Plants have evolved a diverse array of secondary metabolites that play a key role in plant growth and development as well as in interactions with other organisms. Isoprenoids are among the oldest and most diverse family of known low-molecular-weight compounds. Within this family, apocarotenoids, originating from the oxidative cleavage of carotenoids, have recently attracted much attention because these molecules have been shown to influence a wide variety of biological processes in plants (Bouvier et al., 2005; Auldridge et al., 2006). Strigolactones are root apocarotenoids that were previously isolated as a seed-germination stimulant for the detrimental parasitic weeds Striga and Orobanche (Bouwmeester et al., 2003). The very same compounds have recently been shown to act as a symbiotic signal for the phosphate-acquiring arbuscular mycorrhizal (AM) fungi of the phylum Glomeromycota (Akiyama et al., 2005; Besserer et al., 2006). Both AM fungi and parasitic weeds are obligate biotrophs, relying on carbon provided by their hosts to complete their life cycle. Arbuscular mycorrhizal symbioses first evolved 460 million years ago and are known to occur with > 80% of land plant species. It is thought that during the co-evolution of the two partners, strigolactones were selected as a host location signal to allow the AM fungi to recognize the roots of hosts. This ancient signalling mechanism provided an opportunity for the evolutionary later parasitic weeds to wiretap. Despite the central role of strigolactones in the rhizosphere communication among plants, AM fungi and root parasitic weeds (Bouwmeester et al., 2007), little is known about the molecular mechanisms underlying strigolactone-mediated signalling and regulation in these interactions. In this issue of New Phytologist, López-Ráez et al. (pp. 863–874) have provided a new insight into the biogenesis of strigolactones and its regulation by nutrient availability in plants through different approaches such as using a carotenoid biosynthesis inhibitor and an abscisic acid-deficient tomato mutant, using bioassays for parasitic weed germination/AM fungal hyphal branching, and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) analysis.
‘... increased exudation of strigolactones in roots upon phosphate starvation is likely to be a general phenomenon in the hosts of AM fungi.’
To date, nine naturally occurring strigolactones have been identified in plant root exudates (Bouwmeester et al., 2007; Xie et al., 2007, 2008a,b; Matsuura et al., 2008; Fig. 1). They are composed of a tricyclic lactone that connects, via an enol ether bond, to a methylbutenolide ring and have long been regarded as sesquiterpenoids. However, using carotenoid mutants of maize, and inhibitors of isoprenoid pathways on maize, sorghum and cowpea, Matusova et al. (2005) recently showed that the tricyclic lactone of strigolactones for Striga and Orobanche spp. was derived from carotenoids. Specifically, the tricyclic lactone was shown to be derived from the C40 carotenoids that originate from the plastidic, nonmevalonate methylerythritol phosphate (MEP) pathway. The present work by López-Ráez et al. corroborates this further, showing that tomato strigolactones (orobanchol and solanacol) are biosynthetically derived from carotenoids. It is thus likely that a carotenoid origin of strigolactones within the plant kingdom is commonplace.
The oxidative cleavage of carotenoids is catalyzed by a family of carotenoid cleavage dioxygenases (CCDs) (Auldridge et al., 2006). CCDs exhibit specificity for the double bond that they cleave, but many are promiscuous in their substrate choice. Although the nature of substrate carotenoids leading to the tricyclic lactone and the identity of the CCD enzymes involved remains to be determined, a CCD, probably 9-cis-epoxycarotenoid dioxygenase, cleaves 11,12 double bonds of 9-cis-β-carotene to give a putative C15 precursor aldehyde in the proposed biosynthetic pathway (Matusova et al., 2005; Bouwmeester et al., 2007; Fig. 1).
