Plant Hormones

Introduction

Plant hormones auxin and cytokinin are primarily known for their roles in vegetative (indeterminate) growth. This type of growth involves the repetition of structures in a given order once the plant has established the primary apical meristem. Auxin and cytokinin work antagonistically during the vegetative growth of plants, but synergistically in cellular regeneration [1]. Auxin is produced in the apical meristem of the plant, whereas cytokinin is produced in the leaves. Auxins are responsible for apical dominance in vegetative growth, repressing cytokinin from activating dormant axillary (lateral) buds from growing new lateral shoots. Cytokinin is responsible for creating new lateral shoots when the apical meristem is damaged or the concentration of auxin falls below the ability to repress cytokinin [1].

Auxin

Organic and synthetic Auxins [7]

The plant hormone auxin, produced in the apical meristem, can be found in nature as a compound known as indole-3-acetic acid (IAA), and is primarily synthesized from the well-known amino acid tryptophan [4]. There are also synthetic auxins known as 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene acetic acid (NAA) [3].

Auxins are extremely important to plant growth as they are responsible primarily for cellular elongation, root development, and apical dominance during vegetative growth. During apical dominance, auxin produced by the apical meristem creates a downward concentration gradient, preventing axillary buds from activating through cytokinin initiation. This concentration gradient also creates a hormone sink in the roots, where auxin stimulates root development. They are widely used commercially for root induction for plant propagation [3].

Interestingly, auxins can induce epinastic and hyponastic responses through localized auxin concentrations. This causes plant bending in the process known as phototropism, where plants such as sunflowers tend to face direct sunlight to increase photosynthetic function. In this process, the plant will bend toward the localized auxin concentration. It may also induce leaf epinasty, where leaves bend downward "as result of disturbances in their growth, with greater expansion in adaxial cells as compared to abaxial surface cells" [4].

Cytokinin

Organic and synthetic cytokinins [7]

The plant hormone cytokinin, produced in leaf tissues of plants, is primarily found in nature as a compound known as zeatin. Interestingly, "almost all organisms make cytokinin; for example, isopentenyl adenine derivatives found adjacent to the anticodon loop of a subset of tRNAs in most eukaryotes and bacteria" [5]. These were first discovered while searching for factors that promote cell proliferation in plant cells in concert with auxin to regulate cell division and differentiation [5].

Cytokinins are primarily responsible for cellular division and overcoming apical dominance to stimulate lateral shoot formation. If the plant apical meristem becomes damaged, the auxin concentration of the plant falls allowing cytokinin concentrations to accumulate and activate dormant axillary buds [5]. In some cases, the auxin concentration gradient falls below levels of apical dominance near the ground, causing lateral shoot formation in a Christmas-tree like effect.

Plant Callus Formation

Plant callus tissue forms as a result of wounding, followed by cellular regeneration controlled by the ratio of auxin to cytokinin [7]

Callus formation on plants occurs as a result of wounding, infection, or unregulated and undifferentiated cellular regeneration, controlled by concentrations of auxin and cytokinin. Studies have shown "an intermediate ratio of auxin and cytokinin promotes callus induction, while a high ratio of auxin-to-cytokinin or cytokinin-to-auxin induces root and shoot regeneration, respectively" [6]. These hormones are important for infection processes in plants such as agrobacterium where transgenes are inserted into the host genome. The agrobacterium T-DNA has oncogenes which promote production of auxin and cytokinin in order to provide a favorable environment for the bacteria to reproduce and feed. This site is typically referred to as the crown gall. Eventually the plant will die, in which the bacteria are released back into the soil to begin the infection process again.

References

1. Su, Ying-Hua, et al. “Auxin-Cytokinin Interaction Regulates Meristem Development.” Molecular plant, vol. 4, no. 4, Elsevier Inc, 2011, pp. 616–25, doi:10.1093/mp/ssr007.
2. G. Eric Schaller, et al. “The Yin-Yang of Hormones: Cytokinin and Auxin Interactions in Plant Development.” The Plant Cell, vol. 27, no. 1, American Society of Plant Biologists, 2015, pp. 44–63, doi:10.1105/tpc.114.133595.
3. Zaman, Mohammad, et al. “Enhancing Crop Yield with the Use of N‐based Fertilizers Co‐applied with Plant Hormones or Growth Regulators.” Journal of the Science of Food and Agriculture, vol. 95, no. 9, John Wiley & Sons, Ltd, 2015, pp. 1777–85, doi:10.1002/jsfa.6938.
4. Sandalio, Luisa M., et al. “Leaf Epinasty and Auxin: A Biochemical and Molecular Overview.” Plant Science (Limerick), vol. 253, Elsevier Ireland Ltd, 2016, pp. 187–93, doi:10.1016/j.plantsci.2016.10.002.
5. Kieber, Joseph J., and G. Eric Schaller. “Cytokinin Signaling in Plant Development.” Development (Cambridge), vol. 145, no. 4, COMPANY BIOLOGISTS LTD, 2018, p. dev149344–, doi:10.1242/dev.149344.
6. Momoko Ikeuchi, et al. “Plant Callus: Mechanisms of Induction and Repression.” The Plant Cell, vol. 25, no. 9, American Society of Plant Biologists, 2013, pp. 3159–73, doi:10.1105/tpc.113.116053.
7. Berry, James O. “Rec#6 Hormones and Plant Form.” 2021.