Plant Hormones

Plants do not have nervous systems that help them respond to changes happening around them. Instead, they use chemical molecules, known as plant hormones, to control and coordinate growth and responses to their environment. They are regulatory messengers produced by the plant (or sometimes synthetically added) to stimulate an action or behaviour in specific cells. A little goes a long way! These hormones are made in one part of the plant and are transported to others parts, where they have their effect.

Functions of Plant Growth Hormones

Plants are quite sensitive but since they aren't able to move, they have developed a system to help them respond to the various stimuli in their environment. These stimuli include gravity, light, touch, water and changes in seasons.

These responses by the plant to the stimuli in the environment are known as tropisms. These help plants to grow towards a light and water source so that processes such as photosynthesis can take place more efficiently. There are two categories of tropisms:

• Positive tropism: the growth or movement of the plant towards the stimulus.

• Negative tropism: the growth or movement of the plant away from the stimulus.

Types of Plant Growth Hormones

There are five main types of plant hormones that coordinate the growth and development of a plant. These plant hormones all serve different purposes and sometimes have more than one function (see table below). They can work independently or together to help affect plant growth.

Auxins

Auxins are a family of hormones that are very important for the growth of plants and can be found in growing stems and roots where they diffuse to other parts of the plant. They are involved in a few of the growth processes and it is likely that you have seen their effects.

Plants tend to grow (NOT bend) towards the direction of a light source – this is known as positive phototropism. This is due to an unequal distribution of auxins, causing unequal growth rates in the stem. The side that is in the shade contains a higher concentration of auxins, leading to cell elongation and the plant growing towards the light.

When a higher concentration of auxins is found at the tip of the stem, it leads to apical dominance. This is when the main stem of a plant grows more strongly than the side branches. If the auxin-containing tips are cut off, then phototropism will not take place.

Gibberellins and Ethene

Gibberellins are a group of plant hormones that are involved in several different development phases, such as initiating seed germination. They signal to seeds that it is time to sprout, putting a pause on the resting time (dormancy) of the seed or bud. Despite being a completely different hormone, gibberellins, similarly to auxins, also help to promote plant growth by cell elongation. They essentially help to make the stems of plants grow longer.

Ever wonder why when you buy bananas, they ripen so quickly and cause all of the other fruit in the bowl to ripen too? The reason for this is due to ethene. This is an unusual plant hormone, as it is the only plant hormone to exist as a hydrocarbon gas. This means that it can travel through the air, and have an effect on any nearby fruit! Ethene also is involved in cell division, where it controls the process of abscission.

Cytokinin and ABA

Cytokinin is like an anti-ageing cream for plants! It helps to delay the natural ageing process, known as senescence, by encouraging cells to divide. It is sometimes used by florists to delay wilting of cut flowers.

ABA helps to prevent further water loss. It does this by encouraging the stomata to close in times of stress, such as when there is a lack of water.

Plant Rooting Hormone

Auxin, just like with stem growth, is a necessary plant hormone for rooting. It helps plants to establish roots! As discussed above, auxins are involved in phototropism. The growth of the stem goes towards the stimulus of light. However, when it comes to the roots, the unequal distribution of auxins leads to the growth of the roots towards the direction of gravity – known as positive geotropism (or gravitropism) and the stem to grow away from gravity (negative geotropism).

Uses of Plant Hormones

These plant hormones are used in agriculture and horticulture for various reasons to help control plant growth in some way.

Auxins are plant hormones that are used:

• as rooting powder

• for promoting growth in tissue culture (along with cytokinin)

• as weedkiller

Gibberellins are used:

• to speed up germination by ending seed dormancy
• to promote year-round flowering
• to increase fruit size and yield

Ethene is used:

• to control the ripening of fruit