What are plant hormones?
Plant hormones, known as phytohormones (-Phyto means plant in greek), are single-molecule chemical messengers produced by plants in low concentrations, however, they regulate key cellular activities such as embryogenesis, stress tolerance, pattern formation, pathogen defense, regulation of organ size, vegetative development, and reproductive development.
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They essentially oversee vital activities taking the plant from seed to adult plant, through to seed again in reproducing the next generation.
By the mid-20th century the five classic plant hormones of abscisic acid (ABA), auxin, cytokinins, ethylene, and gibberellins had been discovered. In more recent years brassinosteroids, jasmonates, salicylates, and strigolactones have been added to this list.
The action of plant hormones is governed by its synthesis, followed by how it accumulates, which is impacted by the antagonistic processes of conjugation and degradation. Transport also plays a role in the regional accumulation of plant hormones. For some hormones, scientists do not fully understand the underlying mechanisms responsible for its regulation.
In general, we know that the binding of plant hormones to receptors initiates their signaling. Some of these receptors are transmembrane proteins, meaning that once a hormone binds to them they undergo a process of conformational change.
In other receptors, the hormones act as “molecular glue” to enable the interaction of two separate proteins, triggering the processes that follow as a result of this interaction. Protein phosphorylation, proteolysis, and transcription are downstream signaling events that underly a range of processes and vital cellular activities within the plant.
Plant hormones and their functions:
Auxin is one of the most well-known plant hormones. Discovered by Charles Darwin, auxins are known to be a family of plant hormones that are usually produced in the growing stems and roots of a plant and are recognized as inducing stem growth and surpassing root growth.
They cause growth by promoting cell division, causing the plant cells to elongate, auxin is found in abundance in areas of the plant exposed to light, causing it to grow toward the light source. However, roots respond in the opposite way to the presence of auxin. In roots, auxin accumulates in the underneath side, where gravity pulls more strongly.
This causes the underneath side to grow less, causing the root to bend downwards and grow in the direction of gravity, deep into the earth.
Cytokinins are also responsible for the process of cell division in the shoots and roots of plants. They are a family of growth substances derived from the chemical compound adenine, one of the bases of DNA and RNA.
Cytokinins work antagonistically to auxins and are involved in the processes of cell growth and differentiation, as well as axillary bud growth, apical dominance, and leaf senescence. They also oversee the action of nutrient uptake through the roots.
The signaling of the cytokinins is mediated by a two-component phosphorelay system.
Strigolactones are a group of hormones produced in the roots of the plant. Research has revealed two main functions of the strigolactones, the first is that they regulate plant development, the second is that they facilitate symbiotic interactions between the microbes in the soil and the plant’s roots.
They were first identified for this second role, identified as root-exuded compounds found to stimulate the germination of parasitic plant seeds. They were later also identified as endogenous plant hormones, influencing the branching of both root and shoot.
Gibberellins are most commonly known for their impact of promoting growth in the plant, specifically, stem elongation. However, they are also involved in the processes of germination, flowing, flower development, dormancy, and leaf and fruit senescence.
The pathway of the gibberellins has been well documented. Scientists know that the growth-inhibitory DELLA-proteins begin degradation in response to gibberellin perception.
Brassinosteroids are a group of polyhydroxysteroids that promote cell elongation and also play a vital role in pollen development. The most active of the brassinosteroids is brassinolide.
The plasma-membrane localized BRI1 receptor interacts with the brassinosteroids, initiating a protein-kinase cascade, resulting in transcriptional changes. Recently, scientists have identified this group of hormones as potentially having use in anticancer drugs to treat endocrine-responsive cancers.
Ethylene plays a vital role in senescence, or deterioration with age, and fruit ripening, as well as in the plant’s stress responses and its vegetative development.
Ethylene production can be manipulated biochemically, which is often the route taken by farmers who want to delay fruit ripening so that produce is fresh when it reaches the consumer.
ABA (Abscisic acid)
Drought initiates the synthesis of the hormone ABA which results in the expression of the plant’s stress-responsive genes. ABA binds to receptors of the PYR/ PYL/ RCAR protein family, resulting in the activation of protein kinases.
Jasmonates are best known for their activity in responding to tissue wounding. They are lipid-based plant hormones derived from the membrane, the synthesis of which is initiated by wounding or herbivory.
The accumulation of Jasmonates results in the expression of anti-herbivory chemicals and proteins and initiates the plant’s defense responses. They are also involved in growth, photosynthesis, and reproductive development.
Salicylate from a chemical standpoint is a salt or ester of salicylic acid. Salicylate plays a major role in developing a plant’s resistance to pathogens. A pathogen attack will initiate synthesis of the hormone, however, scientists are still unclear on the identity of the salicylate receptor.
Plant hormone cross-talk
A growing body of research is demonstrating that the phytohormones do not have distinct, singular functions, rather that they work together to regulate a range of processes. Between them, there is a lot of cross-talk, and a single process may result from the activity of several plant hormones.
- Joo, J., Bae, Y. and Lee, J., 2001. Role of Auxin-Induced Reactive Oxygen Species in Root Gravitropism. Plant Physiology, 126(3), pp.1055-1060. http://www.plantphysiol.org/content/126/3/1055.short
- Larrieu, A. and Vernoux, T., 2016. Q&A: How does jasmonate signaling enable plants to adapt and survive?. BMC Biology, 14(1). https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-016-0308-8
- Smith, S., 2014. Q&A: What are strigolactones and why are they important to plants and soil microbes?. BMC Biology, 12(1). https://bmcbiol.biomedcentral.com/articles/10.1186/1741-7007-12-19
- Tang, J., Han, Z. and Chai, J., 2016. Q&A: what are brassinosteroids and how do they act in plants?. BMC Biology, 14(1). https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-016-0340-8