Mr. Yagnit Vadhel and Ms. Tanya Vyas,
St. Xavier's College (Autonomous),
Mumbai, Maharashtra, India
Oh! This fruit is still very hard on my digestive system. My mouth is patiently waiting for it to ripen. Oh my god! This fruit is so ripe that it has spoiled my entire fruit basket. How often do we come across fruits and flowers becoming spoiled or leaves falling during autumn and in the winter month? But friend do you know!!! The main culprit behind such a negative change is not just the external environment but, the complex interplay of 1 of the 2 gaseous Phyto hormones. Can you try to think your heads out? Yes, you all correctly pointed out the key player, and it nonother then than the hormone Ethylene. Very simple concerning its structure just CH2=CH2 right? Despite its simple structure, the mechanism of its perception and transduction of the perceived signal is indeed complex.
As a consequence,
This article highlights the major signalling pathway of Ethylene signal transduction in plants.
1: A short overview of the structure of the only gaseous Phytohormone, Ethylene:
Ethene or Ethylene (C2H4) is one of the simplest alkenes, having a double bond between the two carbon atoms, with the remaining two valencies of each carbon being satisfied by two Hydrogens. On account of its SP2 hybridization, the molecule of Ethylene possesses a planar geometry in a trigonal planar orientation.
Image: Colored Ball and stick model of Ethylene.
2: Role of Ethylene in Plant Growth and Development:
Ethylene plays a major role in climacteric fruit ripening, as proved by the dramatic inhibition of the phenomenon when the ACC synthase and ACC oxidase genes responsible for its biosynthesis are suppressed; as well as hastening the ripening process if applied to unripe fruits. Ethylene also influences various other complex physiological processes during the plant’s life cycle including seed germination, flowering, and senescence; thus being generally accepted as a hormone necessary for plant growth and development. Apart from its role in up-regulating and down-regulating the above-mentioned processes, Scientists have also proposed that the hormone acts in a concentration-dependent manner. This implies that the hormone if present in lesser amounts, then it will increase leaf and flower development. On the other hand, if the hormone is present in a higher concentration that it will inhibit the developmental processes. researchers have also indicated that low concentrations of the hormone Ethylene have a positive effect on leaf enlargement. However, the phenomenon is restricted to only some species.
The initial presence of ethylene in a plant is autocatalytic, resulting in further production of the hormone without the need for any other triggering factors. Many morphological changes also occur in the presence of the gas- including softening of the fruit, visible ageing, discolouration, etc. The sensitivity of plants to ethylene varies from species to species.
A: Binding of Ethylene molecule to the ethylene-sensitive receptor on the endoplasmic reticulum:
Once the Ethylene molecule enters the cell through diffusion or by combination with water, it is quickly sensed by the ethylene-sensitive receptors on the endoplasmic reticulum. Different receptors sensitive to Ethylene have been isolated and characterized from a wide array of plant species. However, the most common receptors involved in Ethylene signal perception are ETHYLENE RESPONSE1 and 2 (ETR1 and ETR2), ETHYLENE RESPONSE SENSOR1 and 2 (ERS1 and ERS2), and ETHYLENE INSENSITIVE4 (EIN4).
All these receptors have a Cu ion coordinated in the active site of the receptors. These receptors exist as homo- or hetero-dimers in their active state and tend to bind with the Ethylene molecule through a Cu metal ion present in the binding domain of the protein. In contrast to the normal cell signalling process observed in plants and animals under various environmental conditions, the mechanism of the Ethylene signal perception and transduction is the opposite.
Initially, the Ethylene binding receptors are in their active conformation. On binding with Ethylene through a metal-ligand bond due to the donation of the electrons from Ethylene to the Cu metal ion, the coordination number changes and this results in the locking of the protein structure in an inactive state. Once the signal is perceived by the receptor, downstream signalling commences, involving a chain of molecules, until the signal reaches the transcription factor. This then migrates from the cytoplasm to the nucleus to induce gene expression in response to Ethylene.
B: CTR1 Protein and its role in Ethylene signal transduction:
CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) is a negative regulator of the Ethylene signalling pathway. It physically interacts with the receptors to form an ER-localized complex that represses responses and activation of ethylene.
The exact model of CTR1 repression has not yet been discovered, other than the establishment of the necessity of the serine/threonine kinase activity for the regulation of downstream signalling. The current proposed model predicts that upon ethylene binding, the receptors/ CTR1 signalling complexes are turned ‘‘off’’, presumably by adopting an inactive conformation, thus releasing the repression of the downstream signalling pathway.
This is then followed by various genes regulating the signal response.
Immediately following the deactivation of the CTR1 protein by the Ethylene signal, the most important protein in the signalling process is the EIN2 protein. Only very recently, have we understood the exact role and mechanism of EIN2 action in Ethylene transduction. Studies employing mutant EIN2 protein have exhibited a complete Ethylene insensitivity indicating that the EIN2 class of proteins is very important for relaying the signal to yet another class of transcription factors the EIN3. The molecular mechanism of EIN2 can be explained as follows.
EIN2 is an ER-bound protein directly associated with the CTR1 protein when the protein CTR1 is inactivated by Ethylene, the dephosphorylation of the EIN2 protein ensures that the C terminal of the protein which contains the sites required for activation of yet another transcription facture, the EIN3. Inside the nucleus. Post the role of EIN2 the most fascinating aspect of the entire signal transduction process is the EIN3 and the activation and deactivation of the ethylene signal response genes by various DNA binding proteins that commence inside the nucleus of the plant cell.
C: The EIN3 and other transcription factors along with their mechanism of action:
The signal transmitted from the EIN2 proteins onto the EIN3 type of proteins derived transcription factors forms a very crucial phase in the entire process of Ethylene signal transduction.
The EIN3 is a transcription factor that receives the signal from the EIN2. Additionally, the protein exists in conjugation with other proteins including the EIN3‐LIKE1 (EIL1) and EIL2. Together these proteins directly regulate the activity of the Ethylene response genes including the transcription factor-yielding gene the ERF1. The mechanism of EIN3 and associated proteins is far more interesting to study as compared to the initial stages of the process. The activated genes controlling the activity of ERF1 and many such unidentified transcription factors ultimately impact the upregulation of a plethora of ethylene response genes directly or indirectly. However, the EIN3 protein is very sensitive.
E: Activation of Ethylene response genes: (Partly by Yagnit and Tanya)
EIN3 activation then results in a ‘domino effect’ concerning further hormones. It is itself regulated by the SCF/26S proteasome pathway, which acts as a fine control for the transcriptional cascade. It is usually bound by F-box proteins EIN3 BINDING FACTOR1 (EBF1) and EIN3 BINDING FACTOR2 (EBF2), both of which target it for destruction and repress its functioning.
To prevent the two factors from destroying EIN3, two theories have been proposed: either EIN3 is modified beyond recognition or down-regulates the two EBFs to allow the ethylene signalling pathway to continue. EIN3 is crucial for the full expression of ethylene in a plant, and thus necessary for its further growth and development.
F: Concluding remarks and future directions in this domain:
Our increasing advancements in the domain of biochemistry, stereochemistry, molecular biology, and functional genetics will enhance our knowledge about the signalling pathways regulating Ethylene signal transduction in higher plants. In addition to that, the discoveries of genes directly regulating the Ethylene signal response will pave the way for the design of novel fruit varieties resistant to over-ripening and will ultimately result in the improved shelf life of various species. Furthermore, future studies should target the role of CO2 and various phytohormones along with the central clock of the plant. The mechanism of Ethylene response in stress response will also allow researchers to design species resistant to environmental stresses.
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