PLANT GROWTH REGULATORS

PLANT GROWTH REGULATORS

            Growth regulators, or hormones, are not nutrients, but they influence growth and development. They are  produced naturally in plants. Cultures, however, usually do not manufacture sufficient quantities of growth regulators, sothey must be added selectively to culture media.

The five classical hormones:

Ø  Auxin

Ø  Gibberellins

Ø  Cytokinin

Ø  Abscisic acid

Ø  Ethylene

Additional substances gaining recognition as hormones:

Ø  Polyamines

Ø  Jasmonates

Ø  Salicylic acid

Ø  Brassinosteroid

 

Auxins

Auxins stimulate new cell division, cell enlargement, the formation of shoot apices or buds, the induction of somatic embryogenesis, and may promote rooting. e.g. Indole-3-acetic acid (IAA), Indole-3-butyric acid, 1-Naphtalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-Trichlorophenoxy acetic acid (2,4,5-T) and picloram.

Cytokinins

Cytokinins stimulate cell division, cell and/or shoot differentiation, lateral bud break, etc

Natural : zeatin (ZEA), Isopentenyl Adenine (2iP or IPA), (adenine)

Synthetic: benzylaminopurine (BAP = benzyladenine BA), kinetin (KIN)

Auxin - Cytokinin Interaction

            In case of high auxin-cytokinin ratio, root formation, embryogenesis and callus formation is initialised.

  In case of low auxin-cytokinin ratio, formation of adventitive or axillary shoots is induced.

  The auxin-cytokinin ratio is also essential for chloroplast formation (and for most of another process).

Gibberelins

Gibberelins - naturally occurring plant hormones involved in internode elongation, enhancement of flower, fruit and leaf size, germination, vernalization and others processes in plants. e.g gibberellic acid (GA3)

Abscisic Acid

Abscisic acid (ABA) - A plant hormone involved in abscission, enforcing dormancy, and regulating early stages of embryo development.

Ethylene

Ethylene - A gaseous plant hormone involved in fruit maturation, abscission, and senescence. It is produced by certain tissue cultures.

Brassinosteroids

Ø  Promote shoot elongation at low concentrations

Ø  Strongly inhibit root growth and development

Ø  Promote ethylene biosynthesis and epinasty

Jasmonates

  Jasmonates are represented by jasmonic acid and its methyl ester. Jasmonic acid is considered by some to be a new class of plant growth substance.

  Inhibition of many processes such as tissue culture growth, embryogenesis, seed germination, pollen germination, flower bud formation, chlorophyll formation,

  Differentiation in plant tissue culture, adventitious root formation, breaking of seed dormancy, pollen germination,

Polyamines

There is some controversy as to whether these compounds should be classified with hormones. They appear to be essential in growth and cell division.

Salicylic Acid

SA is thought by some to be a new class of plant growth substances. It promotes flowering, inhibits ethylene biosynthesis and reverses the e