Plant Hormones


Plant hormones coordinate physiology and development between organ systems, which can often be separated by large distances. In this article, we focus on the set of nine small molecule plant hormone families that are present throughout land plants: auxins, gibberellins, jasmonates, salicylates, brassinolides, strigolactones, cytokinins, abscisic acid, and ethylene. We cover the basic aspects of their physiological roles, biosynthesis, signal transduction and agricultural relevance. We note commonalities in the molecular mechanisms of plant hormone perception and signal transduction; for example, hormone‐stabilised protein–protein interactions, that are often linked directly to ubiquitylation of downstream effector proteins are prevalent. Plant hormone signalling pathways preferentially exploit soluble receptors over transmembrane receptors; brassinosteroids are the only hormone family known to act through a classical plasma membrane‐anchored receptor kinase, a modality common in animal signal transduction pathways. Plant hormones provide a complementary set of molecular solutions to the biological problem of communicating information over long distances in multicellular organisms.

Key Concepts

  • Plant hormones are small organic molecules that can act at a distance from their site of synthesis with high potency.
  • At least nine hormones are present throughout most land plants and function in long‐distance communication.
  • Most plant hormones are perceived by soluble receptors.
  • Synthetic plant hormone mimics and inhibitors have been developed for agriculture.

Keywords: hormone; signal transduction; genetic analysis; auxin; strigolactone; brassinosteroid; cytokinin; jasmonic acid; abscisic acid; salicylic acid; ethylene

Figure 1. Structures of plant hormones.
Figure 2. Hormone signalling pathways. The signalling mechanisms of Auxin, JA, GA, and SL involve hormone‐activated targeting of protein substrates through SCF (Skp1‐Cullin‐F‐box) complexes. SCF complexes are a type of E3 ubiquitin ligase that consist of Skp1, Cullin1, and an F‐box protein that confers substrate specificity. E3 ubiquitin ligases work in concert with E1 (not shown) and E2 ubiquitin ligases to attach ubiquitin or polyubiquitin chains to proteins. Polyubiquitylated proteins are rapidly degraded by the 26S proteasome. Two types of SCF‐mediated plant hormone signalling mechanisms have been observed. The hormone may function as molecular glue that facilitates binding between an F‐box protein and its target substrate. Effectively these proteins are co‐receptors. Alternatively, a receptor protein may undergo conformational changes following ligand binding or processing (allosteric regulation) that promote formation of a complex with an F‐box protein and a target. In either case, the target is polyubiquitylated and degraded. The targets of the auxin, JA, and GA pathways function as transcriptional regulators, and it is thought that this is also a function of SL targets. None of the known targets directly bind DNA; instead, they interact with transcription factors and modulate their activity. The targets of auxin, JA, and likely SL signalling are associated with transcriptional repressor complexes. Therefore, degradation of the target protein can relieve transcriptional repression.
Figure 3. Structures of hormone mimics and inhibitors used in agriculture.


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Further Reading

Buchanan BB, Gruissem W and Jones RL (2015) Biochemistry and Molecular Biology of Plants. Chichester, UK: John Wiley & Sons, Ltd.,.

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Cutler, Sean R, and Nelson, David C(Mar 2017) Plant Hormones. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002091.pub2]