Strigolactones: A New Class of Plant Hormones with Multifaceted Roles


Strigolactones (SLs) are terpenoid lactones produced mainly in plant roots and initially identified as seed germination stimulants for parasitic weeds. In 2005, they were described also as boosters of hyphal branching in arbuscular mycorrhizal fungi, and thereby as promoters of arbuscular mycorrhizal symbiosis. In 2008, they emerged as a new class of plant hormones controlling plant architecture through repression of shoot branching. Since then, several new roles were discovered for SLs: in the adaptive responses to a number of environmental stimuli (including light, osmotic stress, interaction with pathogens and nodulating bacteria), and in several aspects of plant development (including seed germination for nonparasitic plants, hypocotyl elongation, reproduction, leaf senescence and nodulation). The biosynthetic and perception/transduction systems of SLs are being elucidated, and the first mechanistic models presented.

Key Concepts:

  • Strigolactones are the key nutrient allocators regulating plant development at the interface between plants, beneficial and detrimental (micro)organisms, and abiotic factors.

  • Strigolactones induce hyphal branching in AM fungi and facilitate the establishment of symbiosis.

  • Strigolactones inhibit shoot branching.

  • Strigolactones affect root architecture and root development depending on nutrients availability.

  • Synthetic SLs (analogues and mimics) are used in pharmacological applications to plants and fungi to decipher the structure–activity relationship.

  • Structure–activity relationship (SAR): Different structures are tested for bioactivity in order to pinpoint which part of the molecule is essential for bioactivity and which can be considered only ‘decoration’.

Keywords: strigolactones; plant hormones; plant development; symbiosis; AM fungi; parasitic plants; shoot branching; hyphal branching; biofertilisers

Figure 1.

Naturally occurring SLs. Natural SLs can be grouped into two families. In one, the absolute configuration of the BCD part (stereochemistry at C3a, C8b and C2′) is the same as parent (+)‐strigol (1); in the other, the stereochemistry is the same as in (−)‐orobanchol (6) (found also in fabyl acetate, ent‐2′‐epi‐orabanchyl acetate and ent‐2′‐epi‐solanacol). In both the families of natural SLs the stereochemistry at C‐2′ remains the same as strigol.

Figure 2.

Some synthetic analogues and mimics of SLs. The difference between analogues and mimics lies on the functional group linked to the D ring. In analogues, as in natural SLs, the D ring is connected to the main core of the molecule through an enol ether bridge (O linked to a double bond). In mimics the D ring is linked to a simpler structure.

Figure 3.

The SL biosynthetic pathway. SLs are derived from carotenoids, and the first steps of their biosynthesis occur in plastids. The main gene products involved in the pathway are indicated; their functions are commented in the text.

Figure 4.

The proposed model of SL perception and signal transduction. D14 binds the SL molecule, hydrolises it and thereby becomes competent to interact with MAX2. MAX2 is an F‐box protein and part of the SCF (Skp, Cullin, F‐box complex) of the proteasome; its interaction with MAX2 in the presence of SLs makes it able to recruit its substrate(s), the repressor proteins of SL responses such as D53 in rice. Once ubiquitinylated, the repressor is committed to be degraded by the proteasome thus initiating signal transduction.



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Further Reading

Bonfante P and Requena N (2013) Dating in the dark: how roots respond to fungal signals to establish arbuscular mycorrhizal symbiosis. Current Opinion in Plant Biology 14: 451–457.

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Nadal M and Paszkowski U (2013) Polyphony in the rhizosphere: presymbiotic communication in arbuscular mycorrhizal symbiosis. Current Opinion in Plant Biology 16: 473–479.

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Prandi, Cristina, and Cardinale, Francesca(Apr 2014) Strigolactones: A New Class of Plant Hormones with Multifaceted Roles. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0023754]