What Are plant Growth Regulators a Detailed Introduction to RGRs
Plant growth regulators are important compounds that impact aspects of plant growth such as root, shoot and embryo development. Plants naturally produce and use these; however, researchers often use plant growth regulators in culture media in order to guide events within plant regeneration.To perform plant tissue culture and regeneration successfully, you’ll need to have a decent overview about what plant growth regulators (PGRs) do, the types of PGRs available and how each of those PGRs interact with plant tissue.
In this article, I’ll give you a brief overview so that you have a good foundation when working with your plants. Be sure to check out some of our other articles about plant regeneration.
What are plant growth regulators?
Plant growth regulators are synthetic or natural compounds that affect developmental or metabolic processes in plant tissues cultured in vitro.
The term "phytohormones'' is commonly used for those compounds naturally produced by plants. Instead, synthetic hormone-like compounds are not considered phytohormones. In some articles, I've seen researchers using the term "PGRs" for the hormones used in vitro. For this article's case, I will use PGRs to refer to all natural and synthetic hormonal compounds added exogenously .
What is the importance of plant growth regulators?
Plant growth regulators are important because they modify and regulate plant growth by altering certain aspects of plant development such as fruit ripening, plant height, seed development, flowering, etc.
Plant growth regulators play a role in nature as well as in biotechnology.
In nature, plant growth regulators control every aspect of plant growth and morphology. Plant growth regulators also help plants face environmental changes and stressors.
In biotechnology, plant growth regulators can be added to improve the quality of plant fruits, plant survival, and propagate plants for breeding. Researchers can also use plant growth regulators to study plant regulation at the molecular level.
What do plant growth regulators do?
Despite being small molecules used in low dosages, plant growth regulators profoundly affect plant growth in vitro affecting cell division, elongation, and regeneration.
You may read about different plant growth regulators like auxins, cytokinins, gibberellins, abscisic acid, and ethylene. Among these PGRs, auxins and cytokinins, and the ratio between them, are relevant for organogenesis and somatic embryogenesis (SE) processes.
Natural PGRs are produced in the plants endogenously. They interplay with the added plant growth regulators (natural or synthetic) to stimulate in vitro development.
However, you must be aware that besides exerting a direct effect on plant cellular mechanisms, many exogenously applied regulators may modify the synthesis, destruction, activation, transport, or specificity of endogenous hormones or other PGRs.
What do plant growth regulators do in plants?
Let’s break down what each natural hormone does in vivo, or in nature, although typically they can have overlapping functions.
Auxins
Influence cell growth expansion and elongation
Stimulate root formation
Induce vascular differentiation
Promote tropic responses
Maintain the apical dominance (the main, central stem of the plant is dominant over other side stems)
Delay leaf senescence (which is when leaves are close to dying in the plant life cycle)
Induce the development of auxiliary buds, flowers, and fruits
Cytokinins
Affect mitosis (chromosomes are separated into two new nuclei) and cytokinesis (cytoplasm of a parental cell is split into two daughter cells)
Promote lateral bud growth and leaf expansion
Delay leaf senescence
Promote chlorophyll synthesis
Enhance chloroplast development
Gibberellins
Promote stem elongation
Induce flowering
Cone initiation (in conifers, the cones are the reproductive organs)
Promote seed germination
Abscisic acid
Regulate seed germination
Induce storage protein synthesis
Modulating water stress
Maintains bud and seed dormancy (seed remains asleep or inactivated)
Slows cell elongation
Regulate the closing of stomatal apertures (reducing transpiration)
Modulate leaf abscission and senescence
Play a role in seed maturation
Ethylene
Promotes the development of root and shoots
In conjunction with other phytohormones, this gas promotes fruit ripening, senescence, and leaf abscission.
What are the 5 plant growth regulators and what are their functions?
The five primary plant growth regulators are auxins, cytokinins, gibberellins, abscisic acid and ethylene. Each plant growth regulator functions in different ways. For instance, Auxins induce calli. Cytokinins stimulate cell division. Gibberellins induce organogenesis especially in adventitious roots. Abscisic acid favors maturation and germination. And ethylene is the main regulator for fruit ripening.
Below are the common PGR actions when added in vitro.
