Have you ever thought about how it is even possible that a plant grows in the direction of light, or how does a seed know it is time to germinate? What makes the fruits in the orchard ripen? Why do the leaves fall from the tree? Furthermore, finally, how to get rid of weeds from your garden? It is all thanks to plant hormones. Here, we will introduce you to the worlds of these clever substances that regulate all the plants’ responses.
Plants are fascinating creatures. Their source of life is energy conversion of light energy into chemical energy. This process is called photosynthesis. During that process, plants produce sugars and carbohydrates. How does it work? Green leaves work like tiny roofs or solar panels that get energy from the light source (natural sunlight or artificial light from LED, light bulb). All this is possible thanks to the green pigment inside them – chlorophyll. Nevertheless, the growth and development of plants are controlled not only by light but also by chemical substances contained within them, called plant hormones or phytohormones. We may distinguish six main groups such as auxins, gibberellins, cytokinins, ethylene, growth inhibitors, including abscisic acid. Sounds complicated? Let us think about what a plant needs, just different chemicals?
Plants may appear to be simple organisms, but they can precisely control all the processes that take place in them. It is not a coincidence that a flower in your pot grows towards the window. Thanks to the particular receptors, they can detect light sources and their direction. In the apex of the shoot, there are plant hormones called auxins. When the light reaches the plant from both sides, auxins are evenly distributed, while when one side is more shaded than the other, they travel there and cause the cells to elongate on that side. Flowers planted at home nearby the window behave the same way, bending directly to the light. Auxins also trigger the formation of lateral roots and vascular differentiation.
Other plant hormones called gibberellins are connected with the seed. When gibberellins are released and water is supplied to the seed, it starts to germinate. Their role does not finish within the sprouting of a seedling. They promote cell elongation and division, so the seedling grows taller and eventually forms a tall plant. Gibberellins can also trigger the flowering and development of buds and some fruits. Fruits developed in a way stimulated by these chemical compounds, such as grapes, bananas, or cucumbers, are seedless. Gibberellins occur naturally but may also be synthetically produced, so their equivalents are commonly used in agriculture and horticulture to increase yields.
Plants are fascinating creatures. Their source of life is energy conversion of light energy into chemical energy. This process is called photosynthesis. During that process, plants produce sugars and carbohydrates. How does it work? Green leaves work like tiny roofs or solar panels that get energy from the light source (natural sunlight or artificial light from LED, light bulb). All this is possible thanks to the green pigment inside them – chlorophyll. Nevertheless, the growth and development of plants are controlled not only by light but also by chemical substances contained within them, called plant hormones or phytohormones. We may distinguish six main groups such as auxins, gibberellins, cytokinins, ethylene, growth inhibitors, including abscisic acid. Sounds complicated? Let us think about what a plant needs, just different chemicals?
Plants may appear to be simple organisms, but they can precisely control all the processes that take place in them. It is not a coincidence that a flower in your pot grows towards the window. Thanks to the particular receptors, they can detect light sources and their direction. In the apex of the shoot, there are plant hormones called auxins. When the light reaches the plant from both sides, auxins are evenly distributed, while when one side is more shaded than the other, they travel there and cause the cells to elongate on that side. Flowers planted at home nearby the window behave the same way, bending directly to the light. Auxins also trigger the formation of lateral roots and vascular differentiation.
Other plant hormones called gibberellins are connected with the seed. When gibberellins are released and water is supplied to the seed, it starts to germinate. Their role does not finish within the sprouting of a seedling. They promote cell elongation and division, so the seedling grows taller and eventually forms a tall plant. Gibberellins can also trigger the flowering and development of buds and some fruits. Fruits developed in a way stimulated by these chemical compounds, such as grapes, bananas, or cucumbers, are seedless. Gibberellins occur naturally but may also be synthetically produced, so their equivalents are commonly used in agriculture and horticulture to increase yields.
Probably each of us has heard about youth hormones. Humans have them, but they are also found in plants, and they are called cytokinins [1]. It is well known that increased production of the green pigment – chlorophyll makes the leaves greener. In turn, cytokinins delay the process of aging and stimulate cell division. Such an illusion of eternal youth has also affected the world of plants. Gardeners also take advantage of the miraculous properties of this hormone. They treat their colorful flowers with sprays containing cytokinins to increase their size and make sure the plants do not wither during the transfer to the shop.
That is not the end of the list of chemical compounds that stimulate the growth or maturing of plants. If you place unripe avocado in a paper bag with a ripening apple, after a few days you will see the avocado ripen as well. It is the best way to observe the activity of ethylene, the only gaseous compound that supports maturing of fruits and vegetables. The already mentioned apple has a high concentration of ethylene that spreads to the avocado and makes it softer, less green. Ethylene is responsible for abscission, a condition when a plant drops its leaves and fruits. It is produced in plenty by a plant in some stress conditions such as drought, flood, physical damage, infection, low or high temperature. The presence of auxins enhances its production, whereas ethylene inhibits the production of auxins [2].
Some compounds work as growth inhibitors. Meristems, the tissue that makes up the plants, provide the possibility of indeterminate growth. However, it does not happen due to the growth inhibitors. The most well-known growth inhibitor is abscisic acid (ABA). It is produced during the autumn when days become shorter. It plays a vital role in the survival of the plants in the winter because it brings them into the rest period, also known as the dormant period. The plants flower in the spring when the growth inhibitors decompose. Sometimes a plant wants to reduce the evaporation of water from the surface of the leaf. That is where the ABA comes into play. It is produced in the roots when the plant experiences drought. Contrary to gibberellins, it can inhibit seed germination [3]. It is worth mentioning that there is a way to kill weeds in your garden with a bit of help from growth inhibitors [4].
Summary
Hormones are chemical substances, which regulate the many biological processes in organisms, including animals, people, and even plants. The main plant hormones called phytohormones include auxins, gibberellins, cytokinins, ethylene, and growth inhibitors. They control everything from plant germination to the ripening of vegetables and fruits. They even decide when they fall off the trees. Each plant is different; knowing the main plant hormones is just the tip of the iceberg to discovering the secrets of the fascinating world of plants.
This article is a joint work of Julia Ławińska (Faculty of Chemistry, University of Warsaw), Agnieszka Pregowska (Institute of Fundamental Technological Research, Polish Academy of Sciences), and Magdalena Osial (Faculty of Chemistry, University of Warsaw) as a part of the Science Embassy project. Image credit – Agnieszka Pregowska
References
[1] Osugi, A., Sakakibara, H. Q&A: How do plants respond to cytokinins and what is their importance? BMC Biol. 13, 102 (2015). DOI: 10.1186/s12915-015-0214-5.
[2] Daszkowska-Golec, A., Szarejko, I. Open or Close the Gate – Stomata Action Under the Control of Phytohormones in Drought Stress Conditions. Frontiers in Plant Science 4, 138 (2013). DOI: 10.3389/fpls.2013.00138
[3] Pandolfini, T. Seedless fruit production by hormonal regulation of fruit set. Nutrients. 2009;1(2):168-177. DOI:10.3390/nu1020168
[4] Paque, S., Weijers, D. Q&A: Auxin: the plant molecule that influences almost anything. BMC Biol, 14, 67 (2016). DOI: 10.1186/s12915-016-0291-0