467 Mini-review Received: 9 July 2008 Revised: 6 October 2008 Accepted: 16 October 2008 Published online in Wiley Interscience: 16 February 2009 (www.interscience.wiley.com) DOI 10.1002/ps.1726 Strigolactones: structures and biological activities Koichi Yoneyama, ∗ Xiaonan Xie, Kaori Yoneyama and Yasutomo Takeuchi Abstract Strigolactones released from plant roots induce seed germination of root parasitic weeds, witchweeds (Striga spp.) and broomrapes (Orobanche spp.), and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. In addition to these functions in the rhizosphere, strigolactones have recently been shown to be a novel class of plant hormones regulating shoot outgrowth. The natural strigolactones identified so far have the common C – D ring moiety, which is thought to be the essential structure for exhibiting biological activity. The introduction of substitutions on the A – B ring moiety of 5-deoxystrigol, the basic strigolactone, affords various strigolactones, e.g. hydroxylation on C-4, C-5 and C-9 leads to orobanchol, strigol and sorgomol respectively. Then, acetylation and probably other derivatisations of these hydroxy-strigolactones would occur. Although the C-2 ′ -(R) stereochemistry was thought to be an important structural feature for potent germination stimulation activity, 2 ′ -epi-strigolactones were found in root exudates of tobacco, rice, pea and other plant species, indicating that at least some plants produce both epimers. c  2009 Society of Chemical Industry Keywords: strigolactone; germination stimulant; parasitic weeds; arbuscular mycorrhizal fungi 1 INTRODUCTION Since the isolation of strigol as a germination stimulant for Striga lutea Lour. from root exudates of a false host cotton (Gossyp- ium hirsutum L.), 1,2 more than ten strigol-related compounds, collectively called strigolactones, have been identified as germi- nation stimulants for root parasitic weeds, witchweeds (Striga spp.), broomrapes (Orobanche spp.) and Alectra. Recently, strigo- lactones were also found as host recognition signals for arbuscular mycorrhizal (AM) fungi, from which plants benefit. 3 Since >80% of land plants form symbiotic relationships with AM fungi, these my- cotrophics are expected to produce and release strigolactones. 4 Surprisingly, not only host plants but also non-hosts of AM fungi such as Arabidopsis sp. 5 and white lupin 6 produce strigolactones. Such a wide distribution of strigolactones in the plant kingdom indicates that strigolactones have other important roles in plants and in rhizosphere communication. Recent findings unveiled such a hidden function of strigolactones as a novel class of plant hor- mones regulating shoot branching. 7,8 This paper will focus on the structures and biological activities of strigolactones. More detailed discussions on the chemistry and regulation of pro- duction of strigolactones can be found in other papers in this issue. 9,10 2 STRUCTURES OF STRIGOLACTONES 2.1 Essential structural features and structural diversity Natural strigolactones isolated to date are shown in Fig. 1. Strigol (1) and strigyl acetate (2) were isolated from cotton root exudates as the first strigolactones. 1,2 Strigol was then identified in the root exudates of Striga hosts, sorghum [Sorghum bicolor (L.) M ¨ onch], maize (Zea mays L.) and proso millet (Pennisetum glaucum R.Br.). 11 Sorgolactone (3) and alectrol were isolated from root exudates of sorghum 12 and cowpea (Vigna unguiculata Auct.) 13 respectively. Isolation of orobanchol (4) and alectrol as the first Orobanche germination stimulants from red clover (Trifolium pretense L.) root exudates clearly demonstrated that both Striga and Orobanche utilise strigolactones as germination cues. 14 Recently, alectrol was identified as orobanchyl acetate (5). 15,16 5-Deoxystrigol (6), the first identified branching factor for AM fungi from Lotus japonicus (Regel) Larsen root exudates, 3 has been detected in root exudates of various plant species, both monocots 17 and dicots, 6 suggesting that the other strigolactones are derived from 5-deoxystrigol (6). 18–20 Indeed, an allylic hydroxylation of 5-deoxystrigol (6) leads to strigol (1) or orobanchol (4), and the third hydroxy-strigolactone, sorgomol (7), 21 is produced by hydroxylation on the homoallylic position. These hydroxy-strigolactones may be acetylated, and conjugations with sugars and amino acids may occur. Further oxidation of the hydroxymethyl group of sorgomol (7) and subsequent decarboxylation affords sorgolactone (3). 20,21 Among the three hydroxy-strigolactones, orobanchol (4) seems to be distributed most widely in the plant kingdom, and various strigolactones that would be derived from orobanchol (4) have been isolated. For example, very recently, 7-oxoorobanchol (8), 7-oxoorobanchyl acetate (9) and 7-hydroxyorobanchol acetate (10) have been detected in flax (Linum usitatissimum L.) and cucumber (Cucumis sativus L.) root exudates. 22 The didehydro- orobanchol isomers and solanacol (11) detected in the root exudates of tobacco (Nicotiana tabacum L.) 23 and tomato ∗ Correspondence to: Koichi Yoneyama, Weed Science Centre, Utsunomiya University, 350 Mine-machi, Utsunomiya 321-8505, Japan. E-mail: yoneyama@cc.utsunomiya-u.ac.jp Weed Science Centre, Utsunomiya University, 350 Mine-machi, Utsunomiya 321-8505, Japan Pest Manag Sci 2009; 65: 467–470 www.soci.org c  2009 Society of Chemical Industry