Auxins
Induce callus (mass of undifferentiated cells) from explants (the portion of plant tissue used in culture techniques)
Favors root and shoot morphogenesis (biological process that causes a cell, tissue or organism to develop its shape)
Are more effective combined with cytokinins
Cytokinins
Stimulate cell division
Release of lateral bud dormancy (inactivated bud)
Induce adventitious bud formation
Often inhibit embryogenesis and root induction
Gibberellins
Induce organogenesis, particularly adventitious roots
In some cases, inhibit shoot, root, and embryo formation
Abscisic acid
Favors maturation and germination of somatic embryogenesis
At high concentrations, inhibits callus growth and organogenesis (buds, roots, embryos)
Favors the maturation and normal growth of somatic embryos
Increases freezing tolerance of grown plants and cell cultures
Ethylene
It is less frequently used as it is naturally produced in all plant cultures
When it is used, it promotes the maturation of tissues
Depending upon the time after subculture, ethylene can stimulate or
inhibit growth and organogenesis
Affects growth of callus and suspension cultures, stem and root elongation, axillary and adventitious bud formation, rooting, and embryogenesis.
Auxin In Vivo vs. In Vitro Functions
The table below shows the different functions of auxins within in vivo plants versus its role in vitro.
Cytokinins In Vivo vs. In Vitro Functions
In vivo | In vitro · Induce callus · Favors root and shoot morphogenesis · Effective combined with cytokinins |
· Influence cell growth · Stimulate root formation · Induce vascular differentiation · Promote tropic responses · Maintain the apical dominance · Induce the auxilliary buds, flowers and fruits |
· Affect mitosis · Promote lateral bud growth · Delay leaf senescence · Promote chlorophyll synthesis · Enhance chloroplast development · Promote leaf expansion | · Stimulate cell division · Release lateral bud dormancy · Induce adventitious bud formation · Can inhibit embryogenesis · Can inhibit root formation |
The table below shows the different functions of gibberellins within in vivo plants versus its role in vitro.
In vivo | In vitro |
· Promote stem elongation · Induce flowering · Cone initiation · Promote seed germination | · Induce adventitious roots · Can inhibit shoot formation · Can inhibi root formation · Can inhibit embryo formation |
The table below shows the different functions of Abscisic acid within in vivo plants versus its role in vitro.
In Vivo · Regulates seed germination · Induces storage protein synthesis · Modulates water stress · Maintains bud and seed dormancy · Slows cell elongation · Modulates leaf abscission and senescence | In Vitro · Favors maturation of somatic embryos · Favors germination of somatic embryos · Increases freezing tolerance |
The table below shows the different functions of Ethylene within in vivo plants versus its role in vitro.
In vivo | In vitro |
· Promotes the development of roots and shoots · Promotes fruit ripening · Promotes fruit senescence · Promotes leaf abscission | · Is less frequently used · Promotes tissue maturation · Affects growth of callus · Affects stem elongation · Affects root elongation · Affects bud formation |
Plant growth regulators commonly used in vitro
Auxins
Plants produce natural types of auxin like Indole butyric acid (IBA) and Indole-3-acetic acid (IAA). These can also be used in vitro. Commonly used synthetic auxins in tissue culture are 2,4-dichlorophenoxyacetic acid (2,4-D), 1-naphthaleneacetic acid (NAA), dicamba ((3,6-dichloro-2-methoxybenzoic acid) and picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid).
Cytokinins
The most commonly used natural cytokinins in plant tissue culture are zeatin, 2-iP 6-(γ,γ-Dimethylallylamino)purine and zeatin riboside. Among synthetic cytokinins are kinetin, benzylaminopurine (BA) and thidiazuron (TDZ).
Gibberellins
There are over 80 different gibberellin compounds in plants, but only gibberellic acid (GA3) and GA4+7 are often used in plant tissue culture.
Abscisic acid
In nature, the most common form of ABA is (S)-(+)-abscisic acid. This compound is often called the cis isomer or simply ABA. The trans isomer has a slight difference in the chemical configuration concerning cis isomer (a carboxyl group is in another direction in the molecule). Then, in vitro, the commercially sold ABA is a 1:1 mixture of the cis and trans-ABA optical isomers.
Ethylene
Ethephon (Ethrel; 2-chloroethylphosphonic acid; or 2-CEPA) can be used as an ethylene-releasing chemical in tissue cultures.
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