Auxin vs Gibberellin
Auxin and gibberellin are two classes of growth regulators/hormones found basically in plants and we can identify some similarities as well as differences between them. Plant growth regulators are mainly responsible for the growth and differentiation of cells, tissues and act as chemical messengers in intercellular communication. Apart from auxins and gibberellins, cytokinins, abscisic acid (ABA), and ethylene are also considered as major plant growth regulators.
What is Auxin?
Auxin is the first group of plant hormones that was discovered by scientists in 1926. Auxin is mainly present in the form of indole acetic acid (IAA) in plants. However, there are other chemical compounds also found in plants that exhibit functions similar to auxins. One of the major functions of the IAA is to stimulate cell elongation of the young shoots. Primary sites of auxin synthesis are shot apical meristems and young leaves. It has been found that developing seeds and fruits also consist of high levels of auxin. It is transported apoplastically through parenchyma cells and translocate through tracheary elements of xylem and sieve elements of phloem. Transportation is known to be unidirectional (polar/basipetal transport) and always occurs from tip to base.
Major functions of the auxins, in brief , are as follows;
• At low concentrations (10-8– 10-4M) auxin travels from shoot apex to cell elongation region and stimulate the stem elongation.
• Enhance apical dominance.
• Initiation of formation of lateral and adventitious roots.
• Fruit development regulation.
• Functions in phototropism (movements according to the light) and gravitropism (movements according to the gravity).
• Promotes vascular differentiation by increasing cambial activity during secondary growth.
• Retards leaf and fruit abscission.
Apart from the 2, 4-dichlorophenoxyacetic acid (2, 4-D), synthetic auxin is commercially used as an herbicide.
Lack of the plant hormone auxin can cause abnormal growth (right)
What is Gibberellin (GA)?
Gibberellins are a group of plant hormones that promotes plant growth mainly through cell elongation. Gibberellins are primarily produced at meristems of apical buds and roots, young leaves, and developing seeds. Translocation of gibberellin is acropetal i.e. base to top.
Some of the major functions of the gibberellins are as follows;
• Gibberellins stimulate cell elongation together with auxins and promote elongation of internodes.
• Increases the fruit size. E.g. seedless grapes.
• Break seed and bud dormancy.
• Enhance the growth of cereal seedlings by stimulating the digestive enzymes such as α-amylase that mobilized stored nutrients.
• Modification of flower sex expression and transition from juvenile to the adult phase.
• Effect on pollen development and pollen tube growth.
Effect of gibberellic acid on cannabis sprout
What is the difference between Auxin and Gibberellin?
There are some similarities as well as differences between these two plant growth regulators.
• Auxins have side chain in its chemical structure while gibberellins do not have side chains.
• Auxins are found only in higher plants while gibberellins are found in some fungi as well. E.g. Gibberella fujikuroi.
• Auxin transportation is basipetal while gibberellin transportation is acropetal.
• Auxin does not promote cell division, but gibberellin promotes the cell division.
• Auxin enhances apical dominance while gibberellin does not effect on apical dominance.
• Auxin does not elongate cells of genetically dwarf plants while gibberellins increase internode elongation of genetically dwarf plants.
• Auxin has no role in breaking seed dormancy, but gibberellins help in breaking dormancy of buds and seeds.
• Both Auxins and gibberellins enhance cell elongation.
As a conclusion, it is clear that auxin and gibberellins together involve in primary growth of the plant and at the same time both are involved in functions specified to each group of hormones.
Images Courtesy:
- Auxin by Pixie (CC BY 2.5)
- Gibberellin effects via Wikicommons (Public Domain)
- Tryptamine structure and Gibberellic acid via Wikicommons (Public Domain)
