Gibberellins and cytokinins

The foolish seedling

Japanese rice farmers knew a disease called bakanae, which means foolish seedling. The infected seedlings grew very tall and thin and pale. Then they fell over and gave no grain.

E. Kurosawa studied the disease in the early twentieth century. He found that the fungus Gibberella fujikuroi made a chemical that went into the rice plant and made it grow tall. Later, other scientists named the chemical gibberellic acid. The chemical was a hormone of the fungus, and it turned out that plants make it too.

Gibberellins

Gibberellins are terpenes. There are more than 100 of them. They are written GA1, GA2, GA3 and so on. GA3, which is gibberellic acid, is the one that was studied first, and it is the most important. Many gibberellins come from fungi, and many from higher plants.

The main gibberellin effect is to make the axis, the stem, longer. A cell in a stem becomes longer, and the stem becomes longer too.

NCERT lists these effects.

• Longer grape stalks. A spray of GA3 makes the stalk of a grape bunch longer. The berries then have room to grow and the bunch is loose.
• Better apples. GA improves the shape of apples.
• Delay of senescence. Senescence is the ageing of a plant part. GA delays it, so fruits can be left on the tree for a longer time. The market can then be supplied for longer.
• Malting. In the brewing industry, GA speeds up malting, which is the germination of barley grains to make the sugar.
• Sugarcane. A spray on sugarcane makes the stems longer, and the yield goes up by as much as 20 tonnes per acre.
• Juvenile conifers. A spray on young conifers hastens the maturity period, so cone production comes sooner.
• Bolting. In plants that grow as a rosette, such as beet and cabbage, the stem is very short until the plant flowers. Bolting is the sudden elongation of the flower stem. GA promotes bolting.

The effects of GA on bakanae and on bolting come from the same action: it stretches the stem.

Cytokinins

Think of a plant cell that must make two cells. It needs a signal for cytokinesis, the splitting of the cell. That signal is a cytokinin.

Folke Skoog and his co-workers found that tobacco pith tissue grew only if a certain substance was in the jelly. They found the substance in old, autoclaved herring sperm DNA. Miller isolated it, and it was named kinetin. Kinetin is a modified form of adenine, a purine base of DNA. It was found by chance, in DNA that had been heated.

The first natural cytokinin was zeatin. It was isolated from maize kernels that were not yet ripe, and also from coconut milk. Cytokinins now include zeatin and other natural compounds and synthetic ones, and all are adenine derivatives.

Cytokinins are made where cells divide fast: the root apices, the developing shoot buds and the young fruits.

What cytokinins do

• New leaves and chloroplasts. Cytokinins help to produce new leaves and chloroplasts in leaves.
• Lateral and adventitious shoots. They promote the growth of lateral shoots, and the formation of adventitious shoots, which are shoots that form in an unusual place, such as on a cutting.
• Apical dominance. They help to overcome apical dominance. Auxin keeps the lateral buds quiet, and cytokinin wakes them.
• Delay of senescence. Cytokinins promote the mobilisation of nutrients, which means that the nutrients move towards the leaf. A leaf kept supplied this way stays green longer, so senescence is delayed.
• Cell division. They promote cell division in the presence of auxin, which is why cell culture needs both.

Look at the last drawing of the figure. A purple dot of cytokinin on a lateral bud makes that bud grow, even though the apical bud is still there.

Auxin and cytokinin together

In plant tissue culture, a piece of plant tissue grows on a jelly that holds both regulators. The result depends on their ratio. This goes beyond the NCERT text, but NEET uses it.

• A high ratio of auxin to cytokinin makes the tissue form roots.
• A low ratio makes it form shoots.
• A middle ratio gives a mass of undifferentiated cells, called callus.

You should find that the marker moves to the left, to shoots, when the cytokinin is greater, and to the right, to roots, when the auxin is greater.

Gibberellins and cytokinins side by side

• Chemistry: gibberellins are terpenes. Cytokinins are adenine derivatives.
• Found from: gibberellins from the rice disease bakanae. Kinetin from autoclaved herring sperm DNA.
• Key examples: GA3 for gibberellins. Kinetin and zeatin for cytokinins.
• Main effect: gibberellins give a longer stem and bolting. Cytokinins give cell division and lateral shoots.
• Senescence: both delay it.
• Apical dominance: gibberellins have no part in it. Cytokinins overcome it.

Try it in Python

The program finds the average stalk length of grape bunches with and without GA3, and the per cent increase.

Practise with new numbers

Common mistakes

• Calling kinetin the first natural cytokinin. Kinetin was found in DNA that had been heated. Zeatin, from maize, was the first natural cytokinin.
• Mixing up the two names. Gibberellins come from Gibberella, a fungus. Cytokinins are about cytokinesis, the division of cells.
• Saying GA makes the plant grow thicker. GA mostly makes the axis longer.
• Thinking GA hastens senescence. Both GA and cytokinins delay senescence. Ethylene hastens it.
• Saying that cytokinin works alone on lateral buds. It works against auxin, as the antagonist, in apical dominance.

Remember these

• Bakanae: the fungus Gibberella fujikuroi, studied by Kurosawa, gave gibberellins. Gibberellins are terpenes. GA3 is the best known, and there are over 100 gibberellins.
• GA: longer stems, long grape stalks, better apples, delayed senescence, faster malting, sugarcane yield, juvenile conifers, bolting.
• Cytokinins are adenine derivatives. Kinetin came from herring sperm DNA. Zeatin came from maize and coconut milk.
• Cytokinins: cell division, new leaves and chloroplasts, lateral and adventitious shoots, overcoming apical dominance, delay of senescence.
• High auxin to cytokinin ratio: roots. Low ratio: shoots.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is bakanae, and what did it lead to?

2. Name three practical uses of gibberellins.

3. What is bolting?

4. From where was kinetin obtained, and what kind of molecule is it?

5. A gardener puts cytokinin on a lateral bud of a healthy plant. What happens, and why?