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, including our GoldBio article about how to optimize plant PGRs which goes into detail about important relationships to look out for when adjusting concentrations.


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 in vitro.


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.

Table of plant growth regulators and their role in different plants


What do plant growth regulators do in vitro?

For the most part, added plant hormones into an artificial (in vitro) environment promotes similar behavior as seen in nature. However, when researchers explored this several years ago, they discovered some differences.

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.

list of plant growth regulators and what they do in plants in in vivo research




Comparing in vivo and in vitro plant growth regulators actions

Table of what Auxins do in vivo and in vitro




Table of what cytokinins do in vitro and in vivo



list of what gibberellins do in vitro and in vivo



List of what abscisic acid does in vitro and in vivo plants



List of what Ehtylene does in plants in vitro and in vivo



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.




References

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Chen, X., Qu, Y., Sheng, L., Liu, J., Huang, H., & Xu, L. (2014). A simple method suitable to study de novo root organogenesis. Front Plant Sci, 5, 208. https://doi.org/10.3389/fpls.2014.00208

Gaspar, T., Kevers, C., Penel, C., Creppin, H., Reid, D., & Thorpe, T. (1996). Plant hormones and plant growth regulators in plant tissue culture. In Vitro Cell. Dev. Biol.--Plan, 32, 272-289.

Jimenez, V. (2001). Regulation of in vitro somatic embryogenesis with emphasis on the role of endogenous hormones. R. Bras. Fisiol. Veg., , 13, 196-223.

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Khan, N., Ahmed, M., Hafiz, I., Abbasi, N., Ejaz, S., & Anjum, M. (2015). Optimizing the concentrations of plant growth regulators for in vitro shoot cultures, callus induction and shoot regeneration from calluses of grapes. J. Int. Sci. Vigne Vin, 49, 37-45.

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Niveshika, J., Aiswarya, A., & Yarmichon, A. (2020). Plant growth regulators used for in vitro micropropagation of Orchids: A research review. International Journal of Biological Research, 8(1), 37-42.

Sharma, H. (2017). Role of Growth Regulators in Micropropagation of Woody Plants-a Review. International Journal of Advanced Research, 5(2), 2378-2385. https://doi.org/10.21474/ijar01/3421

Yang, X., Yang, X., Guo, T., Gao, K., Zhao, T., Chen, Z., & An, X. (2018). High-Efficiency Somatic Embryogenesis from Seedlings of Koelreuteria paniculata Laxm. Forests, 9(12). https://doi.org/10.3390/f9120769.