The strigolactone germination stimulants of the plant-parasitic Striga and Orobanche spp. are derived from the carotenoid pathway

Abstract

The seeds of parasitic plants of the genera Striga and Orobanche will only germinate after induction by a chemical signal exuded from the roots of their host. Up to now, several of these germination stimulants have been isolated and identified in the root exudates of a series of host plants of both Orobanche and Striga spp. In most cases, the compounds were shown to be isoprenoid and belong to one chemical class, collectively called the strigolactones, and suggested by many authors to be sesquiterpene lactones. However, this classification was never proven; hence, the biosynthetic pathways of the germination stimulants are unknown. We have used carotenoid mutants of maize (Zea mays) and inhibitors of isoprenoid pathways on maize, cowpea (Vigna unguiculata), and sorghum (Sorghum bicolor) and assessed the effects on the root exudate-induced germination of Striga hermonthica and Orobanche crenata. Here, we show that for these three host and two parasitic plant species, the strigolactone germination stimulants are derived from the carotenoid pathway. Furthermore, we hypothesize how the germination stimulants are formed. We also discuss this finding as an explanation for some phenomena that have been observed for the host-parasitic plant interaction, such as the effect of mycorrhiza on S. hermonthica infestation.

Figure 1.

Structure of identified strigolactone germination stimulants. A, (+)-Strigol; B, orobanchol; C, sorgolactone; D, synthetic germination stimulant GR24.

Figure 2.

Isoprenoid biosynthetic pathway in higher plants redrawn from Lichtenthaler (1999). GA-3P, d-glyceraldehyde-3-P; MEP, 2-C-methyl-d-erythritol 4-P; IPP, isopentenyl diphosphate; DMAPP, dimethylallyl diphosphate; GPP, geranyl diphosphate; FPP, farnesyl diphosphate; GGPP, geranylgeranyl diphosphate.

Figure 3.

Carotenoid and abscisic acid biosynthetic pathway in maize. Intermediates that are not shown are in brackets. Carotenoid maize mutants (italics) and inhibitors (underlined) at different steps in the pathway that are described in this article are indicated (⇐). IPP, Isopentenyl diphosphate; MEP, 2-C-methyl-d-erythritol 4-P. Redrawn from Cunningham and Gantt (1998), Hirschberg (2001), and Seo and Koshiba (2002).

Figure 4.

Phenotypes of inhibitor-treated maize seedlings and phenotypes of carotenoid maize mutant seedlings. Inbred W22: nontreated control (A), 100 μm fosmidomycin (B), 25 μm fluridone (C), and 10 μm mevastatin (D). Inbred Dent: nontreated control (E), 10 μm fluridone (F), nontreated control grown under dim light (G), 10 μm fluridone grown under dim light (H), inbred Dent nontreated control grown at 21°C (I), 200 μm amitrole grown at 21°C (J), 0.1 mm naproxen (K), 1 mm naproxen (L), 0.1 mm sodium tungstate (M), 1 mm sodium tungstate (N), 0.02 mm abscisic acid (O), and 0.02 mm abscisic acid + 10 μm fluridone (P). Carotenoid mutants cl1 (Q), lw1 (R), y10 (S), al1y3 (T), vp5 (U), y9 (V), and vp14 (W).

Figure 5.

Germination of S. hermonthica seeds induced by root exudates of maize (W22 or Dent, as indicated in the graphs; A–D), cowpea (E and F), and sorghum (G) as affected by treatment of the host seedlings with (A) 100 μm fosmidomycin (Fos), 25 μm fluridone (Flu), and 10 μm mevastatin (Me) compared with untreated control (C). B, 10 μm Fluridone (Flu) applied to plants growing under normal light (Flu) and dim light (FluD) compared with their respective untreated controls (C and CD). C, 200 μm Amitrole (Ami) compared with untreated control (C). Seedlings were grown at 21°C. D, Untreated control (C), 10 μm fluridone (Flu), 0.1 mm naproxen (0.1 N), 1 mm naproxen (1 N), 0.1 mm sodium tungstate (0.1 ST), 1 mm sodium tungstate (1 mm ST), 0.02 mm abscisic acid and 10 μm fluridone (0.02 ABA Flu), 0.02 mm abscisic acid (0.02 ABA), 0.2 mm abscisic acid and 10 μm fluridone (0.2 ABA Flu), and 0.2 mm abscisic acid (0.2 ABA). E, 10 μm Fluridone and nontreated control (C), 10 μm fluridone (Flu), and root exudate of fluridone-treated seedlings combined with 0.01 mg/L GR24 (Flu GR 0.01) compared with 0.01 and 0.1 mg/L GR24 alone (GR 0.01 and GR 0.1). F, 10 μm Fluridone (Flu) on O. crenata germination compared with nontreated control (C). G, 4 μm Fluridone (Flu) compared with nontreated control (C). Within each experiment, the concentrations of root exudates were equalized, by dilution, for differences in root fresh weight. Data represent average germination ± standard error of 8 to 10 individual plants and two or three disks (indicated in each diagram by 8/3, etc.), with about 50 S. hermonthica seeds each per plant. Statistical analysis showed significant difference (P < 0.05) for all treatments, except the treatments with 100 μm fosmidomycin,10 μm mevastatin, and 0.1 mm naproxen.

Figure 6.

A, Phenotypes of mutant y10 seedlings (white) and corresponding wild-type siblings (gray) and germination of S. hermonthica induced by the root exudates of these seedlings. B, Phenotypes of mutant cl1 311AA seedlings (M; white) and corresponding wild-type siblings (N; gray) and germination of S. hermonthica induced by root exudates of these seedlings.

Figure 7.

Average germination of S. hermonthica induced by root exudates of maize mutants lw1, y10, al1y3, vp5, y9, cl1 311AA, and vp14 and their corresponding wild-type siblings (the number of seedlings used for each bioassay is indicated). Statistical analysis showed that lw1, y10, al1y3, vp5, and y9 mutant phenotype induced significantly lower (P < 0.05) germination than the corresponding nonmutant phenotype seedlings. The difference in germination induced by the vp14 mutant was not significant (P = 0.09).

Figure 8.

Putative biogenetic scheme for the formation of strigol, orobanchol, and sorgolactone. CCD, Carotenoid cleavage dioxygenase.