Strigolactones: new plant hormones in action

Abstract

The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.

Keywords: Karrikins; Mode of action; Signal transduction; Strigolactone analogs; Strigolactone mimics; Strigolactones.

Fig. 1

Structures of naturally occurring SLs. Names of the orobanchol family using (+)-strigol as the parent compound are given within brackets

Fig. 2

Simplification of SL structures (all are appreciably active as germinating agents)

Fig. 3

Model for designing SL analogs with germinating activity

Fig. 4

Synthetic analogs of SLs with germinating activity

Fig. 5

Isosteric SL analogs

Fig. 6

Debranones and aroyloxy SL mimics

Fig. 7

Simplified SL structures with activity as a branching factor for AM fungi

Fig. 8

Minimum structure required for shoot branching inhibition in rice

Fig. 9

SLs with an extra methyl group at C-3′ in the D-ring

Fig. 10

Comparison of structural features of SLs and KARs

Fig. 11

Tentative molecular mechanism for the mode of action via an addition–elimination reaction

Fig. 12

Tentative molecular mechanism for the mode of action involving a nucleophilic attack of ester carbonyl of the D-ring of an SL (acyl attack, top line). Idem for KAR (bottom line)

Fig. 13

Proposal for the hydrolysis mechanism of SLs induced by the catalytic triad of Ser-His-Asp

Fig. 14

Schematic presentation of the interaction of an SL with the receptor protein D14

Fig. 15

SLs with functional labels; the bioactiphore is in the boxed part

Fig. 16

Tentative mode of action for SL mimics having an aroyloxy substituent at C-5 the D-ring

Fig. 17

Fluorescein tagged SL mimics having a fluorescence turn-on probe