Plant Hormones Notes

Phytohormones (plant growth regulators) are organic substances produced naturally in higher plants. They control growth or other physiological functions at a site away from the place of production, and they act in minute amounts. The term phytohormone was proposed by Thimann (1948).

Main points

A plant growth regulator is a wider term. It also includes synthetic compounds like NAA and 2,4-D.

Cytokinins (kinetin) are purine derivatives. Kinetin was discovered by Skoog and Miller (1950) from tobacco pith callus, and the term cytokinin was proposed by Letham (1963). Zeatin and coconut milk factor are natural cytokinins. The roles are as follows.

I would add a cytokinin (kinetin) together with an auxin to the medium.

Why: callus is a mass of undifferentiated cells, and its growth needs continuous cell division. Kinetin is the hormone that induces cell division in callus tissue. In many tissue cultures the continued growth of callus takes place only after auxin is added, because auxin supports both cell division and elongation. So the pair restores growth.

The ratio of the two hormones also decides what the callus will form later. High auxin with low kinetin gives roots, and high kinetin with low auxin gives shoot buds.

I would recommend an auxin, for example NAA or 2,4-D sprayed in low concentration.

Mechanism: abscission means the premature fall of leaves, flowers or fruits. It happens when an abscission layer, a zone of weak cells, forms at the base of the fruit stalk. Natural auxin prevents the formation of this layer. When the auxin level in the fruit falls, the layer forms and a wind can easily detach the fruit. Spraying auxin keeps the level high, so the abscission layer does not form and the fruits stay attached to the plant.

Auxins are phytohormones made in the shoot and root apices, and they migrate to the zone of elongation. The name comes from auxeing, meaning to grow. The term auxin was introduced by Kogl and Haagen-Smit (1931), and Went (1928) isolated it from Avena coleoptile tips. IAA is the only natural auxin. The physiological effects are as follows.

Synthetic auxins: IBA, NAA, MENA, MCPA, TIBA, 2,4-D and 2,4,5-T.

Gibberellin was found by the Japanese scientist Kurosawa. Rice seedlings infected by the fungus Gibberella fujikuroi grew taller, thin and pale, and the active substance isolated from them was named gibberellin. It occurs in all parts of higher plants, and immature seeds are especially rich in it. Its precursor is acetate.

Cytokinins are used in agriculture and horticulture in several ways.

So, cytokinins improve the quality of the produce by keeping it fresh, and improve the yield by giving more branches, a better root system and more oil.