Plant Hormones: Shaping Growth, Development and Adaptation to Stress

Plants, with their truly remarkable ability to adapt and thrive in a myriad of diverse and challenging environments, owe a substantial portion of their growth, development, and multifaceted responses to a complex and intricate system of chemical messengers, referred to as plant hormones or phytohormones. These minuscule yet influential organic molecules play an unequivocally vital role in precisely regulating and orchestrating a multitude of physiological processes, thereby empowering plants to not only flourish and acclimate to their ever-changing surroundings but also to react and adjust to a diverse array of signals emanating from the environment. In this article, a comprehensive journey through the intriguing, complex, and multifarious realm of plant hormones will be taken, thus profoundly illuminating their boundless and diverse functional repertoire, while meticulously dissecting and deciphering their profound and far-reaching impact on the existence and life cycle of plants.

The Diversity of Plant Hormones and Their Function

Plant hormones exert control and regulation over the multifaceted tapestry of plant growth, development, and responsive behaviors, by intricately modulating and steering a myriad of cellular processes. Each of the different types of hormones assumes a unique and indispensable role, orchestrating and sculpting variegated facets of the botanical existence, commencing from the pivotal moment of seed germination and persevering through the remarkable journey culminating in ripe fruits and their fruition. Not only that, but they are also a key aspect for the plants to adapt to the environment in order to survive against different conditions.

By the mid-20th century, abscisic acid (ABA), auxin, cytokinin, ethylene, and gibberellins emerged as the primary plant hormones. In recent years, the list has expanded to include brassinosteroids, jasmonates, salicylic acids, and strigolactones (Santner et al., 2009) [Figure 1]. This article will be centered on the five types of hormones that were first studied.

Figure 1: Different types of plant hormones (Santner, 2009).

Auxins are some of the most well-known classes of plant hormones, as they play a pivotal role in the promotion of cell growth and expansion within plants and were the first to be discovered. Auxins are present in all parts of the plants at different concentrations, creating concentration patterns, which are crucial for plant growth and development. These hormones not only contribute to the promotion of cell growth but also to other different development mechanisms, such as phototropism, which is the bending of plants towards light and gravitropism, which is the response to gravity (Figure 2) (Perrot-Rechenmann & Napier, 2005).

Another hormone family having similar effects as auxins is gibberellins. However, despite their functional similarities, they are an entirely separate hormone group with distinct properties. These hormones influence various developmental processes, including the regulation of plant height, fruit growth, and the breaking of dormancy in seeds (Davière & Achard, 2013) [Figure 2].

Figure 2: Table of a summary of the main hormone classes and their activities (BioNinja, 2023).

The cytokinin hormone family is vital for cell division and differentiation. They are involved in controlling shoot and root development, as well as the regulation of nutrient transport. Cytokinins also play a role in delaying the senescence (aging) of leaves and other organs (Li et al., 2021) [Figure 2]. They are normally complementary to auxin, as they generally have opposite effects.

Ethylene is a gaseous hormone that influences various aspects of plant growth and development, including fruit ripening, senescence, and responses to mechanical stress. It also plays a role in root development and the formation of adventitious roots (Stepanova & Alonso, 2005) [Figure 2].

Finally, ABA is known as the "stress hormone" because it is involved in plant responses to environmental stressors such as drought, cold, and salinity. It promotes seed dormancy and inhibits growth during unfavorable conditions. ABA also helps regulate stomatal closure to prevent water loss from leaves; stomata are little pores in the epidermis that regulate plant transpiration and gas exchange (Nakashima & Yamaguchi-Shinozaki, 2013) [Figure 2].

Plant Hormones in Action: Growth and Development

Plant hormones do not function in an isolated way—they often interact and communicate with each other, resulting in complex networks of regulatory pathways (Figure 3). Hormonal crosstalk allows plants to fine-tune their responses to changing environmental conditions and developmental cues (Jaillais & Chory, 2010). For example, the balance between auxins and cytokinins is critical for determining whether a plant produces roots or shoots (Kurepa & Smalle, 2022). These interactions ensure that plants can respond effectively to a wide range of stimuli, such as light, gravity, nutrients, and stress. The intricate web of hormonal signaling enables plants to adapt their growth patterns and physiological responses to maximize their chances of survival and successful reproduction.

