Phytohormones are organic, signalling molecules produced in small amounts in plant tissues. They are responsible for regulating life processes like cell division and differentiation, growth and embryogenesis. They can function in the same tissues they were produced in but are not limited to them and can also travel through the plant.
There are multiple phytohormones that all play an important role in the homeostasis of plants, so here we will further explore some of them and their roles.
Auxins, gibberellins and cytokinins show a predominantly stimulatory effect, while abscisic acid and ethylene have an inhibitory effect.
Auxins were first discovered; they are growth hormones produced in apical parts of the tree and roots. They got their name from the Greek word auxein which means to grow.
Gibberellins are a group of plant hormones found in plants and fungi. The first discovered gibberellin was isolated from the fungus Gibberella.
Gibberellins are produced in chloroplasts and in young leaves of apical buds, meristematic tissues of stems and roots and in seeds, then they are transported through phloem. Their task is to stimulate growth from the seed and blossom. In low temperatures they are also responsible for inducing a state of rest.
Cytokinins are molecules with the organic nitrogen base adenine, responsible for cell cycle regulation and plant development. They are created in the roots from where they travel through the xylem to the leaves and fruits.
Abscisic acid (ABA), which is synthesized in the chloroplasts, regulates growth and development in the period when the plant goes from the active growth phase to the phase of rest. It affects the falling of leaves and fruits, prevents seed germination and plays a key role in water loss prevention.
Ethene, a gaseous hormone, is formed by the breakdown of the amino acid methionine, and because of its poor solubility in the aqueous environment of the cell, it quickly leaves the site of synthesis. It causes fruit ripening and inhibits the elongation of root and stem cells.
Salicylic acid was first isolated from the family Salicaceae, from which it got its name. Its primarily function is protecting the plan in case of extreme temperatures or temperature changes or water induced stress. It’s also responsible for defending the plant against parasitic pathogens. It is used for production of aspirin medication.
Tropism is a plant’s growth response to a specific environmental stimulus. It helps plants grow in ways that improve their chances of survival. The direction of growth depends on the direction of the stimulus. If a plant grows towards the stimulus, it is called a positive tropism and if it grows away from the stimulus, it is a negative tropism.
The most well-known type is phototropism, when plants grow towards light. Gravitropism or geotropism refers to roots growing downward and shoots growing upward in response to gravity. Thigmotropism is growth as a response to touch, like how wines wrap around objects. Hydrotropism is when roots grow toward water sources.
All of these responses are controlled by phytohormones, mostly by auxins. By helping cells grow longer on one side of the plant, auxins cause it to bend. In the roots auxins slow down their growth and make them bend downward.
Tropism shows that even though plants can’t move they can still react to their environment in chemically complex, but fascinating ways.
Written by Tamara Bračko