Characterization of Biosynthetic Pathways for the Production of the Volatile Homoterpenes DMNT and TMTT in Zea mays

Abstract

Plant volatiles not only have multiple defense functions against herbivores, fungi, and bacteria, but also have been implicated in signaling within the plant and toward other organisms. Elucidating the function of individual plant volatiles will require more knowledge of their biosynthesis and regulation in response to external stimuli. By exploiting the variation of herbivoreinduced volatiles among 26 maize (Zea mays) inbred lines, we conducted a nested association mapping and genome-wide association study (GWAS) to identify a set of quantitative trait loci (QTLs) for investigating the pathways of volatile terpene production. The most significant identified QTL affects the emission of (E)-nerolidol, linalool, and the two homoterpenes (E)-3,8-dimethyl-1,4,7-nonatriene (DMNT) and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). GWAS associated a single nucleotide polymorphism in the promoter of the gene encoding the terpene synthase TPS2 with this QTL. Biochemical characterization of TPS2 verified that this plastid-localized enzyme forms linalool, (E)-nerolidol, and (E,E)-geranyllinalool. The subsequent conversion of (E)-nerolidol into DMNT maps to a P450 monooxygenase, CYP92C5, which is capable of converting nerolidol into DMNT by oxidative degradation. A QTL influencing TMTT accumulation corresponds to a similar monooxygenase, CYP92C6, which is specific for the conversion of (E,E)-geranyllinalool to TMTT. The DMNT biosynthetic pathway and both monooxygenases are distinct from those previously characterized for DMNT and TMTT synthesis in Arabidopsis thaliana, suggesting independent evolution of these enzymatic activities.

Selinene Volatiles Are Essential Precursors for Maize Defense Promoting Fungal Pathogen Resistance

“…More specifically, in maize, the use of terpene as a keyword search in Phytozome (https://phytozome.jgi.doe.gov) currently reveals more than 30 Tps gene models. Collective efforts have resulted in the genetic, biochemical, and ecological characterization of approximately half of the maize enzymes encoded by Tps with product specificity in the production of monoterpenes, sesquiterpenes, and diterpenes (Schnee et al, 2002(Schnee et al, , 2006Köllner et al, 2004bKöllner et al, , 2008aDegenhardt et al, 2009;Fu et al, 2016;Richter et al, 2016). Maize terpene volatiles are often highly inducible following insect attack and aid in the attraction of diverse natural enemies both aboveground and belowground (Turlings et al, 1990;Rasmann et al, 2005;Degenhardt et al, 2009a).…”

“…The discovery of further immune-related biochemical traits is certain to continue, given the extreme genetic diversity in maize highlighted by over 8,000 representative transcript assemblies detectable in diverse germplasm that are absent from B73 (Hirsch et al, 2014). To fill existing voids highlighted by comparative genomics, the combined application of metabolomics, mapping, and in vitro biochemistry provides a useful approach to rapidly connect phenotypes with genotypes (Meihls et al, 2013;Handrick et al, 2016;Richter et al, 2016). Our current identification of ZmTps21 as a b-selinene synthase required for b-costic acid production adds to the foundational knowledge of useful maize biochemical pathways that can be combined intentionally to combat complex biotic pressures.…”

Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize

“…1), are part of a complex repertoire of defense-related specialized metabolites, such as phenylpropanoids, oxylipins, benzoxazinoids, and volatile terpenoids (Schnee et al, 2006;Köllner et al, 2008a;Santiago and Malvar, 2010;Ahmad et al, 2011;Huffaker et al, 2011;Schmelz et al, 2011;Christensen et al, 2015;Richter et al, 2016;Wouters et al, 2016). Inducible zealexin and kauralexin production contributes to defense, both aboveground and belowground, against pathogenic fungi, including Fusarium, Aspergillus, and Colletotrichum spp.…”

Network-Guided GWAS Improves Identification of Genes Affecting Free Amino Acids

“…GWAS of the ratio of d-tocotrienol to the sum of g-and a-tocotrienols in maize grain also found a more highly significant association with a known tocochromanol biosynthesis gene than any of the absolute tocotrienol levels . Lastly, a joint-linkage-assisted GWAS of the ratio of two homoterpenes in the maize nested association mapping panel facilitated the identification of a new cytochrome 450 gene involved in the pathway (Richter et al, 2016).Here, we report and compare findings from GWAS performed on a broad range of biochemically based and network-derived FAA ratios calculated from a previous quantification of 18 FAAs in an Arabidopsis association panel (Angelovici et al, 2013). These metabolic ratios integrate both the extensive biochemical knowledge of amino acid pathways and the inherent coordinated behaviors of FAAs in seeds as deduced from a correlation-based network topology.…”

“…GWAS of the ratio of d-tocotrienol to the sum of g-and a-tocotrienols in maize grain also found a more highly significant association with a known tocochromanol biosynthesis gene than any of the absolute tocotrienol levels . Lastly, a joint-linkage-assisted GWAS of the ratio of two homoterpenes in the maize nested association mapping panel facilitated the identification of a new cytochrome 450 gene involved in the pathway (Richter et al, 2016).…”