Structure-activity relationship studies of strigolactone-related molecules for branching inhibition in garden pea: molecule design for shoot branching

Abstract

Initially known for their role in the rhizosphere in stimulating the seed germination of parasitic weeds such as the Striga and Orobanche species, and later as host recognition signals for arbuscular mycorrhizal fungi, strigolactones (SLs) were recently rediscovered as a new class of plant hormones involved in the control of shoot branching in plants. Herein, we report the synthesis of new SL analogs and, to our knowledge, the first study of SL structure-activity relationships for their hormonal activity in garden pea (Pisum sativum). Comparisons with their action for the germination of broomrape (Phelipanche ramosa) are also presented. The pea rms1 SL-deficient mutant was used in a SL bioassay based on axillary bud length after direct SL application on the bud. This assay was compared with an assay where SLs were fed via the roots using hydroponics and with a molecular assay in which transcript levels of BRANCHED1, the pea homolog of the maize TEOSINTE BRANCHED1 gene were quantified in axillary buds only 6 h after application of SLs. We have demonstrated that the presence of a Michael acceptor and a methylbutenolide or dimethylbutenolide motif in the same molecule is essential. It was established that the more active analog 23 with a dimethylbutenolide as the D-ring could be used to control the plant architecture without strongly favoring the germination of P. ramosa seeds. Bold numerals refer to numbers of compounds.

Figure 1.

Natural SLs and analogs. Ac, Acetyl.

Figure 2.

Known SL analogs and d’orenone. Boc, t-Butoxycarbonyl.

Figure 3.

Synthesis of D-modified GR24 analogs. Ac, Acetyl; Bn, benzyl; Bu, n-butyl; Bz, benzoyl; Me, methyl; MW, microwave irradiation; r.t., room temperature; Tf, trifluoromethanesulfonyl.

Figure 4.

Synthesis of GR5 analogs and SL mimic 31. Et, Ethyl; r.t., room temperature; tBu, t-butyl.

Figure 5.

Synthesis of bioisostere 33 of GR24. DMF, Dimethylformamide; Me, methyl; r.t., room temperature.

Figure 6.

Synthesis of reduced and oxidized GR24 analogs. DMDO, Dimethyl dioxirane; Me, methyl; PTSA, p-toluenesulfonic acid; r.t., room temperature.

Figure 7.

A, Chemical stability of solanacol and solanacyl acetate in a solution of ethanol:water (1:4, v/v) at 21°C (pH 6.7). B, Chemical stability of GR24 and 23 in a solution of ethanol:water (1:4, v/v) at 21°C (pH 6.7).

Figure 8.

Activity of (+)- and (−)-solanacol and (+)- and (−)-solanacyl acetate. Lengths of axillary buds are shown at 10 d after direct application of a solution at 1 µm at node 4. Data are means ± se (n = 24). CTL 0, Control 0.

Figure 9.

Comparative activity of SL analogs 23 and 2′-epi-23. Lengths of axillary buds are shown at 10 d after direct application at node 3. Data are means ± se (n = 24). CTL 0, Control 0.

Figure 10.

Dose-dependent activity of the most active SL analogs GR24, GR5, and 23 for branching inhibition. Lengths of axillary buds are shown at 10 d after direct application at node 3. Data are means ± se (n = 24). CTL 0, Control 0.

Figure 11.

Bioassay on bud outgrowth using hydroponic culture. A comparison of the activity of GR24 and 23 at 1 µm is shown. Axillary bud lengths are shown at each node of rms1 plants 2 weeks after treatment. Lram Ni-j, Ramification length at node i. j = 1 means the main branch at this node, and j = 2 means the second branch at this node when visible. Data are means ± se (n = 12). Values that have the same letter (a or b) are not significantly different at P = 0.05. CTL 0, Control 0.

Figure 12.

Effect of different SLs (1 µm) on PsBRC1 transcript levels in axillary buds at node 3 of rms1 plants. Plants were treated by direct application on the bud. A, Length of the axillary bud 10 d after treatment at node 3. Data are means ± se (n = 12). B, PsBRC1 transcript levels in the axillary bud 6 h after treatment. Data are means ± se (n = 3 pools of 30 plants). Values that have the same letter (a–c) are not significantly different at P = 0.05. CTL 0, Control 0.

Figure 13.

EC₅₀ (mol L⁻¹) of various SL analogs toward P. ramosa seed germination.

Figure 14.

Effects of SL structural modifications on bud outgrowth inhibition in the pea model. Ac, Acetyl; Boc, t-butoxycarbonyl; Me, methyl.