Auxin vs. Gibberellin

Attribute	Auxin	Gibberellin
Chemical Structure	Indole-3-acetic acid (IAA) and other derivatives	Gibberellic acid (GA) and other gibberellins
Function	Promotes cell elongation, root initiation, and apical dominance	Stimulates stem elongation, seed germination, and flowering
Transport	Unidirectional polar transport from shoot tips to roots	Bidirectional transport within plants
Location	Primarily synthesized in shoot apical meristem and young leaves	Synthesized in actively growing tissues, including shoot apical meristem, young leaves, and developing seeds
Role in Plant Growth	Regulates tropisms, apical dominance, and root development	Controls stem elongation, seed germination, and flowering time
Interaction with Other Hormones	Interacts with cytokinins, abscisic acid, and ethylene	Interacts with auxins, cytokinins, abscisic acid, and ethylene
Commercial Applications	Used as rooting hormones, herbicides, and in tissue culture	Used in agriculture to promote plant growth, increase fruit size, and induce seedlessness

Introduction

Auxin and gibberellin are two important plant hormones that play crucial roles in various aspects of plant growth and development. While both hormones are involved in regulating plant growth, they have distinct attributes and functions. In this article, we will explore the characteristics of auxin and gibberellin, highlighting their similarities and differences.

Origin and Discovery

Auxin was first discovered by Charles Darwin and his son Francis in the late 19th century. They observed that the tips of coleoptiles (the protective sheath covering the emerging shoot) in grass seedlings were responsible for bending towards light. Later, Fritz Went identified the chemical responsible for this phenomenon and named it "auxin." On the other hand, gibberellin was discovered in the early 20th century by Eiichi Kurosawa, a Japanese scientist studying a disease called "foolish seedling" in rice plants. He isolated a compound from the fungus causing the disease and named it "gibberellin" after the Japanese word "gibberellic acid," meaning "foolish."

Chemical Structure

Auxin is a generic term for a group of plant hormones, with indole-3-acetic acid (IAA) being the most common and biologically active form. It has a simple chemical structure consisting of an indole ring and a carboxylic acid group. In contrast, gibberellins are a large family of plant hormones with over 130 different forms identified to date. They have a complex structure characterized by a gibberellin skeleton, which includes a tetracyclic diterpenoid ring system.

Functions

Auxin plays a crucial role in various aspects of plant growth and development. It promotes cell elongation, particularly in the stem, by loosening the cell wall and allowing water uptake. Auxin also influences root development, apical dominance (the inhibition of lateral bud growth by the terminal bud), and tropisms (plant responses to environmental stimuli such as light and gravity). Additionally, auxin is involved in fruit development and ripening, leaf abscission (shedding), and the formation of adventitious roots.

Gibberellins, on the other hand, are primarily involved in regulating stem elongation. They stimulate cell division and elongation in the internodes, leading to increased plant height. Gibberellins also promote seed germination, breaking dormancy and initiating the growth of the embryo. They are responsible for the elongation of stems in response to light, known as the "shade avoidance response." Furthermore, gibberellins influence flowering, fruit development, and the production of enzymes involved in various metabolic processes.

Transport and Distribution

Auxin is synthesized primarily in the apical meristem (the growing tip of the plant) and young leaves. It is then transported downwards through the stem, mainly in a polar manner, from the shoot apex to the root. This polar transport is facilitated by specialized proteins called PIN proteins, which actively transport auxin across cell membranes. Once in the target tissues, auxin can move laterally, influencing various growth processes.

Gibberellins, on the other hand, are synthesized in various plant organs, including young leaves, shoot tips, and developing seeds. Unlike auxin, gibberellins are transported both acropetally (upwards) and basipetally (downwards) through the plant. The transport mechanism of gibberellins is not fully understood, but it is believed to involve passive diffusion and active transport processes. Once distributed, gibberellins act on target tissues to promote growth and development.

Interactions with Other Hormones

Auxin and gibberellins interact with other plant hormones to coordinate plant growth and development. Auxin and cytokinins, another class of plant hormones, have a synergistic relationship. They work together to promote cell division and differentiation, leading to the formation of new tissues and organs. Auxin also interacts with abscisic acid (ABA), a hormone involved in stress responses, to regulate seed dormancy and germination.

Gibberellins, on the other hand, interact with other hormones such as auxin and ethylene. Gibberellins and auxin have a complex relationship, with gibberellins promoting the synthesis of auxin and auxin influencing the biosynthesis of gibberellins. Gibberellins also interact with ethylene, a hormone involved in fruit ripening and senescence, to regulate various developmental processes.

Applications in Agriculture

Auxin and gibberellins have significant applications in agriculture. Auxin-based herbicides are commonly used to control weed growth by inducing uncontrolled growth in susceptible plants. Auxin is also used in tissue culture techniques to promote the formation of callus (undifferentiated plant cells) and the rooting of plant cuttings. Additionally, synthetic auxins are used as growth regulators to control fruit thinning, prevent premature fruit drop, and promote fruit development.

Gibberellins find extensive use in agriculture as well. They are applied to crops to increase plant height, promote flowering, and improve fruit size and quality. Gibberellins are used to overcome seed dormancy in certain crops, ensuring uniform germination. They are also employed in brewing and malting industries to promote the elongation of barley stems and increase grain yield.

Conclusion

Auxin and gibberellin are two essential plant hormones that regulate various aspects of plant growth and development. While auxin primarily influences cell elongation, root development, and tropisms, gibberellins are primarily involved in stem elongation, seed germination, and flowering. Both hormones interact with other plant hormones to coordinate growth processes. Understanding the attributes and functions of auxin and gibberellin is crucial for unraveling the complex mechanisms underlying plant growth and for developing effective strategies in agriculture.