Plant growth regulators and auxins

A plant has no nerves, but it still sends messages

When a window lets light in from one side, the plant on the sill bends towards it. Nobody told the stem. The plant has no nerves and no brain. The message is a chemical.

A plant growth regulator, or PGR, is a small chemical that the plant makes, and that controls growth and development in a very small amount. You will also meet the names plant hormone and phytohormone for the same thing. Hormones work like this: they are made in one place, they travel a short way, and they change what the cells do at the place where they arrive.

NCERT describes the PGRs as small, simple molecules of very different chemical kinds. There are five groups, and each group comes from a different kind of chemistry.

Promoters and inhibitors

The five groups fall into two teams by the work they do.

• Growth promoters take part in cell division, cell enlargement, tropic growth, flowering, fruiting and seed formation. They are the auxins, the gibberellins and the cytokinins.
• Growth inhibitors take part in the response to wounds and stress, and in dormancy and abscission. The inhibitor of this team is abscisic acid.
• Ethylene could sit in either team, but it mostly slows growth, so we count it with the inhibitors.

A PGR seldom works alone. One PGR can have many effects. Two PGRs can have the same effect. Some pairs work together, which is synergism, and some pairs work against each other, which is antagonism. The result also depends on how much of the PGR there is, and on the tissue and its age.

How auxin was found: a coleoptile bends

The experiments in this lesson all use a coleoptile. A coleoptile is the hollow protective sheath that covers the first leaf of a grass seedling, such as oat or canary grass. It grows quickly, it is straight, and it is easy to cut. That is why it is a favourite in the laboratory.

Phototropism is growth that bends a plant towards light. Charles Darwin and his son Francis Darwin watched canary grass coleoptiles grow towards a window. Then they covered and cut parts of the coleoptile, with the light coming from one side only.

Read the picture like a detective.

• The intact coleoptile bends towards the light.
• With the tip cut off, there is no bending.
• With an opaque cap on the tip, there is no bending. The light cannot reach the tip.
• With a transparent cap, the coleoptile bends. The tip sees the light.
• With the lower part covered and the tip bare, it still bends.

So the tip is the part that senses the light. The bending, however, happens below the tip. The Darwins decided that the tip makes some influence that travels down. They did not know what it was.

Boysen-Jensen went one step further. These experiments are not in the NCERT text, but NEET asks about them.

• He cut off the tip and put it back with a block of gelatin between them. The coleoptile still bent. Gelatin lets a chemical pass, so the signal is chemical, not a nerve-like impulse.
• A thin plate of mica, which no chemical can cross, stopped the bending when it was on the shaded side. The signal travels down the shaded side.
• Mica on the lit side did not stop the bending.

Then Paal cut off the tip and put it back on one side of the stump, in the dark. The stump bent away from the side that carried the tip. The side below the tip grew faster. So the substance makes cells grow longer.

Went isolates the substance

F. W. Went finished the story, using the oat coleoptile, Avena sativa.

1. He cut off tips and stood them on agar, a jelly. The substance moved from the tips into the agar.
2. He cut the tip off a coleoptile and put a small piece of that agar on one side of the stump, in the dark. The stump bent away from the block.
3. A plain block of agar, which had never touched a tip, caused no bending. This control shows that the substance, and not the agar, causes the bending.
4. The angle of curvature is larger when the block has more of the substance. The angle can be measured, so the substance can be counted. This is the Avena curvature test.

The substance was auxin. The name comes from the Greek word auxein, which means to grow. Auxin was first isolated from human urine, and later from plants. Went showed that auxin is made in the tips of coleoptiles.

You should find that the curvature rises with the number of tips at first and then flattens, and that a centred block gives no curvature at all.

Kinds of auxin

Auxin is made mainly in the growing apices of stems and roots. From there it moves to the places where it acts.

• IAA (indole-3-acetic acid) and IBA (indole butyric acid) are natural auxins. They have been isolated from plants.
• NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic auxins. Chemists made them, but they act like the natural ones.

Auxins are used a great deal in farming and gardening.

What auxins do

• Rooting. Auxins help stem cuttings to form roots. Gardeners dip the cut end of a stem in auxin to propagate a plant.
• Flowering. Auxins promote flowering in pineapple.
• Fruit and leaf drop. Auxins prevent the early drop of fruits and leaves. They also promote the abscission of older, mature leaves and fruits. Abscission is the shedding of a part of a plant.
• Apical dominance. In most plants the growing apical bud stops the lateral buds from growing. This is apical dominance. When the tip is cut off, the lateral buds grow. Tea planters and gardeners use this to make bushy tea bushes and hedges.

• Seedless fruit. Auxins can cause parthenocarpy, which is the growth of a fruit without fertilisation, as in tomato.
• Weed killer. 2,4-D kills dicot weeds, but it does not harm the mature monocot crop. A gardener uses it to keep a lawn free of broad-leaved weeds.
• Xylem and cell division. Auxin controls the differentiation of xylem and helps cells to divide.
• Cell elongation. The general work of auxin is to make cells longer, as Paal saw.

The same auxin, a different dose

Roots, buds and stems do not need the same amount of auxin. A dose that makes a stem grow faster can stop a root from growing.

Read the graph. Roots react to the smallest amount. Buds need more. Stems need the most. For each organ a little promotes growth, and too much holds it back. A weed killer works this way: the dose is far too high for the weed.

Try it in Python

The program goes through the experiments of Darwin and Boysen-Jensen. A coleoptile bends only if the tip is there, the tip sees the light, and the signal can pass down.

Practise with new numbers

Common mistakes

• Saying that the tip bends. The tip senses the light. The region below the tip bends.
• Mixing up the direction. In Went's test the stump bends away from the block. A one-sided cap on the tip does not change that.
• Calling 2,4-D a natural auxin. IAA and IBA are natural. NAA and 2,4-D are synthetic.
• Saying that auxin only promotes. A high dose inhibits, and roots stop growing at a dose that is good for a stem.
• Forgetting the control. The plain agar block shows that the agar is not the cause.

Remember these

• A PGR is a small chemical made by the plant that controls growth in a very small amount.
• Auxins are indole compounds, gibberellins are terpenes, cytokinins are adenine derivatives, ethylene is a gas, and abscisic acid is a carotenoid derivative.
• Auxins, gibberellins and cytokinins promote growth. Abscisic acid inhibits. Ethylene mostly inhibits.
• The tip of a coleoptile makes auxin and senses light. Went showed auxin with agar blocks and the Avena curvature test.
• Auxin: rooting of cuttings, flowering in pineapple, apical dominance, parthenocarpy, 2,4-D weed killer for dicots, xylem differentiation, delay of early fruit and leaf drop.

Check yourself

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

1. Which part of a coleoptile senses one-sided light, and which part bends?

2. In Went's experiment, why was a plain agar block placed on a second coleoptile?

3. A cut coleoptile has an auxin block on its left half and stands in the dark. In which direction does it bend?

4. Name two natural auxins and two synthetic auxins.

5. How does 2,4-D clear a lawn of broad-leaved weeds and leave the grass?