Biosynthesis and Emission of Stress-Induced Volatile Terpenes in Roots and Leaves of Switchgrass (Panicum virgatum L.)

Abstract

Switchgrass (Panicum virgatum L.), a perennial C4 grass, represents an important species in natural and anthropogenic grasslands of North America. Its resilience to abiotic and biotic stress has made switchgrass a preferred bioenergy crop. However, little is known about the mechanisms of resistance of switchgrass against pathogens and herbivores. Volatile compounds such as terpenes have important activities in plant direct and indirect defense. Here, we show that switchgrass leaves emit blends of monoterpenes and sesquiterpenes upon feeding by the generalist insect herbivore Spodoptera frugiperda (fall armyworm) and in a systemic response to the treatment of roots with defense hormones. Belowground application of methyl jasmonate also induced the release of volatile terpenes from roots. To correlate the emission of terpenes with the expression and activity of their corresponding biosynthetic genes, we identified a gene family of 44 monoterpene and sesquiterpene synthases (mono- and sesqui-TPSs) of the type-a, type-b, type-g, and type-e subfamilies, of which 32 TPSs were found to be functionally active in vitro. The TPS genes are distributed over the K and N subgenomes with clusters occurring on several chromosomes. Synteny analysis revealed syntenic networks for approximately 30–40% of the switchgrass TPS genes in the genomes of Panicum hallii, Setaria italica, and Sorghum bicolor, suggesting shared TPS ancestry in the common progenitor of these grass lineages. Eighteen switchgrass TPS genes were substantially induced upon insect and hormone treatment and the enzymatic products of nine of these genes correlated with compounds of the induced volatile blends. In accordance with the emission of volatiles, TPS gene expression was induced systemically in response to belowground treatment, whereas this response was not observed upon aboveground feeding of S. frugiperda. Our results demonstrate complex above and belowground responses of induced volatile terpene metabolism in switchgrass and provide a framework for more detailed investigations of the function of terpenes in stress resistance in this monocot crop.

The foxtail millet (Setaria italica) terpene synthase gene family

“…SiTPS1 and SiTPS2 are co-located on chromosome 1 and show 82% protein sequence identity. Yet, unlike SiTPS1, SiTPS2 lacks syntenic orthologs in several analyzed genomes and appears to be devoid of a plastidial transit peptide, supporting the emergence of SiTPS2 through a more recent gene duplication event and possible alteration of compartmentalization and associated substrate restriction as also proposed for functionally redundant acyclic TPSs in switchgrass and snapdragon (Antirrhinum majus) (Nagegowda et al, 2008;Muchlinski et al, 2019). Similarly, two d-cadinene synthases, SiTPS19 and SiTPS27, showed catalytic redundancy, yet distinct patterns of pathway regulation.…”

“…In particular, the potent pest-and disease-protective functions of terpenoids in maize and rice have been well established (reviewed in Schmelz et al, 2014;Murphy and Zerbe, 2020). In addition, diverse terpenoid metabolic networks comprising both common and unique pathway branches and products have been described in wheat and switchgrass, suggesting similar physiological roles (Wu et al, 2012;Zhou et al, 2012;Pelot et al, 2018;Muchlinski et al, 2019). projected increased harvest losses caused by shifting environmental pressures and associated biotic and abiotic stressors that can overwhelm the natural defense systems of plants (Chakraborty and Newton, 2011;de Sassi and Tylianakis, 2012;Leng and Hall, 2019).…”

“…Among these, the catalytically redundant SiTPS1 and SiTPS2 of the TPS-g clade showed dual activity, converting FPP into trans-nerolidol and GGPP into geranyllinalool. Related TPS-g enzymes identified, for example, in Arabidopsis, switchgrass, tomato (Solanum lycopersicum) and maize function in forming the geranyllinalool-derived 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) with antifeedant activity (Ament et al, 2004;Herde et al, 2008;Richter et al, 2016;Muchlinski et al, 2019). SiTPS1 and SiTPS2 are co-located on chromosome 1 and show 82% protein sequence identity.…”

“…Putative functions of these TPSs in sesquiterpenoid metabolism were further supported by the absence of the N-terminal RRx 8 W motif characteristic of mono-TPSs and the lack of plastidial transit peptides in at least 19 of the identified TPS candidates. Close phylogenetic relationships of several S. italica TPS candidates with known sesqui-TPSs from related monocot species, such as maize and switchgrass, suggested similar sesqui-TPS activities (Schnee et al, 2002;K€ ollner et al, 2008a;K€ ollner et al, 2008b, K€ ollner et al, 2009Muchlinski et al, 2019).…”

“…Syntenic TPS networks on S. italica chromosomes 3, 6, 7 and 9 are mostly conserved on the respective homologous chromosomes 3, 6, 7 and 9 of S. viridis and P. hallii, consistent with the expected collinearity between genomes of closely related species (Table S3). Select syntenic network associations included genes of known function, including switchgrass 1,8-cineole synthase (PvTPS3, chromosome 3), b-bisabolene synthase (PvTPS17, chromosome 6), and a germacrene D-synthase (PvTPS55, chromosome 9) (Muchlinski et al, 2019). Additionally, TPSs on S. italica chromosome 6 featured syntenic orthologs mapping to one chromosome in S. viridis (chromosome 6), P. hallii (chromosome 6), B. distachyon (chromosome 3), S. bicolor (chromosome 7) and Z. mays (chromosome 10), the latter including ZmTPS10 producing a-bergamotene and b-farnesene (K€ ollner et al, 2009).…”