Strigolactones and their crosstalk with other phytohormones

Abstract

Background: Strigolactones (SLs) are a diverse class of butenolide-bearing phytohormones derived from the catabolism of carotenoids. They are associated with an increasing number of emerging regulatory roles in plant growth and development, including seed germination, root and shoot architecture patterning, nutrient acquisition, symbiotic and parasitic interactions, as well as mediation of plant responses to abiotic and biotic cues.

Scope: Here, we provide a concise overview of SL biosynthesis, signal transduction pathways and SL-mediated plant responses with a detailed discourse on the crosstalk(s) that exist between SLs/components of SL signalling and other phytohormones such as auxins, cytokinins, gibberellins, abscisic acid, ethylene, jasmonates and salicylic acid.

Conclusion: SLs elicit their control on physiological and morphological processes via a direct or indirect influence on the activities of other hormones and/or integrants of signalling cascades of other growth regulators. These, among many others, include modulation of hormone content, transport and distribution within plant tissues, interference with or complete dependence on downstream signal components of other phytohormones, as well as acting synergistically or antagonistically with other hormones to elicit plant responses. Although much has been done to evince the effects of SL interactions with other hormones at the cell and whole plant levels, research attention must be channelled towards elucidating the precise molecular events that underlie these processes. More especially in the case of abscisic acid, cytokinins, gibberellin, jasmonates and salicylic acid for which very little has been reported about their hormonal crosstalk with SLs.

Keywords: Abscisic acid; D3/MAX2; D53/SMXL; GR24; auxin; cytokinins; ethylene; gibberellins; jasmonates; salicylic acid; strigolactone interactions; strigolactone signalling.

Fig. 1.

Chemical structures of canonical strigolactones, karrikin and gr24 (a synthetic strigolactone).

Fig. 2.

Structures of non-canonical strigolactones.

Fig. 3.

Strigolactone biosynthetic pathway originating from β-carotene in plastids. ① D27 isomerise all-trans-β-carotene to form 9-cis-β-carotene; ② CCD7 cleavage of 9-cis-β-carotene to yield 9-cis-β-apo-10′-carotenal and β-ionone; ③ carlactone is produced from 9-cis-β-apo-10′-carotenal by CCD8; ④ carlactone is exported from the plastid into the cytoplasm; MAX1 catalyses the biosynthesis of ⑤ carlactonoic acid from carlactone and the conversion of carlactonoic acid to ⑥ methyl carlactonoate; ⑦ 5-deoxystrigol; ⑧ strigol directly as seen in moonseed; ⑨ strigol via 5-deoxystrigol as seen in cotton; ⑩ 4-deoxyorobanchol, ⑪ orobanchol directly in cowpea, and ⑫ orobanchol via 4-deoxyorobanchol as in rice.

Fig. 4.

A model of strigolactone perception and signal transduction pathway. ① SL binds in the catalytic cleft of D14 and induces a destabilization/conformational change in D14; ② SL-induced destabilization of D14 facilitates D14 interaction with SCF^D3; ③ SCF^D3-D14 recruits a target protein (e.g. D53 or SMXLs); ④ D53/SMXLs undergo polyubiquitination; ⑤ proteasomal degradation; ⑥ D14 deactivates the bound SL molecule by hydrolytic cleavage of the enol–ether bridge and releases the ABC and D rings.

Fig. 5.

Major growth and developmental processes mediated by strigolactones in plants. , stimulatory effect/regulation; , inhibitory effect/regulation.

Fig. 6.

Strigolactone interactions with abscisic acid, auxin, cytokinin, ethylene and salicylic acid. Abbreviations: ABA, abscisic acid; BRC1, BRANCHED1; CK, cytokinin; COP1, CONSTITUTIVE PHOTOMORPHOGENESIS1; HY5, ELONGATED HYPOCOTYL5; PDR1/6, PLEIOTROPIC DRUG RESISTANCE1/6; PIN, PIN-FORMED; SHY2, SHORT HYPOCOTYL2; SA, salicylic acid; SLs, strigolactones. , stimulatory effect/regulation; , possible stimulatory effect; , inhibitory effect/regulation.