Plant Hormones

Keywords

Hormones, plants, auxins, gibberellins, cytokinin, ethene, ethylene, seed germination, cell elongation, phytohormones.

Introduction

Plants have a selection of hormones, also called phytohormones, and PGRs (Plant Growth Regulators). that provide and stimulate growth, reproduction and also help with which direction the plant grows in. It is linked to photosynthesis, and we will look at the main three hormones in the plant system. The same hormones are also present in algae.

The Three Main Hormones

A brief look at the hormones associated with plants, which includes:

Auxins: stimulates plant growth, and works with tropisms. They were the first group of hormones in plants to be discovered, and are found in the roots and shoots of new plant growth.

Gibberellins: first found in fungi called Gibberella fujikuroi, gibberellins stimulate seed germination and return a plant out of dormancy.

Ethylene/Ethene: a strong hormone that promotes ripening of fruit, particularly in bananas.

There are five main types of plant hormones. They include auxins, gibberellins, cytokinin, Ethylene and abscisic acid. They allow the plant to grow, reproduce, respond to environmental changes and also when the plant dies.

Auxins - A Closer Look

Derived from the Greek word 'Auxein', which means to grow, auxins were the first group of plant hormones to be discovered. Found mainly in the tips of roots and shoots, they allow the plant to grow freely. There are three types of auxin that help control the growth of the plant.

Functions of the hormone include promoting cell growth and elongation, initiation of roots in cuttings and grafts, helping cell division and differentiation, and inducing parthenocarpy - producing fruit without prior fertilisation. They also promote phototropism, which enables the plant to bend toward the sunlight (or artificial light if presented.

Other functions include the slower release of fruit, leaves and flowers from the plant. While environmental factors might do this abruptly (if there's a strong wind, for instance), the plant will keep these attached until it is necessary for them to leave. A deciduous plant (which releases it's leaves during autumn) will know that the weather is turning colder and the plant will prepare for a level of dormancy in order to protect itself and prepare itself for the coming winter months.

Gibberellins - A Closer Look

Originally isolated from the fungus Gibberella fujikuroi, gibberellins were later found in other plants. These hormones control growth.

Gibberellins will cause seed germination. So when you plant a seed in the ground or in a pot, these hormones will activate because there are nutrients and the right conditions with which it can grow. Similar to the parthenocopy described above, gibberellins also promote the growth of seedless fruits, such as grapes and bananas (yes, bananas used to have seeds running through the middle).

These hormones also regulate stem elongation, making sure that they do not grow too much at a time. When this happens though, it is usually because of conditions around the plant are necessary for it to grow in this manner. You can often find plants (particularly vegetables like runner beans) become 'leggy' - when they grow too much at a time to find the sunlight. Gibberellins should compensate for this, and slow the growth down once it reaches what it needs.

Ethylene/Ethene

So, go down to your local supermarket, and you buy a load of fruit - let's see. Apples, pears, peaches, nectarines, and bananas. You then get home, and you place them in a fruit bowl (typically) together. But then a couple of days later, you notice that the peaches and nectarines have shrivelled, and the pears have gone very soft and are growing mould on them. But why?

Well, it's the bananas. They give off a gas called ethylene, which contains ethene. This is a ripening hormone for fruit. While many fruit have ethene in them, and they ripen at a slower speed, bananas like to speed up that process - imagine a child that's eaten a lot of sugary products and is bouncing around the room like a loon - by releasing this gas. Great for the bananas, as they will go black and become sweeter, but a disaster for the other fruit, which will just end up going over-ripe and ruin in the bowl.

The best thing you can do with the bananas? You can buy fruit bowls with a hook on them. Place the bananas on this hook, and the ethene will dissipate upwards away from the other fruit (well, mostly), preventing them from over-ripening.

Ethylene/ethene is also used in producing polyethylene, which is a type of plastic made from oil. It is used to make clear plastics for washing up bottles, liquid soap bottles, plastic bags and more. Being the most common plastic used, you will notice on a lot of products you buy that are made from it have the PE sign imprinted on the bottom or on the bottles.