Plant Growth and Development: An Overview

Plant Growth and Development - Introduction

• Chapter 13 explores plant growth and development in flowering plants.
• Structures such as roots, stems, leaves, flowers, fruits, and seeds arise in an orderly sequence.
• Trees grow in height and girth, and leaves, flowers, and fruits appear/fall periodically.
• Development combines growth and differentiation following events with a precise order.
• These processes form a body with roots, leaves, branches, flowers, fruits, and seeds, eventually leading to death.
• Seed germination is the first step, occurring under favorable environmental conditions.
• Seeds enter suspended growth without these conditions, resuming activities when conditions improve.
• The factors governing these processes are both intrinsic and extrinsic to the plant.

Growth

• Growth involves irreversible permanent increases in the size of an organ, its parts, or a cell.
• Metabolic processes (anabolic and catabolic) accompany growth, using energy.
• Plant growth is unique, retaining the capacity for unlimited growth.
• Meristems present at certain locations enable this continuous growth.
• Meristem cells divide and self-perpetuate, with their products losing division capacity to form the plant body.
• This addition of new cells is the "open form" of growth.
• Root and shoot apical meristems are responsible for primary growth, which elongates plants along their axis.
• Lateral meristems (vascular cambium and cork-cambium) in dicots and gymnosperms cause secondary growth, increasing the girth of organs.

Measurable Growth

• Growth at the cellular level primarily increases protoplasm.
• Measuring protoplasm directly is hard, so growth is measured through parameters like fresh weight, dry weight, length, area, volume, and cell number.
• A single maize root apical meristem can produce more than 17,500 new cells per hour.
• Cells in a watermelon may increase in size up to 350,000 times.
• Growth is expressed as increased cell number or size.
• Pollen tube growth is measured by length, while surface area indicates growth in a dorsiventral leaf.

Phases of Growth

• Growth is divided into meristematic, elongation, and maturation phases.
• The root and shoot apices have constantly dividing cells, representing the meristematic phase.
• These cells are rich in protoplasm, with large, conspicuous nuclei, thin, primary cell walls, and abundant plasmodesmatal connections.
• The meristematic zone is where elongation occurs, marked by vacuolation, cell enlargement, and cell wall deposition.
• The maturation phase is away from the apex, where cells reach maximal size with wall thickening and protoplasmic modifications.

Growth Rates

• Growth rate measures increased growth per unit of time, expressed mathematically.
• Arithmetic and geometrical growth are two types of growth rate increases.
• In arithmetic growth, one daughter cell continues dividing while the other matures.
• Root elongation at a constant rate exemplifies arithmetic growth, producing a linear curve when plotted.
• Geometrical growth involves an initial slow phase (lag) and a rapid exponential phase.
• Progeny cells retain the ability to divide if there are are adequate nutrients.
• Limited nutrient supply leads to a stationary phase, forming a sigmoid or S-curve when growth is plotted over time.
• A sigmoid curve shows growth in a natural environment and is common in plant cells, tissues, and organs.
• Exponential growth is written as W1 = W0ert, with W1 as final size, W0 as initial size, r as growth rate, t as time, and e as the natural logarithm base.
• The relative growth rate is also the plant's ability to produce new material. The final size depends on the initial size.

Conditions for Growth

• Necessary conditions for growth: water, oxygen, and nutrients.
• Cell enlargement requires water, with turgidity aiding extension.
• Growth and development are linked to water status, which provides the medium for enzyme activity.
• Oxygen provides metabolic energy for growth.
• Nutrients (macro and micro elements) synthesize protoplasm and act as an energy source for the plant.
• Optimum temperature ranges are necessary for growth.
• Environmental signals like light and gravity also affect growth.

Differentiation, Dedifferentiation, and Redifferentiation

• Cells from root apical, shoot-apical meristems, and cambium differentiate and mature to perform specific functions.
• Differentiation involves cells undergoing structural changes in their walls and protoplasm.
• To form a tracheary element, cells lose protoplasm and develop lignocellulosic secondary walls for water transport.
• Living differentiated cells regain the capacity to divide under certain conditions.
• Dedifferentiation forms meristems like interfascicular cambium and cork cambium from parenchyma cells.
• These meristems/tissues can divide and produce cells that lose the capacity to divide but mature, known as redifferentiation.
• Differentiation is open in plants because cells/tissues from the same meristem have different structures at maturity.
• The final cell/tissue structure is also determined by its location.

Development

• Development includes all changes in an organism’s life cycle, from seed germination to senescence.
• Plant development is the ability to change the kind of structures that are formed during life in response to the enviroment, known as plasticity.
• Heterophylly demonstrates this, where juvenile plant leaves differ in shape from mature plants.
• Leaves produced in air versus water in buttercup represent heterophyllous development due to environment.