The intricate dance of plant hormones orchestrates various aspects of growth and development, from seed germination to flowering and fruit development (Santner et al., 2009) [Figure 3].

Figure 3: Plant hormones structures and functional interactions (Jaillais & Chory, 2010).

Seed germination is a pivotal event that marks the beginning of a plant's journey that is controlled by the interplay between two key hormones: ABA and gibberellins. ABA maintains seed dormancy, ensuring that germination occurs only under favorable conditions. This is achieved by inhibiting the synthesis of enzymes that break down stored starches, keeping the seed in a quiescent state. When conditions become conducive to growth, the balance shifts, and then gibberellins take the stage, triggering a cascade of events. These hormones stimulate the production of enzymes that break down starches into sugars, providing the energy needed for the embryonic plant to emerge from the seed coat. This energy powers cell division, elongation, and the expansion of young shoots, propelling the seedlings upwards and toward the light (Penfield, 2017).

The transition from vegetative growth to flowering is also worth noting. Plant hormones are key players in coordinating this intricate dance of growth and differentiation. Gibberellins and cytokinins stimulate cell division, elongation, and the development of specialized tissues, ensuring that each floral component is formed with precision. In addition, when fertilization happens, the ovary of the flower begins its transformation into a fruit (Izawa, 2021). The pinnacle of fruit development is the enchanting process of ripening. This transformative stage marks the culmination of complex hormonal interactions and biochemical changes that result in the fruit's vibrant colors, enticing aromas, and delectable flavors. Ethylene, often referred to as the "ripening hormone" takes center stage during this phase. As the fruit reaches maturity, it releases ethylene, which triggers a series of enzymatic reactions. These reactions lead to the breakdown of cell walls, softening the fruit's texture, and the conversion of starches into sugars, enhancing its sweetness. Simultaneously, pigments responsible for color development become more prominent, enhancing the color of the fruits (Liu et al., 2015).

Plant Hormones in Action: Response to Stress

Plants encounter a myriad of environmental challenges, both abiotic (not derived from living organisms), such as drought and salinity, and biotic (derived from living organisms), such as herbivore attacks and pathogen infections (Suzuki et al., 2014) [Figure 4]. Plant hormones act as messengers that convey stress signals and orchestrate appropriate responses.

Figure 4: Plants are often exposed to various types of biotic and abiotic stresses (Singh, 2021).

For example, in response to drought, abscisic acid (ABA) has a key role. It orchestrates the closure of stomata (Lehman & Or, 2015). By closing stomata, plants minimize water loss through transpiration, a vital adaptation that conserves their limited water supply. Furthermore, ABA triggers a suite of molecular responses that enable plants to acclimate to arid conditions. Genes encoding proteins involved in water retention and drought tolerance are activated, ensuring the plant's endurance in the face of water scarcity (Lim et al., 2015).

In addition, the attack from pathogens such as bacteria, fungi, or viruses activates the plant's defense mechanisms. In these cases the hormone salicylic acid (SA) emerges as a key hormone for the plant's immune response, which stimulates the expression of genes that encode antimicrobial proteins, fortifying the plant's defenses against invaders (Ding & Ding, 2020). SA also orchestrates the production of reactive oxygen species (ROS), which act as molecular weaponry against pathogens (Saleem et al., 2021).

Applications in Agriculture

The discovery and understanding of plant hormones have profound implications for agriculture, where they have transformed the landscape of crop production and food security. Using different varieties of the same plant or crop with more or less production of some of the hormones mentioned before has led to precise crop production with different traits that are beneficial for enhancing crop yield, quality, and resilience (Figure 5).

Figure 5: Using different plant varieties with modified hormone concentrations can lead to better crop production (Nield, 2016).

For example, the role of gibberellins in promoting stem elongation has led to the development of dwarf and semi-dwarf varieties of cereal crops, such as rice and wheat. These varieties, with reduced gibberellin response, are characterized by shorter stature and stronger stems, making them less prone to lodging—a phenomenon where tall plants collapse due to their own weight (Plaza-Wüthrich, 2016). In another case, manipulating ABA levels could yield drought-tolerant plants, offering a lifeline to regions where water scarcity threatens agricultural productivity (Zhou, 2019).

