24 Aug 2021

Auxin: The Rooting Hormone

Anjali Singh, MS

As a content and community manager, I leverage my expertise in plant biotechnology, passion for tissue culture, and writing skills to create compelling articles, simplifying intricate scientific concepts, and address your inquiries. As a dedicated science communicator, I strive to spark curiosity and foster a love for science in my audience.

Table of Contents

Introduction

Plant growth regulators are chemical substances that play a vital role in the growth and development of plants. They act as chemical messengers that facilitate cell to cell communication to induce the growth and differentiation of plant cells, tissues, and organs.

Some of the functions performed by these hormones include increasing branching, suppressing shoot growth, increasing return bloom, removing excess fruit, or altering fruit maturity. But these responses also depend on some factors, like how well the chemical is absorbed by the plant, tree vigor and age, dose, timing, cultivar, and weather conditions before, during, and after application.

The five main classes of plant growth regulators include: auxins, cytokinin, gibberellins, abscisic acids, and ethylene.

This article is dedicated to the auxins, their role in plants’ growth and development, and their use in the tissue culture processes.

What are Auxins?

Auxins are a growth-promoting substance that is involved in the shoot and root tips and promotes cell division, stem and root growth. Moreover, they also have the ability to affect plant orientation by promoting cell division to one side of the plant in response to sunlight and gravity.

Structurally and naturally, it’s an indole acetic acid (IAA) that possesses an indole ring and a carboxylic acid function. IAA is the most studied auxin that is naturally synthesized by plants for the development in natural conditions.

How was Auxin discovered?

Auxin (IAA) was isolated from maize by chemists during the 1930s. But, the journey of its discovery started in 1881 with Francis and Charles Darwin. They performed an experiment on coleoptiles that showed they bend towards the light source when illuminated with light from one direction. They hypothesized the presence of a mobile signal that promotes the elongation of grass coleoptiles.

Then later, Boyen-Jensen (1913), Paal, and Went (1928) used the same experiment system to show that the bending was promoted by a mobile signal that was hydrophilic in nature, and this signal was finally identified as IAA.

Functions of Auxins

Auxins participate in several developmental changes in the plants. The response also depends on environmental cues like unidirectional light or gravity force that cause uneven distribution of auxin leading to uneven responses in different parts.

Auxin governs the form and shape of the plant body, direction, and strength of growth of all organs, their mutual interaction, and stimulates the intake of water causing plants to bend by building turgor pressure.

A few more functions of auxins are given below:

Stimulating Shoot Elongation: Auxins stimulate gibberellins for shoot elongation that eventually increases plant length. The hormones increase the distance between nodes, spacing the branch points further apart.
Controlling Seedling Orientation: Cells grow more in the areas where auxins are concentrated, promoting cell division to one side of the plant. So, in seedlings, the Auxins will move downward due to gravity and laterally, away from light.
Stimulating Root Growth and Development: Auxins are involved in the growth of pre-existing roots, lateral root initiation or root branching, and adventitious root formation. But, if the source of the auxin is removed by cutting the shoot tip, the roots will be less stimulated and the growth of shoots will be supported.
Promoting Fruit Development: Auxins are required for fruit growth and development and delays fruit senescence. It’s also involved in the formation of fruits when the seeds are removed and fruit with unfertilized seeds (parthenocarpy).
Wound Response: Auxin is involved in the formation and organization of phloem and xylem. And, when the plant is injured, it induces cell differentiation and regeneration of vascular tissues.
Apical Dominance: Auxin induces shoot apical dominance. They stimulate the synthesis of ethylene in the axillary buds, causing the inhibition of their growth.

What is the Role of Auxin in Tissue Culture?

Under in vitro conditions, plants can’t synthesize hormones for their growth and development and that’s why exogenous hormone is required.

In horticulture, auxins, especially NAA and IBA, are commonly applied to stimulate root initiation when rooting plant cuttings. However, high concentrations of auxin inhibit root elongation and instead enhance adventitious root formation. When the root tip is removed, the formation of the secondary root is also inhibited.

In tissue culture conditions, auxins are used to induce root development. Sometimes, a high ratio of auxin to cytokinin is required for the proper organ development in some species.

Three auxins are widely used in tissue culture, based on the suited protocol. It includes 1-Naphthaleneacetic acid (NAA), Indoleacetic acid (IAA), and Indole-3-butyric acid (IBA) hormones.

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Happy Culturing!