How Do Hormones Influence Plant Growth and Development?
Definition
Plant growth is a process characterized by the irreversible change in the size of cells and organs that is a result of cell division and enlargement. Plant development is the process of progression from seed germination to maturation.
Growth and Development are often used to mean the same thing in discussions in colloquial language. However scientifically speaking, growth and development are two distinctly different events in the organization and formation of a mature plant and its body structures. Plant growth and development are very lengthy and very complicated processes. Both begin at germination and last all through the lifetime of the plant.
What Do You Mean by Growth and Development?
Development is defined as the process of progressing from an earlier to a later stage in the process of maturation. For example, development occurs when from a fertilized egg of a single cell a fully matured plant is formed. In the process of plant development processes like growth, morphogenesis and differentiation (formation of tissues with specialised and/or specific functions) take place. How a plant develops is determined by the interaction of the inherited genetic material and the environment.
On the other hand growth is defined as the irreversible change in cell size due to cell elongation. It is also used to explain the process of increase in plant organs' size due to cell division and elongation. Elongation in cells requires change in the elasticity of cell walls along with an increase in the size of vacuoles and/or water content of the vacuole. Growth can be of two types:
Determinate:
Growth is called determinate when an organ or a part of or the whole of the organism stops growing after reaching a certain size. For example, plant leaves and flowers mostly show determinate growth.
Indeterminate:
Growth is said to be indeterminate when the cells of the organ, part or organism continues to divide indefinitely. Plants as a whole have indeterminate growth.
What are the Processes of Growth and Development in Plants?
Two important processes involved in the growth and development of plants are:
Differentiation
Dedifferentiation
Differentiation:
Differentiation is the process by which undifferentiated cells transform into cells with specialised functions and having distinct morphological and physiological characteristics. All cells in the plant body have the same genetic makeup. The only thing separating the cells of different parts or organs from each other in morphology and physiology is the way the cells undergo differentiation. Differentiation happens by overexpressing or expressing or repressing certain genes. The differentiation of the cells is dependent on the location of the cells. For example, cells are placed towards the ground or water source form the root, while the cells exposed to the sun form shoot cells. Root cells do not differentiate into flower cells, and nor do shoot cells transform into root cells.
Dedifferentiation:
Dedifferentiation is the exact opposite of differentiation. In this process, the mature and differentiated cells of the plant are stimulated under specific conditions to divide and become undifferentiated and lose all the specific characteristics it acquired previously. These dedifferentiated cells again differentiate. This is noted when the plant tissues undergo damage.
Unlike animals, plants show indeterminate growth. As animals grow, when they turn into a fully matured animal they stop growing. When they are growing the different parts of the animal grow until they reach a size which is determined by their genetic make-up. Plants on the other hand never stop growing. They continue to grow all through their lifespan. As young plants grow older, the growth gets restricted to their meristematic tissues. Meristematic tissues are young, actively growing/dividing tissues found at the apexes of the plant.
How the Growth of Tissue Happens while Plants Grow?
The continuous growing patterns of the plant result in the formation of two types of tissues, they are:
Primary tissue
Secondary tissue
The Primary Tissues:
The primary tissues are involved with apical growth. Example: apical meristems
The Secondary Tissues:
The tissues involved in the lateral growth of the plant are called the secondary tissues. Example: lateral meristems.
What is Apical Meristem?
Apical meristems are zones of cell division, made up of readily dividing cells, which are responsible for the increase in the height of the plant in both the shoot and root region. It is responsible for growth in the length of the primary plant body. This is because primary tissues develop from primary meristems. Apical meristems have cells rapidly dividing and elongating. Rapid cell elongation is possible due to rapid enlargement of the vacuoles in the primary tissue cells. This results in the stem and roots increase in girth till they reach a maximum size. The maximum size is determined by the elasticity of the cells. After reaching the maximum size, the primary cells do not grow any further. Herbaceous plants only have primary meristems. This is why they only grow in axial length, while their girth doesn't grow so much in proportion.
However in Woody plants we see that they grow enormous in size as well as in girth. This is because the secondary tissues formed by the lateral meristems provide strength and protection. Secondary tissues develop around the periphery of the roots and shoots of the plant.
What are Plant Growth Regulators?
The plant growth regulators are hormones which control the growth of plants. The plant hormones are small organic molecules with low molecular weight and a wide variety of chemical composition.
What are the Types of Plant Growth Hormones?
Depending upon the primary trigger or conditions of synthesis, plant growth regulators are classified to be of 2 types:
Healing plant growth promoters
Growth promoting plant growth regulators
Healing Plant Growth Promoters:
These plant growth promoters act on the plant when there is stress of some sort. These plant growth promoters are simple organic compounds which are produced in response to wounds and/or stresses. The stresses can be of the biotic origin of the abiotic origin. The biotic stressors include biological agents like animals, insects, pests and so on. The abiotic stressors include other factors like temperature, moisture and so on.
Growth Promoting Plant Hormones:
These are simple compounds called plant growth promoters. The plant growth regulators have diversified chemical properties and constitution. These PGRs help in the growth of plants. Examples of plant growth promoters include auxins and cytokines.
