Phytohormones

• E. Starling coined the term “hormone” in connection with the discovery of Secretin.

• Hormones are organic compounds secreted in small quantities that regulate the plant’s activities after being translocated to the organ or site of reaction where they show their specific effect.

• Five major kinds of plant hormones are :

Auxins

Cytokinins

Gibberellins

Ethylene

Abscisic acid

Auxins

• 1898 - proposed by Charles Darwin in “The power of movements in plants” while working on Canary grass (Phalaris cannariensis).

• 1928 - W. Went discovered auxins by Avena curvature test.

• 1931 - Kogl and Haagen Smit isolated Auxin-A (Auxintriolic acid)

• 1934 - Kogl and Haagen Smit isolated Auxin-B (Auxinolonic acid)

• 1935 – isolated heteroauxins.

Natural auxin : Indole-3-acetic acid (IAA)

Synthetic auxins : Nephthalene acid (NAA), indole butyric acid (IBA), indole propionic acid (IPA), 2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-Trichlorobenzoic acid (2,4,5-T) etc.

Funtions of Auxins

• Elongation of the cell.

• Apical dominance

• Root initiation (IAA, NAA, IBA, IPA )

• Herbicides ( 2,4-D, 2,4,5-T)

• inhibit sprouting of potato

• induce flowering (NAA,  2,4-D)

• abscission of fruits.

• stimulate cell wall synthesis and parthenocarpy.

Cytokinins ( Kinetins )

• 1940s - extracted from coconut milk.

Adenine-type cytokinis : kinetin, zeatin, 6-benzylaminopurine (BA )

Phenylurea-type cytokinis : diphenylurea, thidiazuron (TDZ)

Functions of Cytokinins

• Activates cell division.

• Acts in concert with Auxins :  Formation of callus , cell elongation in roots, embryos & fruit

• Acts in reverse with Auxins :  promote axillary bud, inhibits branch roots

• Prevents ageing and senescence in plants.

Gibberellins

•1926 -  isolated from a fungus Gibberella fugikuroi in the “bakanae disease”infected rice plant.

• > 100 naturally occurring gibberellins

Functions of Gibberellins

• induces seed germination

• stimulate stem elongation and leaf expansion.

• break dormancy in potato tubers and tree buds in winter.

• Induces flowering in LD plants

• causes parthenocarpy in pome fruits

• increases the number of male flowers  in moneecious plants

Ethylene

• gaseous hormone.

• synthesized from methionine.

• released from roots, shoot meristems and ripening fruits.

Functions of Ethtylene:

• induces ripening of fruits.

• accelerates abscission of leaves, stems, flowers and fruits.

• induces flowering in pineapple.

• Induces epinasty in leaves.

Abscisic acid (ABA)

•  produced in mature leaves, stem, fruits and seeds.

Functions of Abscisic acid

• accelerates abscission.

• inhibits seed germination

• also accelerates senescence in leaves.

• causes the closing of stomata

• Inhibits fruit ripening