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Regulation of the formation of germination stimulants of parasitic weed seeds in tomato roots

Final Activity Report Summary - TOMGERMSTIM (Regulation of the formation of germination stimulants of parasitic weed seeds in tomato roots)

Host plant root exudates contain germination stimulant (strigolactones) for parasitic weed seeds which play an important role in the interaction between host and parasitic plants. Moreover, these secondary metabolites act also as hyphal branching factor for arbuscular mycorrhiza (AM) spores (Bouwmeester et al., 2007). Strigolactones are very important for the lifecycle of both organisms, however so far very little is known about their biosynthetic origin and pathway. We have demonstrated that the strigolactones produced by tomato roots are also derived from carotenoids (López-Ráez et al., in press), as we previously showed for other plant species like maize, cowpea and sorghum (Matusova et al., 2005). Therefore, it can be stated that the carotenoid origin of the strigolactones is a general fact in the plant kingdom. In addition, we have shown that tomato's strigolactones present germination stimulatory activity for Orobanche ramosa seeds and hyphal branching activity for Gigaspora spp, and that their de novo biosynthesis is promoted under limited phosphate (Pi) conditions (López-Ráez et al., in press). Moreover, by LC-MS/MS we have demonstrated that the major tomato strigolactones in the root exudates are orobanchol, the recently described solanacol (Xie et al., 2007) and two to three didehydro-orobanchol isomer(s) (López-Ráez et al., in press).

Although the branching point from the carotenoid pathway for strigolactone biosynthesis has not yet been identified, our previous results have suggested that the ABC-part of the strigolactones is derived from carotenoid cleavage by the action of carotenoid cleavage dioxygenases (CCDs,) and/or 9-cis-epoxycarotenoid dioxygenases (NCEDs) enzymes (Matusova et al., 2005; Bouwmeester et al., 2007). Genes encoding tomato CCDs and NCEDs have been cloned, and their gene expression patterns checked by real time qRT-PCR. In addition, constructs with the corresponding genes to knock out/down their expression, using the RNAi technology, have been made and used to make both transient and stable transformation. No clear results were obtained by transient transformation, and stable transformation is currently being carried out.

On the other hand, by microarrays analysis more than 200 genes induced in tomato roots under limited Pi conditions, which we have shown to promote strigolactone biosynthesis, have been identified. Further characterisation of these genes is currently being performed to find genes involved in the so far unknown biosynthetic pathway for the strigolactones.