FAQs on Plant Growth and Development: Understanding Apical Meristems and Hormones
1. What is the difference between growth and development in plants?
Growth in plants refers to an irreversible, permanent increase in the size of an organ or its parts, or even of an individual cell. It is a quantitative process. In contrast, development is a much broader term that includes all the changes an organism goes through during its life cycle, from seed germination to senescence. Development includes both growth and differentiation, making it a qualitative process. For a deeper understanding, explore the detailed explanation on Growth and Development.
2. What are the main phases of growth in a plant?
Plant growth is typically divided into three main phases:
- Meristematic Phase: This is the phase of cell division. The cells in the root and shoot apex are constantly dividing. These cells are rich in protoplasm and have large nuclei.
- Elongation Phase: The newly formed cells undergo enlargement. This phase is characterised by vacuolation, new cell wall deposition, and an increase in cell size, leading to the elongation of the plant axis.
- Maturation Phase: In this phase, the elongated cells differentiate and mature to perform specific functions. The cell walls thicken, and protoplasmic modifications occur, resulting in the formation of specialised tissues.
3. What are Plant Growth Regulators (PGRs) and what are the major types?
Plant Growth Regulators (PGRs), or phytohormones, are small, simple molecules of diverse chemical composition that regulate plant growth and development. They are broadly divided into two main groups based on their functions:
- Growth Promoters: These are involved in growth-promoting activities like cell division, cell enlargement, flowering, and fruiting. Examples include Auxins, Gibberellins, and Cytokinins.
- Growth Inhibitors: These are involved in growth-inhibiting activities like dormancy and abscission, and they help plants respond to stress. Examples include Abscisic acid (ABA) and Ethylene.
4. What is the primary role of Auxins in plants?
The primary role of auxins is to promote the elongation of cells, particularly in stems and coleoptiles. They are crucial for several developmental processes, including apical dominance (where the central stem grows more than lateral stems), initiating rooting in stem cuttings, promoting flowering (e.g., in pineapples), and preventing the early drop of fruits and leaves. Synthetic auxins are also widely used as herbicides in agriculture. For a complete overview, see our page on Auxin.
5. What is the importance of Gibberellins in the agriculture industry?
Gibberellins have significant applications in agriculture and horticulture. Their ability to cause an increase in length and size is widely used to:
- Increase the length of grape stalks, allowing for larger fruit development.
- Elongate and improve the shape of fruits like apples.
- Delay senescence (ageing), thus extending the market period for produce.
- Speed up the malting process in the brewing industry.
- Increase sugarcane yield by increasing the length of the stem internodes.
6. Why is plant growth described as "open" and indeterminate?
Plant growth is called open because plants continuously add new cells and organs (like leaves, flowers, roots) to their body throughout their life. It is termed indeterminate because, unlike animals that stop growing after reaching a certain size, plants can grow indefinitely. This unique characteristic is due to the presence of meristems—localised regions of actively dividing, undifferentiated cells—at their root and shoot tips.
7. Can a mature, differentiated plant cell become capable of division again? Explain the process.
Yes, a living, differentiated plant cell that has lost the capacity to divide can regain it under certain conditions. This is a remarkable aspect of plant plasticity. The process involves:
- Dedifferentiation: The process by which mature, differentiated cells (like parenchyma) revert to a meristematic state and regain the power of division. This is seen during wound healing or the formation of cork cambium.
- Redifferentiation: The cells produced through dedifferentiation divide and then once again mature to perform specific functions, losing their capacity to divide. This leads to the formation of new specialised tissues.
8. What is the difference between photoperiodism and vernalisation?
Both are environmental cues that regulate flowering, but they are stimulated by different factors:
- Photoperiodism is the developmental response of plants to the relative lengths of light and dark periods (photoperiods). It classifies plants into short-day, long-day, and day-neutral categories based on the critical day length required for flowering.
- Vernalisation is the promotion of flowering by exposing a plant to a period of prolonged cold temperature. This ensures that vegetative growth is completed before flowering begins, often after winter.
9. Why is Abscisic Acid (ABA) known as the "stress hormone" in plants?
Abscisic Acid (ABA) is called the "stress hormone" because its synthesis increases dramatically when plants face adverse environmental conditions like drought, waterlogging, or high salinity. It helps the plant survive by:
- Stimulating the closure of stomata to minimise water loss through transpiration.
- Promoting seed dormancy, which prevents germination during unfavourable conditions.
- Inhibiting overall plant growth, thus conserving energy and resources for survival.
10. How do the roles of Cytokinins and Auxins balance each other to control plant development?
Cytokinins and auxins exhibit an interesting synergistic and antagonistic relationship, where their relative concentration (the auxin/cytokinin ratio) dictates the developmental outcome. In plant tissue culture, this balance is critical:
- A high auxin to cytokinin ratio typically stimulates root formation (rhizogenesis).
- A high cytokinin to auxin ratio generally promotes shoot development (caulogenesis).
- An intermediate ratio often leads to the proliferation of an undifferentiated cell mass known as a callus.
