Understanding Phytohormones

Plant hormones, called phytohormones, are chemical compounds that circulate through the plant, signaling and regulating its life cycle from seedling to maturity.

In animals, hormones are “chemical messengers” produced by the endocrine glands. While plants do not have hormone-producing glands, phytohormones influence similar physiological processes as mammalian hormones, including growth, reproduction and even senescence.[1]

In this blog post, we’ll discuss the importance of phytohormones, the specific types of phytohormones and the benefits of their exogenous application from natural sources.

Why phytohormones matter

Phytohormones are derived from secondary metabolites[2] and synthesized in low concentrations[3] throughout the plant, including the meristem, leaves, roots and shoots.[4] They can act locally at the site of production or travel through the vascular system to other tissues, coordinating an integrated response across the plant.[5] A single phytohormone can regulate multiple processes, while some processes may involve several phytohormones.[6]

Phytohormones influence physiological responses such as stem growth, cell division, flowering and fruit ripening.[7] Unlike enzymes, which are proteins that catalyze specific chemical reactions, phytohormones serve as messengers that signal various biological processes to happen. Enzymes accelerate chemical reactions; hormones trigger them.

One of their most important roles is responding to biotic and abiotic stressors. Since plants cannot escape threats, phytohormones coordinate stress tolerance[8] through crosstalk, a process in which they interact synergistically, antagonistically or additively. This allows plants to mount complex responses tailored to specific stressors, which would not be achievable by a single phytohormone. For instance, leaves produce metabolites and exchange gases that influence phytohormonal crosstalk, with younger and older leaves responding differently to stress.[9]

Phytohormones are needed only at particular stages in the crop life cycle. When no longer required, they disengage. Plants can even break them down chemically.[10]

Types of phytohormones

The seven main types of phytohormones are auxins, abscisic acid, cytokinins, ethylene, gibberellins, brassinosteroids and jasmonates. In addition, strigolactones have been recently discovered. See the table below.

Phytohormone

Synthesis and transport

Roles in plant physiology

Auxins (e.g., indole-3-acetic acid (IAA))

Produced primarily in the apical meristem of growing roots, stems, young leaves and buds. Transported from cell to cell, but transport to the roots likely involves the phloem.

  • Stimulate cell division.
  • Elongate plant stems.
  • Differentiate vascular tissue.
  • Delay fruit ripening and leaf senescence.
  • Enhance apical dominance.
  • Induce fruit setting in some fruits.
  • Promote flowering and growth of flower parts.

Cytokinins

Synthesized in the root tips and developing seeds. Transported from the roots to the shoots through the xylem.

  • Stimulate cell division.
  • Elongate plant stems.
  • Differentiate vascular tissue.
  • Delay fruit ripening and leaf senescence.
  • Enhance apical dominance.
  • Induce fruit setting in some fruits.
  • Promote flowering and growth of flower parts.

Abscisic acid (ABA)

Synthesized in most plant cells, with the highest levels in roots, mature leaves and seeds. Transported in the xylem from the roots and in the phloem from the leaves.

  • Close stomata during drought stress.
  • Inhibit shoot growth, which may be a response to drought stress.
  • Promote storage protein synthesis in seeds.

Gibberellins

Synthesized in the meristem of apical bud and young shoot tissues. Transport is limited, but some gibberellins are likely transported in the xylem and phloem.

  • Stimulate stem elongation.
  • Induce bolting in long-day plants.
  • Promote seed germination.
  • Create male flowers in dioecious plants.

Brassinosteroids

Synthesized in all plant tissues and act locally near the site of synthesis. Transport is limited through the xylem or phloem.

  • Stimulate cell division and elongation, especially in stems.
  • Promote fertility.
  • Inhibit root growth and development.
  • Develop vascular tissue.

Jasmonates

Present in all tissues.

  • Deter insect feeding by synthesizing proteinase inhibitors.
  • Inhibit seed germination and growth.
  • Ripen fruits.
  • Form tubers.
  • Promote senescence and abscission.
  • Form pigments.

Table 1. Main phytohormones, their sites of synthesis, how they are transported and some of the roles they play in plant physiology. Source: Davies, Peter J. 2010. “The Plant Hormones: Their Nature, Occurrence, and Functions.” In Plant Hormones: Biosynthesis, Signal Transduction, Action! Springer Science + Business Media B.V.

Exogenous application of phytohormones

While plants produce their own phytohormones, they can also benefit from phytohormones sourced from other plants, such as kelp and alfalfa.

