Role of terpene synthases in the defense of Medicago truncatula against Aphanomyces euteiches

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Plants as sessile organisms interact with a variety of organisms, including pathogens such as the oomycete Aphanomyces euteiches, which causes root rot in legumes. The early contact of Medicago truncatula roots with zoospores of A. euteiches leads to an enhanced expression of genes encoding terpene synthases (TPSs). Among them, MtTPS10 was identified as a multi-product sesquiterpene synthase specifically expressed in roots and only upon contact with zoospores of oomycetes. MtTPS10 products, such as the major product himachalol, contribute to the resistance of M. truncatula roots to A. euteiches but are modified in planta. First candidate cytochrome P450 enzyme (CYP-A) converting himachalol has been identified. Furthermore, analysis of different ecotypes of M. truncatula showed that there is often a correlation between MtTPS10 expression levels and resistance to A. euteiches. One exception is line 368, which is highly resistant to A. euteiches but completely lacks MtTPS10 expression. Transcriptome profiling revealed the expression of a previously uncharacterized terpene synthase (MtTPS-X) in line 368. In the renewal of the project the following hypotheses will be tested: (i) MtTPS-X contributes to the immunity of M. truncatula line 368; (ii) transgenic expression of MtTPS10 and MtTPS-X increases resistance of potato plants to Phytophthora infestans; and (iii) CYP-A converts MtTPS10 products in planta. It is planned to functionally proof the role of MtTPS-X in line 368 in defense against A. euteiches by transgenic approaches. Furthermore, MtTPS10 and MtTPS-X will be constitutively expressed in potato plants followed by testing their resistance to P. infestans. In addition, the function of CYP-A in the conversion of himachalol in planta will be analyzed by characterization of mutant plants. These studies are expected to greatly enhance our understanding of the role of TPSs and their products in root defense responses and will be important not only for potential control of oomycete-induced root diseases, but for control of oomycete-induced plant diseases in general.