Model Answer
0 min readIntroduction
Ethylene (C₂H₄) is a gaseous plant hormone crucial for various developmental processes, including fruit ripening, senescence, abscission, and responses to stress. It is unique among plant hormones due to its gaseous nature and relatively simple chemical structure. Its biosynthesis is tightly regulated and responds to both developmental cues and environmental stimuli. Understanding ethylene biosynthesis and its effects, such as the ‘triple response’, is vital for optimizing agricultural practices and comprehending plant adaptation mechanisms. This answer will schematically present ethylene biosynthesis and describe the ‘triple response’ observed in plants.
Ethylene Biosynthesis
Ethylene biosynthesis is a complex pathway primarily occurring in ripening fruits and stressed tissues. It involves two key enzymes: ACC synthase (ACS) and ACC oxidase (ACO). The pathway can be summarized as follows:
- Methionine is converted to S-adenosylmethionine (SAM) by methionine adenosyltransferase.
- SAM is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). This is considered the rate-limiting step.
- ACC is converted to ethylene by ACC oxidase (ACO).
The following diagram illustrates the pathway:
Regulation of Biosynthesis: Ethylene biosynthesis is regulated at multiple levels. ACS gene expression is upregulated by various stimuli, including wounding, flooding, and pathogen attack. ACO activity is also regulated, and its expression increases during fruit ripening. Furthermore, feedback inhibition mechanisms exist, where ethylene itself can influence the expression of ACS and ACO genes.
The ‘Triple Response’ due to Ethylene
The ‘triple response’ is a characteristic growth pattern exhibited by etiolated seedlings (grown in the dark) when exposed to ethylene. It consists of three distinct morphological changes:
1. Inhibition of Stem Elongation
Ethylene inhibits the longitudinal extension of stems. This is due to the hormone’s effect on cell division and cell elongation in the apical meristem. Ethylene reduces the sensitivity of cells to auxin, a hormone promoting stem elongation.
2. Radial Swelling of the Stem (Hypocotyl)
Ethylene induces radial expansion of the hypocotyl (the stem below the cotyledons). This swelling is caused by increased cell elongation in the radial direction, resulting in a thicker stem. The mechanism involves changes in cell wall properties and the activation of specific genes involved in cell expansion.
3. Apical Hook Formation
Ethylene causes the apical hook, a characteristic curvature of the stem tip, to become more pronounced. This hook protects the delicate apical meristem as the seedling emerges from the soil. Ethylene promotes differential growth on the inner and outer sides of the hypocotyl, leading to the hook formation. This is thought to be an adaptation to facilitate penetration through the soil.
Physiological Mechanisms: The triple response is mediated by ethylene signaling pathways. Ethylene binds to ethylene receptors, initiating a signaling cascade that ultimately leads to changes in gene expression. Key components of the ethylene signaling pathway include:
- Ethylene Receptors (ETR1, ERS1, ETR2, EIN4): These receptors are located in the endoplasmic reticulum membrane.
- CTR1: A kinase that negatively regulates the ethylene signaling pathway.
- EIN2: A membrane protein that relays the ethylene signal.
- EIN3/EIL1: Transcription factors that activate the expression of ethylene-responsive genes.
Adaptive Significance: The triple response is believed to be an adaptive mechanism that helps seedlings overcome physical obstacles in the soil. The inhibited stem elongation reduces the risk of damage during soil penetration, while the radial swelling provides increased strength. The apical hook protects the meristem from mechanical stress.
Experimental Evidence: The triple response was first observed by N.G. Sanjana in 1958 while studying the effects of petroleum gas on pea seedlings. This led to the discovery of ethylene as a plant hormone.
Conclusion
Ethylene biosynthesis is a crucial pathway regulating plant development and responses to stress. The ‘triple response’ exemplifies the profound effects of ethylene on seedling growth and highlights its adaptive significance. Understanding these processes is essential for manipulating plant growth and improving crop yields. Further research into the intricacies of ethylene signaling will undoubtedly reveal new insights into plant physiology and development.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.