Phytohormones Classification, Function and Mechanism of Action

Phytohormones Classifications

Phytohormones, also known as plant hormones, are intrinsic chemical compounds within plants that regulate a myriad of physiological processes, acting as pivotal signaling molecules governing growth, development, and responses to environmental cues. These compounds, produced in specific plant tissues, traverse the plant, exerting their effects in different regions. The classification of plant hormones includes several major classes: auxins, gibberellins, cytokinins, abscisic acid, ethylene, and brassinosteroids, each playing distinct roles in processes like cell elongation, seed germination, and stress response, crucial for the overall health and adaptation of plants in their ever-changing habitats.

Mechanism of Action of Plant Hormones: Auxins

Auxins, for instance, hold a critical position in plant physiology, especially in triggering cell elongation, particularly in stems and coleoptiles, vital for upward growth towards light sources.The mechanism of action of plant hormones like auxins  reveals that they are essential regulators in plants, influencing their development and responses to environmental changes. Moreover, they are instrumental in fostering root growth and branching, augmenting the plant’s ability to effectively absorb essential nutrients and water from the soil. Collectively, these functions underscore the pivotal role auxins play in shaping and perpetuating plant development. Phytohormone signaling pathways, such as auxin distribution through polar transport and gradient formation within plant tissues, are crucial for maintaining these developmental processes.

Auxins commence their effects by binding to specific receptors on the plasma membrane of plant cells, initiating auxin signaling that prompts diverse cellular responses. The binding of auxins to receptors initiates the breakdown of Auxin/Indole-3-Acetic Acid (Aux/IAA) proteins via the ubiquitin-proteasome pathway, which serve as transcriptional repressors hindering the expression of auxin-responsive genes. This process involves auxin transport mechanisms, where specific proteins like PIN and AUX1 mediate polar auxin transport, establishing gradients that influence physiological processes. As Aux/IAAs degrade, they release Auxin Response Factors (ARFs), acting as transcriptional activators that bind to auxin-responsive elements (AREs) within target gene promoters. This detailed mechanism of action of plant hormones through auxin signaling orchestrates the expression of genes crucial for cell elongation, root development, and phototropic responses, thus impacting overall cell growth and the formation of various plant organs. Exogenous auxin application can further influence fruit ripening and physiological processes, affecting gene regulation and hormonal interactions relevant to fruit development and quality.

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Cytokinins and Cell Division

Cytokinins hold a pivotal role in plant biology, actively stimulating cell division across various plant tissues and significantly contributing to overall growth and development. Moreover, the role of phytohormones in stress response is evident, as cytokinins demonstrate an impressive capacity to delay leaf senescence, the natural aging process, by extending the photosynthetic period, thereby significantly sustaining the overall vigor of the plant.

Cytokinins begin their action by binding to receptors on the endoplasmic reticulum (ER) membrane of plant cells, initiating a two-component signaling system that activates histidine kinases. These activated kinases then transfer phosphate groups to histidine phosphotransfer proteins (AHPs), which, upon phosphorylation, pass on the phosphate group to type-B response regulators (RRs), acting as transcription factors. Subsequently, these activated type-B RRs translocate to the nucleus, where they govern the expression of specific target genes involved in cell division, shoot growth, and apical dominance. This intricate regulatory cascade orchestrated by cytokinins ultimately fosters increased cell division and lateral shoot growth, highlighting their central role in shaping plant development.

Gibberellins in Plant Development

Gibberellins hold a multifaceted role in plant physiology. They crucially promote stem elongation by facilitating both cell division and expansion, a pivotal factor in determining plant height. Additionally, these hormones play a key role in breaking seed dormancy and initiating germination by activating hydrolytic enzymes that break down stored nutrients, providing essential energy for growth. Moreover, gibberellins are pivotal in the transition from vegetative to reproductive growth, triggering the flowering process in various plant species.

Gibberellins instigate their impact by binding to receptors situated within the cytoplasm of plant cells, forming a receptor-gibberellin complex. This complex then translocates to the nucleus, engaging with the gibberellin-insensitive dwarf (GID1), a nuclear protein. Through this interaction, DELLA proteins undergo degradation via the proteasome pathway. As suppressors of gibberellin signaling, the breakdown of DELLA proteins relieves the inhibition of growth-promoting genes, facilitating stem elongation, seed germination, and flowering induction. This intricate process underscores the pivotal role of gibberellins in shaping diverse facets of plant growth and development.