In contrast with the example mentioned before, where hormone pathways were modified to create plants producing more or less of a specific hormone, the hormones themselves can be also used externally to have an effect. Cytokinins have played a pivotal role in tissue culture techniques, which allow the rapid development of plants from a small piece of plant tissue. This technology has immense implications for conserving endangered species, producing disease-free plants, and rapidly propagating high-value crops (Hill & Schaller, 2013). Ethylene, often seen as both a blessing and a challenge in agriculture, has been harnessed to influence fruit ripening. By controlling ethylene levels in storage environments, farmers can ensure that fruits mature uniformly and can be delivered to consumers at optimal ripeness (Qi, 2021). In addition, by harnessing auxins, farmers can enhance root development in cuttings, enabling the propagation of new plants from fragments. This technique has enabled the cloning of plants with desirable traits, preserving their genetic makeup across generations (Figure 6) (Guan, 2019).

Figure 6: Auxin stimulates rooting. On the left plants not treated with auxin, on the right plants trated with auxin (Tropical Trees: Propagation and Planting Manuals).

Conclusion

Plant hormones have a very important impact on the remarkable diversity of forms and functions seen in the plant kingdom. These chemical messengers orchestrate growth, development, and responses to environmental challenges that allow plants to survive during their lifetime. The interactions between different classes of hormones create a finely tuned symphony that guides plants through their life cycles. As the understanding of plant hormones continues to deepen, the scientific community into how these organisms adapt and thrive in diverse environments. From germination to senescence, from bending towards light to withstanding drought, plant hormones are the choreographers of plants' dance. By unravelling their secrets, new avenues are unlocked by modifying the hormone pathways or using the hormones themselves to improve agriculture, conservation, and the future well-being of our planet.

Bibliographical References

Davière, J.-M., and Achard, P. (2013). Gibberellin signaling in plants. Development 140 (6): 1147–1151. https://doi.org/10.1242/dev.087650 Ding, P., and Ding, Y. (2020). Stories of salicylic acid: A plant defense hormone. Trends in Plant Science. 25 (6): 549–565. https://doi.org/10.1016/j.tplants.2020.01.004

Guan, L., Tayengwa, R., Cheng, Z.M., Peer, W.A., Murphy, A.S., and Zhao, M. (2019). Auxin regulates adventitious root formation in tomato cuttings. BMC Plant Biology. 19, 435. https://doi.org/10.1038/s41598-019-39013-8

Hill, K., and Schaller, G.E. (2013). Enhancing plant regeneration in tissue culture: a molecular approach through manipulation of cytokinin sensitivity. Plant Signaling & Behaviour. 8(10): e25709. https://doi.org/10.4161/psb.25709

Izawa, T. (2021). What is going on with the hormonal control of flowering in plants? Plant Journal. 105, 431–445. https://doi.org/10.1111/tpj.15036 Jaillais, Y., and Chory, J. (2010). Unraveling the paradoxes of plant hormone signaling integration. Nature Structural Molecular Biology. 17, 642–645. https://doi.org/10.1038/nsmb0610-642 Kurepa, J., and Smalle, J.A. (2022). Auxin/cytokinin antagonistic control of the shoot/root growth ratio and its relevance for adaptation to drought and nutrient deficiency stresses. International Journal of Molecular Science, 23 (4), 1933. https://doi.org/10.3390/ijms23041933

Lehmann, P., and Or, D. (2015). Effects of stomata clustering on leaf gas exchange. New Phytologist, 207, 1015–1025. https://doi.org/10.1111/nph.13442 Li, S.-M., Zheng, H.-X., Zhang, X.-S., and Sui, N. (2021). Cytokinins as central regulators during plant growth and stress response. Plant Cell Reports, 40, 271–282. https://doi.org/10.1007/s00299-020-02612-1 Lim, C.W., Baek, W., Jung, J., Kim, J.-H., and Lee, S.C. (2015). Function of ABA in stomatal defense against biotic and drought stresses. International Journal of Molecular Science, 16 (7), 15251–15270. https://doi.org/10.3390/ijms160715251 Liu, M., Pirrello, J., Chervin, C., Roustan, J. P., and Bouzayen, M. (2015). Ethylene control of fruit ripening: revisiting the complex network of transcriptional regulation. Plant Physiologist, 169 (4), 2380–2390. https://doi.org/10.1104/pp.15.01361 Nakashima, K., and Yamaguchi-Shinozaki, K. (2013). ABA signaling in stress-response and seed development. Plant Cell Reports, 32, 959–970. https://doi.org/10.1007/s00299-013-1418-1 Penfield, S. (2017). Seed dormancy and germination. Current Biology, 27 (17), R874–R878. https://doi.org/10.1016/j.cub.2017.05.050 Perrot-Rechenmann, C., and Napier, R.M. (2005). Auxins. Vitamins & Hormones, 72, 203–233. https://doi.org/10.1016/s0083-6729(04)72006-3

