vascular system (xylem and phloem) or through diffusion.
   • ABA transport is influenced by environmental factors, particularly water availability.

4. Mechanisms of Action:

   • ABA exerts its effects by binding to specific receptors in plant cells, which trigger various cellular responses.
   • One of the main pathways involves ABA receptors activating protein phosphatases, which in turn modulate the activity of ion channels and transporters. This leads to changes in ion fluxes and cellular processes such as stomatal closure.
   • ABA also influences gene expression, leading to the synthesis of proteins involved in stress tolerance and adaptive responses.

5. Role in Commercial Agriculture:

   • ABA is used in agriculture to enhance stress tolerance in crops exposed to adverse conditions, such as drought or salt stress.
   • ABA-based treatments can help improve seed germination and seedling establishment under stressful conditions.

Ethylene

Ethylene is a simple gaseous plant hormone that plays a pivotal role in various aspects of plant growth, development, and response to environmental stimuli. It is involved in regulating processes such as fruit ripening, senescence, leaf abscission, and responses to mechanical stress. Ethylene is often referred to as the “ripening hormone” because of its prominent role in fruit ripening.

1. Functions:

   • Fruit Ripening: Ethylene is a key regulator of fruit ripening. It induces the expression of genes responsible for fruit softening, color changes, and the production of aroma compounds, making the fruit more appealing for consumption.

   • Senescence: Ethylene promotes the aging and senescence of plant tissues, including leaves and flowers. It triggers processes like chlorophyll degradation and cell wall breakdown in aging tissues.

   • Leaf Abscission: Ethylene plays a role in leaf abscission, which is the shedding of leaves from the plant. It helps initiate the abscission process by weakening the cells at the base of the leaf petiole.

   • Responses to Mechanical Stress: Ethylene can induce various responses to mechanical stress, such as the thickening of stems and the formation of adventitious roots in response to injury.

   • Seed Germination: In some species, ethylene can stimulate seed germination, particularly in seeds that have a hard seed coat.

2. Synthesis:

   • Ethylene is synthesized mainly from methionine, an amino acid, through a series of enzymatic reactions.
   • The key enzyme involved in ethylene synthesis is ACC synthase, which converts S-adenosylmethionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC).
   • ACC is then converted to ethylene by the enzyme ACC oxidase.

3. Mechanisms of Action:

   • Ethylene exerts its effects by binding to specific receptors in plant cells, known as ethylene receptors.
   • In the absence of ethylene, these receptors inhibit the downstream signaling pathway.
   • When ethylene is present, it binds to the receptors, causing a conformational change that allows them to activate downstream signaling components.
   • The key component activated is the transcription factor called ethylene response factor (ERF), which regulates the expression of ethylene-responsive genes involved in various cellular processes.

4. Role in Commercial Agriculture:

   • Ethylene is used in agriculture and horticulture to ripen fruits, such as tomatoes, bananas, and avocados, after they are harvested. Ethylene gas can be applied to controlled environments to induce uniform and predictable ripening.

   • Ethylene inhibitors, such as 1-MCP (1-methylcyclopropene), are used to delay the ripening of fruits and vegetables during transportation and storage.

Cytokinin

Cytokinins are a class of plant hormones that play a significant role in regulating various aspects of plant growth and development. They are essential for processes such as cell division, shoot and root development, and nutrient uptake. Cytokinins often work in conjunction with other plant hormones like auxins to maintain proper growth and balance in plants.

1. Function:

   • Cell Division and Growth: Cytokinins are primarily known for their role in promoting cell division and growth. They stimulate the division of cells in the shoot and root meristems, leading to increased shoot and root development.

   • Shoot Development: Cytokinins help regulate the development of lateral shoots (branches) and promote the growth of axillary buds. This contributes to the overall bushiness of the plant.

   • Root Development: Cytokinins also influence root development by promoting the formation of lateral roots and root elongation. They work in conjunction with auxins to balance root and shoot growth.

   • Nutrient Uptake: Cytokinins can enhance the uptake of essential nutrients, such as potassium, by the plant. This is particularly important for nutrient transport from the root to the shoot.

   • Delay in Senescence: Cytokinins can delay leaf senescence, helping to maintain the health and function of leaves for a longer period.

2. Synthesis:

   • Cytokinins are synthesized in various plant tissues, including roots, developing seeds, and young leaves.
   • The primary biosynthesis pathway involves the conversion of adenosine monophosphate (AMP) to isopentenyladenosine monophosphate (iPMP), which is then converted to various forms of cytokinins.

3. Transport:

   • Cytokinins can be transported within the plant in both the xylem (upward movement) and the phloem (downward movement).
   • Their transport is influenced by factors such as light and environmental conditions.

4. Mechanisms of Action:

   • Cytokinins exert their effects by binding to specific receptors in plant cells, known as cytokinin receptors.
   • Upon binding, these receptors activate a signaling pathway that ultimately leads to changes in gene expression and cellular processes.
   • One of the key responses to cytokinins is the activation of cell division-related genes, which promote cell growth and division.

5. Role in Commercial Agriculture:

   • Cytokinins are used in plant tissue culture techniques to promote the proliferation of plant cells and the development of shoots and roots.
   • They are also used in agriculture and horticulture to enhance crop yield, quality, and fruit size, particularly in crops like tomatoes, grapes, and tobacco.