In Planta Variation of Volatile Biosynthesis: An Alternative Biosynthetic Route to the Formation of the Pathogen-Induced Volatile Homoterpene DMNT via Triterpene Degradation in Arabidopsis Roots

Abstract

Plant-derived volatile compounds such as terpenes exhibit substantial structural variation and serve multiple ecological functions. Despite their structural diversity, volatile terpenes are generally produced from a small number of core 5-to 20-carbon intermediates. Here, we present unexpected plasticity in volatile terpene biosynthesis by showing that irregular homo/ norterpenes can arise from different biosynthetic routes in a tissue specific manner. While Arabidopsis thaliana and other angiosperms are known to produce the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) or its C 16 -analog (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene by the breakdown of sesquiterpene and diterpene tertiary alcohols in aboveground tissues, we demonstrate that Arabidopsis roots biosynthesize DMNT by the degradation of the C 30 triterpene diol, arabidiol. The reaction is catalyzed by the Brassicaceae-specific cytochrome P450 monooxygenase CYP705A1 and is transiently induced in a jasmonate-dependent manner by infection with the root-rot pathogen Pythium irregulare. CYP705A1 clusters with the arabidiol synthase gene ABDS, and both genes are coexpressed constitutively in the root stele and meristematic tissue. We further provide in vitro and in vivo evidence for the role of the DMNT biosynthetic pathway in resistance against P. irregulare. Our results show biosynthetic plasticity in DMNT biosynthesis in land plants via the assembly of triterpene gene clusters and present biochemical and genetic evidence for volatile compound formation via triterpene degradation in plants.

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

“…The biosynthetic pathway of DMNT presented here is consistent with previous feeding experiments utilizing deuterated nerolidol, which was converted to DMNT in nine dicotyledonous plant species (Donath and Boland, 1995). A very different pathway of DMNT biosynthesis was described for Arabidopsis roots, in which this compound is formed by degradation of the triterpene arabidiol (Sohrabi et al, 2015). In maize roots, the formation of DMNT by this or any other pathway was not observed .…”

“…degradation (Sohrabi et al, 2015). The P450 enzyme involved in this alternate pathway, CYP705A1, has no apparent sequence similarity to maize CYP92C5 and CYP92C6 (Figure 14).…”

“…Due to the inactive TPS2-Tzi8 allele, we would not expect any DMNT formation in this inbred line. The remaining, low levels of DMNT in Tzi8 (Supplemental Figure 4) may be caused by other terpene synthases that often produce minor amounts of (E)-nerolidol or the degradation of triterpenes (Sohrabi et al, 2015). Further evidence for the crucial role of TPS2 in the formation of linalool, nerolidol, DMNT, and TMTT was provided by a tps2/3::DS transposon insertion line in the W22 background (B.S08.0585; http://acdstagging.org/).…”

“…Instead, DMNT is produced in the Arabidopsis roots via degradation of the triterpene arabidiol (Sohrabi et al, 2015). Although (E)-nerolidol synthases have been identified in many other plant species (Degenhardt et al, 2009), no enzymes converting this intermediate into DMNT have been found in planta.…”

Plants under dual attack : consequences for plant chemistry and parasitoid behavior

“…All these stressors induce volatile induction via the same general pathways, though there is a degree of attacker specificity in the phytohormonal 'signal signature' (De Vos et al, 2005;Tumlinson & Engelberth, 2008). Phytopathogens can also induce the emission of volatile compounds which are typically and traditionally associated with herbivory, such as GLVs, (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene ((E,E)-TMTT) (Shiojiri et al, 2006;Attaran et al, 2008;Scala et al, 2013) or (E)-DMNT (Sohrabi et al, 2015), all of which show that these volatile compounds can also have important roles in direct plant defense against pathogens. We showed that B. nigra plants emit high levels of GLVs (Fig 4b), particularly (Z)-3-hexen-1-ol, when challenged with the virulent pathovar.…”