Plant Hormones | Lucent for GK - UPSC PDF Download

Introduction

Plant hormones, also known as phytohormones, are essential signal molecules produced within plants that regulate various cellular processes. They play a crucial role in determining the growth, development, and overall physiological responses of plants. Unlike animals, plants lack specialized glands for hormone production, and instead, each cell has the capability to synthesize these vital compounds. In this article, we will explore the different types of plant hormones and their specific roles in plant growth and development.

Auxins: Orchestrating Plant Growth

Auxins are a group of plant hormones that exert diverse effects on plant growth. They regulate processes such as cell elongation, phototropic responses, secondary growth, apical dominance, and the development of leaf traces and fruit. Indole-3-acetic acid (IAA) is one of the most important natural auxins. Synthetic auxins, such as 2,4-D and 2,4,5-T, are commonly used as weed killers. Auxins act as weak organic acids that actively participate in cell division and elongation, thereby promoting overall plant growth. Fascinatingly, the application of certain auxins on flowers can stimulate fruit formation without fertilization or seed development, a phenomenon known as artificial parthenocarpy.

Gibberellins: Orchestrating Seed Germination and Growth

Gibberellins (GAs) comprise a diverse group of chemicals produced naturally within plants and by fungi. These hormones were first discovered when researchers observed abnormal growth in rice plants caused by a fungus called Gibberella fujikuroi. Gibberellins play a crucial role in seed germination by activating enzymes that mobilize stored food reserves, supporting the growth of new cells. These hormones are also involved in promoting flowering, increasing cellular division, and facilitating growth in germinated seeds. Additionally, GAs counteract the inhibitory effects of another hormone called abscisic acid (ABA) on shoot growth and dormancy.

Cytokinins: Orchestrating Cell Division and Delaying Senescence

Cytokinins (CKs) are plant hormones that influence cell division and promote shoot formation. They were initially referred to as "kinins" when they were isolated from yeast cells. Cytokinins play a crucial role in delaying senescence, which is the natural aging process in plants. These hormones also facilitate auxin transport throughout the plant, affect the length of internodes (the segments between leaves or buds), and regulate leaf growth. The balance between auxins and cytokinins, represented by the A/C ratio, is pivotal in determining major growth periods during a plant's lifetime. Additionally, cytokinins, in conjunction with another hormone called ethylene, promote the abscission of leaves, flower parts, and fruits.

Ethylene: Orchestrating Growth and Environmental Responses

Ethylene is a gaseous plant hormone that forms as a result of the breakdown of methionine, a compound present in all plant cells. While ethylene has limited solubility in water and doesn't accumulate within the cells, it diffuses out of the cells and escapes into the atmosphere. Ethylene's effectiveness as a hormone depends on the balance between its production rate and its rate of escaping the plant. Ethylene influences cell growth and shape, and its production increases when a growing shoot encounters an obstacle underground, preventing elongation and causing the stem to swell. This response enables the plant to grow around the obstruction. Ethylene also plays a role in regulating leaf expansion and the geotropic response of stems (growing upright).

Abscisic Acid: Orchestrating Growth Regulation and Stress Responses

Abscisic acid (ABA) is a hormone involved in regulating various physiological processes in plants. It activates the vascular cambium during cell division, which slows down stem growth. ABA can also inhibit sprouting in seeds and buds. Additionally, in drought conditions, ABA prompts the closure of pores (stomata) on leaves, reducing the rate of water evaporation. However, the role of ABA in leaf abscission (shedding) is still under investigation and remains inconclusive.

Conclusion

Plant hormones, with their complex interactions and regulatory functions, are indispensable for plant growth, development, and responses to environmental stimuli. From auxins and gibberellins that promote growth and flowering to cytokinins that control cell division and delay senescence, each hormone plays a vital role in shaping the life cycle of plants. Ethylene and abscisic acid contribute to environmental adaptations and stress responses, further highlighting the intricate web of plant hormone signaling. Understanding the mechanisms and effects of these hormones enhances our knowledge of plant biology and offers insights into potential applications for crop improvement and agriculture.