Coupling of the methylbutenolide ring to the tricyclic lactone via the enol ether will lead to 5-deoxystrigol, which itself is the first completed product in the strigolactone biosynthesis capable of acting as a host location signal for AM fungi and parasitic weeds. 5-Deoxystrigol can also serve as a precursor in the biosynthetic pathway, providing greater structural diversity of strigolactones. Strigol, orobanchol and sorgomol are derived from 5-deoxystrigol, possibly by P450-catalyzed hydroxylation at C-5, C-4 and C-9, respectively. The two former strigolactones are further converted by O-acetylation into strigyl acetate and orobanchyl acetate, the latter of which has recently been demonstrated to be alectrol (Matsuura et al., 2008; Xie et al., 2008a). Sorgolactone can be biosynthesized from sorgomol via oxidative decarboxylation at C-9. This suggests that acidic strigolactones with a carboxylic acid functionality could be present as a member of strigolactones. Solanacol is the first strigolactone that possesses an aromatic benzene ring in the tricyclic lactone moiety (Xie et al., 2007). It is very difficult to envisage a carotenoid origin of the aromatic ring, but this compound is also likely to be derived from carotenoids, as suggested by its similar behavior to the other strigolactones in tomato roots upon various treatments.
Regulation of strigolactone production by nutrient availability
There have been several observations suggesting that the nutrient status of plants, especially phosphate availability, affects the production and exudation of strigolactones by roots (Bouwmeester et al., 2007). The first quantitative LC-MS/MS analysis of strigolactones in the exudates of roots grown under different nutrient conditions was conducted on red clover, which is a host for both AM fungi and the parasitic weed Orobanche minor (Yoneyama et al., 2007a). In the plant, reduced supply of phosphate, but not of the other elements such as nitrogen, potassium, magnesium and calcium, significantly promotes the release of orobanchol by the roots. The same research group also showed that nitrogen deficiency, as well as phosphate deficiency, promotes the production and exudation of 5-deoxystrigol in the roots of sorghum, which is a host for both AM fungi and the parasitic weed Striga hermonthica (Yoneyama et al., 2007b). López-Ráez et al. found, by LC-MS/MS analysis, that phosphate starvation markedly increased the exudation of orobanchol, solanacol and two or three didehydro-orobanchol isomers in the roots of tomato, which is a host for both AM fungi and the parasitic weeds Orobanche ramosa and Orobanche aegyptiaca. Taken together, increased exudation of strigolactones in roots upon phosphate starvation is likely to be a general phenomenon in the hosts of AM fungi.
The plastid-localized carotenoid biosynthetic pathway is now known to play a key role in strigolactone biosynthesis, but the nature of intermediate metabolites and the identity of the enzymes involved are yet to be determined. Strigolactone-defective mutants, or the modification of strigolactone biosynthesis through genetic engineering, will be essential not only for elucidating the biosynthetic pathway of strigolactones, but also in providing a definitive answer to the question of whether strigolactones are essential in AM symbiosis.
Another major task is to identify the strigolactone receptor(s) and strigolactone-activated signalling pathways in AM fungi and parasitic weeds. Strigolactones show potent activity at very low concentrations, suggesting a highly sensitive perception system for strigolactones present in these two organisms. Labelled strigolactone analogues have been synthesized to aid the isolation and purification of the strigolactone receptor (Reizelman et al., 2003), but as yet the receptor remains to be elucidated. Further study will provide insights into the origin and evolution of strigolactone receptors in AM fungi and parasitic weeds.
LC-MS/MS analysis, in combination with parasitic weed germination and AM fungal hyphal branching bioassays, has facilitated the search and identification of known and novel strigolactones. In addition to nine natural strigolactones, several novel molecules have been detected from tomato, pea, carrot, tobacco, Chinese milk vetch, eggplant, cucumber, linseed and sorghum by LC-MS/MS analysis (Yoneyama et al., 2006). Theoretical considerations based on the ubiquitous occurrence of AM symbiosis suggest that strigolactones are present throughout the host plants of AM fungi, including angiosperms, gymnosperms, pteridophytes (including psilotophytes and lycopods) and some mosses. However, the very recent discovery of orobanchol in the root exudates of Arabidopsis thaliana, a nonhost of AM fungi but a host of O. aegyptiaca (Goldwasser et al., 2008), indicates that strigolactones are distributed beyond the host range of AM fungi, suggesting these molecules to be a common component in plants. Furthermore, LC-MS/MS analysis of sorghum clearly showed that strigolactones are not restricted in roots. Sorghum shoots contain trace amounts of 5-deoxystrigol, the level of which, unlike in the roots, was not affected by both nitrogen and phosphate availability (Yoneyama et al., 2007b). These facts imply that strigolactones may have other biological functions in plants that are yet to be revealed.