Auxins and Gibberellins

Plant Growth Regulators (PGRs), also known as phytohormones, are small, simple molecules of diverse chemical composition, which regulate physiological processes in plants. They are broadly classified into two groups based on their functions: plant growth promoters (e.

g., auxins, gibberellins, cytokinins) and plant growth inhibitors (e.g., abscisic acid, ethylene). Auxins and gibberellins fall into the former category, playing pivotal roles in promoting various aspects of plant growth and development.

Conceptual Foundation of Plant Hormones

Plant hormones act as chemical messengers, coordinating cellular activities and developmental programs across the plant body. Their effects are often concentration-dependent, and they frequently interact synergistically or antagonistically to fine-tune growth responses. Understanding these interactions is key to comprehending plant development.

Auxins: The Growth Initiators

Discovery and Types:

The concept of a growth-promoting substance in plants emerged from observations of phototropism (bending towards light). Charles Darwin and his son Francis Darwin, in their 1880 book 'The Power of Movement in Plants,' noted that the coleoptile of canary grass bent towards light only if its tip was exposed. They concluded that some 'influence' was transmitted from the tip to the elongating region below.

Later, F.W. Went, in 1928, isolated this substance from the tips of oat coleoptiles and named it auxin (from the Greek 'auxein,' meaning 'to grow'). He demonstrated its effect on cell elongation using the Avena curvature test.

Natural Auxins: — The most common and physiologically active natural auxin is Indole-3-acetic acid (IAA). Other natural auxins include Indole-3-butyric acid (IBA).
Synthetic Auxins: — Several synthetic compounds mimic auxin activity, such as Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D), and 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T). These are often used in agriculture and horticulture due to their stability and potency.

Synthesis and Transport:

Auxins are primarily synthesized in the apical meristems of shoots, young leaves, and developing seeds. The amino acid tryptophan is the primary precursor for IAA synthesis. Auxins exhibit a unique characteristic called polar transport, meaning they move unidirectionally, typically from the morphological apex to the base (basipetal transport), through parenchyma cells, not via the phloem or xylem.

This active transport mechanism is crucial for establishing auxin gradients that regulate development.

Physiological Effects:

1
Cell Elongation: — Auxins promote the elongation of cells, particularly in stems and coleoptiles, by increasing cell wall plasticity (acid growth hypothesis) and water uptake. This is the primary mechanism behind phototropism and gravitropism.
2
Apical Dominance: — The presence of a dominant apical bud inhibits the growth of lateral (axillary) buds. This phenomenon, known as apical dominance, is largely mediated by auxin produced in the apical meristem. Removal of the apical bud (decapitation) releases the lateral buds from inhibition, leading to bushier growth.
3
Root Initiation: — Auxins promote the initiation of adventitious roots in stem cuttings, a property widely exploited in plant propagation. Higher concentrations of auxin, however, can inhibit root elongation.
4
Parthenocarpy: — Auxins can induce parthenocarpy (development of fruit without fertilization) in some plants, such as tomatoes, leading to seedless fruits.
5
Abscission Prevention: — Young leaves and fruits produce auxins that prevent their premature abscission (shedding). As they mature, auxin levels decrease, making them more susceptible to abscission.
6
Flowering: — Auxins can promote flowering in some plants (e.g., pineapples) and inhibit it in others.
7
Herbicides: — Synthetic auxins like 2,4-D are widely used as selective herbicides. They act as 'super auxins,' causing uncontrolled, abnormal growth in broad-leaved weeds, leading to their death, while monocotyledonous crops (like wheat, maize) are relatively resistant.
8
Xylem Differentiation: — Auxins play a role in the differentiation of xylem elements.

Mechanism of Action (Acid Growth Hypothesis):

Auxins promote cell elongation by stimulating proton pumps (H+-ATPases) in the plasma membrane. This pumps protons into the cell wall, lowering its pH. The acidic environment activates cell wall-loosening enzymes (e.g., expansins), which loosen the cellulose microfibrils, allowing the cell to take up water and expand under turgor pressure.

Gibberellins: The Elongation Specialists

Discovery and Types:

Gibberellins were first discovered in Japan in the 1920s by E. Kurosawa, who was investigating the 'bakanae' (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi. Infected rice seedlings grew abnormally tall and slender. Kurosawa isolated the active substance from the fungal exudate. Later, Yabuta and Sumiki isolated the crystalline form of this substance and named it gibberellin.

There are over 100 types of gibberellins identified so far, denoted as GA1, GA2, GA3, and so on. Gibberellic acid (GA3) is the most thoroughly studied and widely used gibberellin.

Synthesis:

Gibberellins are synthesized from the mevalonic acid pathway, primarily in young leaves, developing seeds, and root tips.

Physiological Effects:

1
Stem Elongation: — The most striking effect of gibberellins is their ability to cause dramatic elongation of internodes, leading to increased plant height. This is particularly evident in genetically dwarf varieties of plants (e.g., dwarf peas, maize), which can grow to normal height when treated with GAs. This effect is due to both cell elongation and increased cell division.
2
Seed Germination: — Gibberellins play a crucial role in breaking seed dormancy and promoting germination. In cereal grains (e.g., barley), GA stimulates the synthesis and secretion of $\alpha$-amylase and other hydrolytic enzymes in the aleurone layer. These enzymes break down stored food reserves (starch) in the endosperm, providing nutrients for the developing embryo.
3
Bolting: — In rosette plants (e.g., cabbage, beet), which exhibit restricted stem growth and a cluster of leaves at the base, gibberellins induce bolting – the rapid elongation of the internodes just prior to flowering.
4
Fruit Growth and Development: — GAs promote fruit growth, especially in grapes, leading to increased fruit size and elongated bunches. They can also delay senescence (aging) in some fruits.
5
Flowering: — Gibberellins can promote flowering in long-day plants (LDPs) under non-inductive short-day conditions and can substitute for the cold requirement (vernalization) in some plants.
6
Malting Industry: — GA3 is used in the malting industry to speed up the malting process in brewing, as it enhances the production of $\alpha$-amylase.
7
Juvenility: — GAs can reverse juvenility in some plants, allowing early flowering.

Mechanism of Action:

Gibberellins exert their effects by regulating gene expression. They bind to receptor proteins, leading to the degradation of DELLA proteins, which are repressors of GA-responsive genes. This allows the transcription of genes involved in growth and development, such as those for cell elongation and enzyme synthesis.

Common Misconceptions and NEET-Specific Angle

Auxins are only for roots, Gibberellins only for shoots: — While auxins are critical for root initiation and gibberellins for stem elongation, both hormones have diverse effects across the plant. Auxins also promote shoot elongation and fruit development, while gibberellins influence seed germination and flowering.
PGRs always promote growth: — While auxins and gibberellins are growth promoters, other PGRs like abscisic acid inhibit growth, and ethylene can have both promoting and inhibiting effects depending on the context. Even high concentrations of auxins can inhibit growth (e.g., root elongation).
PGRs act independently: — Plant development is a result of complex interactions and balances between different PGRs. For example, apical dominance involves an auxin-cytokinin interaction.

For NEET, it's crucial to remember specific examples of applications (e.g., 2,4-D as a herbicide, GA3 for malting and grape size, NAA/IBA for rooting cuttings, IAA for parthenocarpy in tomatoes). Understanding the discovery stories and the specific physiological effects associated with each hormone is also frequently tested. Pay attention to the concentration-dependent effects and the interplay between different hormones.