Abscisic Acid (ABA)

ABA, a vital hormone in plants, coordinates responses to stressors like drought, salinity, and extreme temperatures, highlighting the critical role of phytohormones in stress response. This hormone enhances stress tolerance and enables plants to thrive in challenging environmental conditions. Additionally, it governs stomatal closure, crucial for conserving water during scarcity, and influences seed dormancy and germination, ensuring optimal conditions for seed growth in alignment with environmental cues.

ABA initiates cellular responses by engaging specific receptors associated with G-proteins on the plasma membrane of plant cells. This interaction triggers G-protein activation, initiating a complex signaling cascade that generates secondary messengers such as calcium ions and reactive oxygen species. These phytohormones signaling pathways, mediated by ABA, subsequently trigger protein phosphorylation events, intricately altering gene expression. This modified gene expression, in a domino effect, activates stress-responsive genes that facilitate stomatal closure. This adaptive mechanism aids plants in water conservation during drought and in navigating diverse stress conditions, highlighting the intricate and dynamic nature of ABA-mediated plant responses.

Ethylene and Fruit Growth

Ethylene orchestrates multiple plant processes: initiating fruit ripening by breaking down cell walls, leading to observed softening and color changes; promoting leaf and flower senescence, aiding aging and facilitating natural shedding; and influencing root growth and gravitropism, shaping root response to gravity and orientation.

Ethylene permeates the cell membrane and binds to receptors situated within the endoplasmic reticulum (ER) of plant cells, initiating various physiological responses. The binding of ethylene to its receptors sets off the activation of the transcription factor EIN3 (ethylene-insensitive 3), which subsequently prompts the expression of genes associated with the "constitutive triple response." This distinct growth pattern manifests as a thickened and shortened stem, radial swelling, and lateral expansion. As a result of ethylene-triggered constitutive triple response, plants adapt to mechanical stress and thrive in challenging environments, showcasing an adaptive mechanism crucial for survival.

Brassinosteroids

Brassinosteroids serve a pivotal role in promoting cell elongation and division, profoundly influencing overall plant growth and development. Furthermore, these hormones bolster a plant's resilience against diverse stresses like heat, cold, and salinity, enabling plants to flourish even in harsh environmental conditions.

Through binding to cell surface receptors, brassinosteroids initiate a signaling cascade that involves phosphorylation events with Brassinosteroid Signaling Kinase (BSK), leading to the activation of BRI1-Associated Kinases (BAK1). Consequently, BAK1 activates the transcription factor BES1 (Bri1-EMS-Suppressor 1), pivotal in orchestrating the expression of genes associated with cell elongation, division, and stress responses, resulting in enhanced growth and stress tolerance in plants.

Jasmonates

Jasmonates are key players in coordinating plant defense responses against herbivores and pathogens by primarily inducing the synthesis of defensive compounds such as secondary metabolites and proteinase inhibitors. This induction acts as a protective shield, strengthening plants against potential threats posed by herbivores and pathogens.

The perception of jasmonates occurs through cell membrane receptors, initiating a signaling cascade within the plant. Upon activation of these receptors, Jasmonate ZIM-Domain Proteins (JAZ) undergo degradation through the ubiquitin-proteasome pathway. This degradation of JAZ proteins leads to the release of transcription factors, notably MYC2, which then regulate the expression of genes associated with defense responses. Consequently, the activated transcription factors govern the production of defense-related compounds, including secondary metabolites and proteinase inhibitors. This robust defense response acts as a reinforcement, enhancing the plant's resilience against herbivores and pathogens.

Salicylic Acid

Salicylic acid acts as the initiator of Systemic Acquired Resistance (SAR), a pivotal defense mechanism in plants. This process empowers plants to fend off pathogens by priming different parts of the plant, ensuring a swifter and more robust defense response.

Upon pathogen perception, plants often trigger the production of salicylic acid as a response to attack or infection. This versatile compound assumes a crucial role in inducing the expression of diverse pathogenesis-related (PR) genes, encoding proteins that actively contribute to the plant's defense against pathogens. The presence of salicylic acid serves as the catalyst for SAR, initiating a comprehensive defense response throughout the entire plant structure. This orchestrated process ensures the plant's resistance to potential future attacks by pathogens.