Research shows that exogenous[11] application of phytohormones can help plants survive and grow under stressors like pathogens or high salt concentrations.[12] For example, pre-treating cannabis seeds with gibberellins has been shown to enhance drought tolerance.[13]

The effects of phytohormones often depend on their interactions. For instance, auxins and cytokinins can work antagonistically, with research on rice showing that auxins inhibit tiller bud growth, while cytokinins promote it. A low auxin-to-cytokinin ratio also enhances shoot induction, while a high ratio promotes root growth.[14]

Another example of phytohormones working against each other came from a study on corn, which revealed that higher levels of the phytohormone zeatin riboside improved tillering, while higher levels of auxins, abscisic acid and gibberellins suppressed it and accelerated tiller death.[15] By contrast, exogenous application of zeatin promoted tiller bud growth in winter wheat.[16]

In cannabis, several phytohormones regulate trichome formation and help synthesize secondary metabolites, and exogenous applications may alter the plant’s chemical profile. One study found that foliar application of methyl jasmonate and salicylic acid at 0.1 millimolar increased the accumulation of cannabidiolic acid and Δ9-tetrahydrocannabinolic acid in leaves by nearly 60%, while methyl jasmonate alone raised CBDA levels in inflorescences by about 16% (Figure 1).[17]

Conclusion

Phytohormones are invaluable regulators of plant growth, development and stress adaptation. While plants naturally produce phytohormones, exogenous application can provide additional benefits for plant health and productivity. However, this practice requires diligent research, as some phytohormones may have counterproductive effects. For cannabis growers, research suggests that phytohormone application can enhance secondary metabolite production, increasing the crop’s economic value.

Figure 1. CBDA and THCA produced by phytohormone treatments in cannabis flowers and leaves. Abbreviations: salicylic acid (SA), methyl jasmonate (MeJA), gamma-aminobutyric acid (GABA). Figure source: See footnote 17.

Emerald Harvest Team

[1] ScienceDirect. “Phytohormone.” Accessed December 5, 2024. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/phytohormone.

[2] Ibid.

[3] Davies, Peter J. 2010. “The Plant Hormones: Their Nature, Occurrence, and Functions.” In Plant Hormones: Biosynthesis, signal transduction, action!. Springer Science+Business Media B.V.

[4] Bajguz, Andrzej, and Alicja Piotrowska-Niczyporuk. 2023. “Biosynthetic Pathways of Hormones in Plants.” Metabolites 13 (8): 884. https://doi.org/10.3390/metabo13080884.

[5] Davies, Peter J. 2010. “The Plant Hormones: Their Nature, Occurrence, and Functions.” In Plant Hormones: Biosynthesis, signal transduction, action!. Springer Science+Business Media B.V.

[6] ScienceDirect. “Phytohormone.” Accessed December 5, 2024. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/phytohormone.

[7] Davies, Peter J. 2010. “The Plant Hormones: Their Nature, Occurrence, and Functions.” In Plant Hormones: Biosynthesis, signal transduction, action!. Springer Science+Business Media B.V.

[8] ScienceDirect. “Phytohormone.” Accessed December 5, 2024. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/phytohormone.

[9] Aerts, Niels, Marciel Pereira Mendes, Saskia C.M. Van Wees. 2020. “Multiple levels of crosstalk in hormone networks regulating plant defense.” The Plant Journal 105 (2): 489-504. https://doi.org/10.1111/tpj.15124.

[10] Wahab, Abdul, Gholamreza Abdi, Muhammad Hamzah Saleem, et. al. 2022. “Plants’ Physio-Biochemical and Phyto-Hormonal Responses to Alleviate the Adverse Effects of Drought Stress: A Comprehensive Review.” Plants 11 (13): 1620. https://doi.org/10.3390/plants11131620.

[11] https://www.merriam-webster.com/dictionary/exogenous.

[12] Policarpo Tonelli, Fernanada Maria, Flavia Cristina Policarpo Tonelli, and Moline Severino Lemos. 2023. “Chapter 20 – Exogenous application of phytohormones to increase plant performance under stress.” In Phytohormones and Stress Responsive Secondary Metabolites. Academic Press. https://doi.org/10.1016/C2021-0-00172-3.

[13] Du, Guanghui, Hanxue Zhang, Yang Yang, Yinhong Zhao, Kailei Tang, and Feihu Liu. 2022. “Effects of Gibberellin Pre-Treatment on Seed Germination and Seedling Physiology Characteristics in Industrial Hemp under Drought Stress Condition.” Life 12 (11): 1907. https://doi.org/10.3390/life12111907.

[14] Liu, Yang, Junxu Xu, Yanfeng Ding, Qiangsheng Wang, Ganghua Li, and Shaohua Wang. 2011. “Auxin inhibits the outgrowth of tiller buds in rice (Oryza sativa L.) by downregulating OsIPT expression and cytokinin biosynthesis in nodes.” Australian Journal of Crop Science. 5(2): 169-174. https://www.cropj.com/liu_5_2_2011_169_174.pdf.

[15] Ru-Fang, Wang, Ji-Wang Zhang, Peng Lu, Shu-Ting Dong, Peng Liu, Bin Zhao. 2012. “Effects of Endogenous Hormones on Tiller Development Process of Different Maize Varieties.” Scientia Agricultura Sinica 45 (5): 840-847. https://doi.org/10.3864/j.issn.0578-1752.2012.05.003.

[16] Can, Tie, Xiangping Meng, Xiaoli Liu, et. al. 2018. “Exogenous Hormonal Application Regulates the Occurrence of Wheat Tillers by Changing Endogenous Hormones.” Frontiers in Plant Science 9. https://doi.org/10.3389/fpls.2018.01886.

[17] Garrido, Jose, Saleta Rico, Carolina Corral, et. al. 2022. “Exogenous application of stress-related signaling molecules affect growth and cannabinoid accumulation in medical cannabis (Cannabis sativa L.).” Frontiers in Plant Science 13: https://doi.org/10.3389/fpls.2022.1082554.

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