Grade 12 Plant Hormones

Even though both plant and animal hormones act as chemical messengers in the organism, plant hormones differ from animal hormones in several ways. Plant hormones have varying effects depending on the receptor’s location in the plant. In plants, unlike in animals, there is often a great deal of interaction between the different hormones to bring about the most appropriate response. In animals, it is common for one specialized gland or cell to produce a hormone. In plants a hormone may be produced throughout the plant.

Chemicals known as plant hormones or plant growth regulators are responsible for most communication within plants. Unlike animal hormones, plant growth regulators are not produced in specialised cells within glands, but in a variety of tissues. They move in the plant either directly from cell to cell (by diffusion or active transport) or are carried in the phloem sap or xylem sap. Some may not move far from their site of synthesis and may have their effects on nearby cells.

Here are two examples:

auxins

which influence many aspects of growth including elongation growth which determines the overall length of roots and shoots

gibberellins

which are involved in seed germination and controlling stem elongation.

5 Main Plant Growth Substances

Auxins

Promote primary growth (lengthening) by promoting cell elongation and increasing the rate of cell division

Promote apical dominance – whereby the apex / tip of a plant grows while the lateral buds remain undeveloped

Auxin concentrations may change in response to directional stimuli (i.e. play a key role in tropisms)

Cytokinins

Promote cell division (cytokinesis) and ensure roots and shoots grow at equal rates

Promotes secondary growth (thickening) and help to control the rate of branching by a plant

Cytokinins are also involved in stimulating the growth of fruit

Gibberellins

Triggers germination in dormant seeds (initiates plant growth)

Gibberellin also causes stem elongation by promoting cell elongation and cell division

Ethylene

A gas which acts as a plant hormone and stimulates maturation and ageing (senescence)

It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening)

It also contributes to the loss of leaves (abscission) and the death of flowers

Abscisic Acid

Abscisic acid (ABA) principally functions to inhibit plant growth and development

It promotes the death of leaves (abscission) and is responsible for seed dormancy

It generally initiates stress responses in plants (like winter dormancy in deciduous plants)

Abscisic acid controls the closing of stomata and hence regulates water loss in plants

Tropism

Plants respond to the environment by tropisms. Plants use hormones to control the growth o stems and roots.

Both the rate and the direction o growth are controlled.

The direction in which stems grow can be inuenced by two external stimuli: light and gravity.

Stems grow towards the source o the brightest light or in the absence of light they grow upwards, in the opposite direction to gravity.

These directional growth responses to directional external stimuli are called tropisms.

Growth towards the light is called phototropism and growth in response to gravitational orce is called gravitropism.

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Auxin

Plants make several chemicals known as auxins, ofwhich the principal one is IAA (indole 3-acetic acid). Here, we refer to this simply as ‘auxin’ inthe singular. Auxin is synthesised in the growing tips(meristems) of shoots and roots, where the cells are dividing. It is transported back down the shoot, or up the root, by active transport from cell to cell, and also to a lesser extent in phloem sap.

How Auxin works

Watch this video

The Sequence of Events in the Stem that cause it to bend towards the light is as follows:

Giberellins

Gibberellins are plant growth regulators that are synthesised in most parts of plants. They are present in especially high concentrations in young leaves and in seeds, and are also found in stems, where they have an important role in determining their growth.

Stem Elongation

The height of some plants is partly controlled by their genes.
For example, tallness in pea plants is affected by a gene with two alleles; if the dominant allele, Le, is present, the plants can grow tall, but plants homozygous for the recessive allele, le, always remain short. The dominant allele of this gene regulates the synthesis of the last enzyme in a pathway that produces an active form of gibberellin, GA1.

Active gibberellin stimulates cell division and cell elongation in the stem, so causing the plant to grow tall.

A substitution mutation in this gene gives rise to a change from alanine to threonine in the primary structure of the enzyme near its active site, producing a non-functional enzyme. This mutation has given rise to the recessive allele, le. Homozygous plants, lele, are genetically dwarf as they do not have the active form of gibberellin.
Applying active gibberellin to plants which would normally remain short, such as cabbages, can stimulate them to grow tall.

Seed Germination

Gibberellins are involved in the control of germination of seeds, such as those of wheat and barley.
Figure 15.38 shows the structure of a barley seed. When the seed is shed from the parent plant, it is in a state of dormancy; that is, it contains very little water and is metabolically inactive.
This is useful because it allows the seed to survive in adverse conditions, such as through a cold winter, only germinating when the temperature rises in spring. The seed contains an embryo, which will grow to form the new plant when the seed germinates. The embryo is surrounded by endosperm, which is an energy store containing the polysaccharide starch. On the outer edge of the endosperm is a protein-rich aleurone layer.
The whole seed is covered by a tough, waterproof, protective layer (Figure 15.38). The absorption of water at the beginning of germination stimulates the embryo to produce gibberellins. These gibberellins diffuse to the aleurone layer and stimulate the cells to synthesise amylase.
The amylase mobilises energy reserves by hydrolysing starch molecules in the endosperm, converting them to soluble maltose molecules.
These maltose molecules are converted to glucose and transported to the embryo, providing a source of carbohydrate that can be respired to provide energy as the embryo begins to grow.
Gibberellins cause these effects by regulating genes that are involved in the synthesis of amylase.
In barley seeds, it has been shown that application of gibberellin causes an increase in the transcription of mRNA coding for amylase.
It has this action by promoting the destruction of DELLA proteins that inhibit factors that promote transcription (Chapter 16 page 391). (DELLA stands for the first five amino acids in the primary sequence of these proteins.)

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