Plaza-Wüthrich, S., Blösch, R., Rindisbacher, A., Cannarozzi, G., and Tadele, Z. (2016). Gibberellin Deficiency Confers Both Lodging and Drought Tolerance in Small Cereals. Frontiers in Plant Science, 7, 643. https://doi.org/10.3389%2Ffpls.2016.00643

Qi, Y., Li, C., Li, H., Yang, H., and Guan, J. (2021). Elimination or Removal of Ethylene for Fruit and Vegetable Storage via Low-Temperature Catalytic Oxidation. Journal of Agricultural and Food Chemistry, 69, 10419–10439. https://doi.org/10.1021/acs.jafc.1c02868

Saleem, M., Fariduddin, Q., and Castroverde, C.D.M. (2021). Salicylic acid: a key regulator of redox signalling and plant immunity. Plant Physiologist and Biochemistry, 168, 381–397. https://doi.org/10.1016/j.plaphy.2021.10.011 Santner, A., Calderon-Villalobos, L.I.A., and Estelle, M. (2009). Plant hormones are versatile chemical regulators of plant growth. Nature Chemical Biology, 5, 301–307. https://doi.org/10.1038/nchembio.165 Stepanova, A.N., and Alonso, J.M. (2005). Ethylene signaling pathway. Science's STKE 2005:cm3 https://doi.org/10.1126/stke.2762005cm3 Suzuki, N., Rivero, R.M., Shulaev, V., Blumwald, E., and Mittler, R. (2014). Abiotic and biotic stress combinations. New Phytologist, 203, 32–43. https://doi.org/10.1111/nph.12797

Zhou, Y., He, R., Guo, Y., Liu, K., Huang, G., Peng, C., Liu, Y., Zhang, M., Li, Z., and Duan, L. (2019). A novel ABA functional analogue B2 enhances drought tolerance in wheat. Scientific Reports 9, 2887. https://doi.org/10.1038/s41598-019-39013-8

Visual Sources

Cover Image: Uncover the Basics of Plant Growth Hormones. [Image]. Miller Chemical. Retrieved August 09th, 2023, from https://www.millerchemical.com/blog/plant-growth-hormone-basics/ Figure 1: Santner 2009. Different type of plant hormones. [Image]. Nature Chemical Biology. Retrieved August 09th, 2023, from https://www.nature.com/articles/nchembio.165

Figure 2: Table with a summary of the main hormone classes and their activities. [Image]. BioNinja. Retrieved August 09th, 2023, from https://ib.bioninja.com.au/higher-level/topic-9-plant-biology/untitled-2/plant-hormones.html Figure 3: Jaillais and Chory, 2010. Plant hormones structures and functional interactions. [Image]. Nature Structural & Molecular Biology. Retrieved August 09th, 2023, from https://www.nature.com/articles/nsmb0610-642 Figure 4: Singh 2021. Plants are often exposed to various types of biotic and abiotic stresses. [Image]. Plant Cell Reports. Retrieved August 09th, 2023, from https://link.springer.com/article/10.1007/s00299-021-02727-z

Figure 5: Nield, 2016. Using different plant varieties with modified hormone concentrations can lead to better crop production [Image] Sci News. Retrieved August 29th, 2023 from https://www.sci.news/biology/biotechnology-crop-yield-04413.html

Figure 6: Auxin stimulates rooting. On the left plants not treated with auxin, on the right plants trated with auxin. [Image]. Tropical Trees: Propagation and Planting Manuals. Retrieved August 24th, 2023 from https://www.fao.org/3/AD231E/AD231E05.htm