Plant Growth Regulators - Characteristics

• Development includes growth and differentiation that is controlled through intrinsic and extrinsic factors.
• The former include intracellular (genetic) or intercellular factors (chemicals like plant growth regulators)
• Extrinsic factors include light, temperature, water, oxygen and nutrition.
• Plant growth regulators (PGRs) are small molecules of diverse composition, such as:
  • Indole compounds (indole-3-acetic acid, IAA).
  • Adenine derivatives (N6-furfurylamino purine, kinetin).
  • Carotenoid derivatives (abscisic acid, ABA).
  • Terpenes (gibberellic acid, GA3).
  • Gases (ethylene, C2H4).
• PGRs are divided into two groups based on their functions:
  • One group promotes growth, including cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation (auxins, gibberellins, and cytokinins).
  • The other group responds to wounds and abiotic/biotic stresses, including dormancy and abscission (abscisic acid).
  • Ethylene is largely an inhibitor but fits in either classification.

Discovery of Plant Growth Regulators

• The discovery of each PGR group occurred by accident.
• Charles and Francis Darwin studied the coleoptiles of canary grass and discovered that they display a phototropic response.
• Auxin’s isolation from oat seedling coleoptile tips was conducted by F.W. Went.
• Gibberella fujikuroi, a fungal pathogen, causes Bakanae disease in rice seedlings.
• E. Kurosawa found that sterile filtrates in the fungus display symptoms of the disease in seedlings of rice.
• Those substances were later known as gibberellic acids.
• Skoog et al. found that callus proliferated from internodal sections of tobacco shoots if in addition to the introduction of nutrients, vascular tissues, yeast, coconut milk or DNA were added.
• Miller et al. later crystallized the promoting active substance of cytokinesis and referred to is as kinetin.
• In the mid 1960's three kinds of inhibitors were characterized by several researchers which were later named Inhibitor-B, abscission II and dormin which were later identical and renamed abscisic acid (ABA).
• H.H. Cousins confirmed the hastened ripening of stored bananas when near ripening oranges due to a volatile substance that was later identified as the gaseous ethylene.

Physiological Effects of Plant Growth Regulators - Auxins

• Auxins were first isolated from human urine, with the term applying to indole-3-acetic acid (IAA).
• Naturally derived and synthetic compounds of auxins possess growth regularities.
• They are generally produced from growing stems/roots and move to regions of influence.
• Auxins like IAA and indole butyric acid (IBA) have been derived from plants.
• NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are examples of synthetic auxins used in horticultural practices.
• Auxins initiate stem cutting, which promotes plant propagation, and flowering in pineapples.
• Auxins also aid in minimizing fruit/leaf drop in the early growth stages while promoting abscission later on.
• Apical dominance describes when growing buds suppresses the growth among lateral buds. Shoot tips (decapitation) tend to increase the growth of lateral buds.

Physiological Effects of Plant Growth Regulators - Gibberellins

• Gibberellins (GAs) are promotory kind of PGR with over 100 varieties known from fungi and higher plants.
• Denoted at GA1, GA2, GA3 etc. and Gibberellic acid (GA3) are one of the most intensely studied form.
• GAs are acidic that causes physiological responses, such as increasing the length of grapes stalks axis or improving the shape of of fruits.
• GAs promotes delayed senescence and are used to extend time for market, like in malting processes for brewing.
• GA increases length of stems to boost by an increasing yield by 20 tonnes per acre in sugar cane crops that store carbohydrate as sugar in their stems.
• GA promotes the maturity period of conifers and bolting ( or elongation of inter-nodes) just before flowering.

Physiological Effects of Plant Growth Regulators - Cytokinins

• Cytokinins affect cytokinesis specifically and were discovered from herring sperm cells as kinetin.
• Cytokinins cannot be derived naturally within plants.
• Isolation of zeatin with cytokinin like activities can be observed within coconut milk and corn kernels.
• In the presence of zeatin, cytokines promoting cell division and some synthetic cells have been identified.

Physiological Effects of Plant Growth Regulators - Ethylene

• Ethylene is a straightforward gaseous PGR that is developed tissues going through senescence.
• Ethylene impacts seedlings by causing horizontal growth, the axis swells, and forms apical hooks in dicot seedlings.
• It speeds up respiration during the ripening of fruits, or respiratory climactic.
• Ethylene can break dormancy in seeds and buds by rapidly elongating the internode.
• Ethylene initiates flowering/fruit sets and increases root/hair growth that expands the plants absorption surface.
• Ethephon is used as a compound of ethylene to hasten ripening within fruits and accelerate abscission of flowers. It also helps thin cotton, thin the flowers of cherry blossoms and promotes the number of female flowers in cucumbers.

Physiological Effects of Plant Growth Regulators - Abscisic Acid (ABA)

• Abscisic acid (ABA) regulates abscission and dormancy but it has wide effects on growth/development.
• ABA generally works as a plant inhibitor by causing the stomata to close and tolerance to plant stresses.
• ABA also increases development of seeds,maturation and dormancy. By inducing dormancy, ABA withholds seeds by ensuring it has resistance from desiccation and any other growth factors.

Plant Growth and Development - Summary Points

• Growth, differentiation, and development are closely connected and broadly, development is the sum of growth and differentiation.
• Development in plants depends on both intrinsic and extrinsic factors.
• PGRs has shown roles within phases of growth with roles that range from antagonistic, synergistic, or individualistic.
• There might be events in the lives of the plant that PGR will influence.
• Remember, the influence of PGRs on events is an intrinsic control, alongside genomic control, plus extrinsic factors that temperature, light, and PGR all influence.
• Vernalisation, seed germination, and plant movements are all events controlled by PGRs.