Abstract
SUMMARYTerpenoid metabolism plays vital roles in stress defense and the environmental adaptation of monocot crops. Here, we describe the identification of the terpene synthase (TPS) gene family of the panicoid food and bioenergy model crop foxtail millet (Setaria italica). The diploid S. italica genome contains 32 TPS genes, 17 of which were biochemically characterized in this study. Unlike other thus far investigated grasses, S. italica contains TPSs producing all three ent‐, (+)‐ and syn‐copalyl pyrophosphate stereoisomers that naturally occur as central building blocks in the biosynthesis of distinct monocot diterpenoids. Conversion of these intermediates by the promiscuous TPS SiTPS8 yielded different diterpenoid scaffolds. Additionally, a cytochrome P450 monooxygenase (CYP99A17), which genomically clustered with SiTPS8, catalyzes the C19 hydroxylation of SiTPS8 products to generate the corresponding diterpene alcohols. The presence of syntenic orthologs to about 19% of the S. italica TPSs in related grasses supports a common ancestry of selected pathway branches. Among the identified enzyme products, abietadien‐19‐ol, syn‐pimara‐7,15‐dien‐19‐ol and germacrene‐d‐4‐ol were detectable in planta, and gene expression analysis of the biosynthetic TPSs showed distinct and, albeit moderately, inducible expression patterns in response to biotic and abiotic stress. In vitro growth‐inhibiting activity of abietadien‐19‐ol and syn‐pimara‐7,15‐dien‐19‐ol against Fusarium verticillioides and Fusarium subglutinans may indicate pathogen defensive functions, whereas the low antifungal efficacy of tested sesquiterpenoids supports other bioactivities. Together, these findings expand the known chemical space of monocot terpenoid metabolism to enable further investigations of terpenoid‐mediated stress resilience in these agriculturally important species.
Highlights
Foxtail millet (Setaria italica) is a grain crop of the grass family (Panicoideae; Poaceae) that was domesticated from its wild relative green foxtail (Setaria viridis) in northern China over 8700 years ago (Lu et al, 2009)
Most terpene synthase (TPS) genes mapped to S. italica chromosomes 1, 6 and 7, whereas no TPS genes were located on chromosomes 4 and 5 (Table S2)
SiTPS34 and SiTPS35 clustered most closely with characterized ent-copalyl pyrophosphate (CPP) synthases of general metabolism (e.g. Z. mays ANTHER EAR 1, ZmAn1) or specialized metabolism (e.g. Z. mays ANTHER EAR 2, ZmAn2), respectively, and contained the catalytic His–Asn dyad conserved among ent-CPP synthases involved in GA biosynthesis (Harris et al, 2005; Ko€ksal et al, 2014; Potter et al, 2014; Lemke et al, 2019), suggesting related functionalities (Figures 1a and S1)
Summary
Foxtail millet (Setaria italica) is a grain crop of the grass family (Panicoideae; Poaceae) that was domesticated from its wild relative green foxtail (Setaria viridis) in northern China over 8700 years ago (Lu et al, 2009) Valued for their nutritional benefits, climate resilience and low-input cultivation, foxtail millet and related small millets are cultivated in many arid and semi-arid regions of Asia, southern Europe, India, South America and North Africa (Goron and Raizada, 2015; Pant et al, 2016). Rice accumulates distinct groups of stress-inducible diterpenoid phytoalexins with potent antimicrobial efficacy against major rice pathogens such as rice blast (Magnaporthe oryza) (Xu et al, 2012; Bagnaresi et al, 2012; Han et al, 2015; Lu et al, 2018). Species-specific arrays of stress-elicited volatile sesquiterpenoids and non-volatile diterpenoids have been identified in maize and shown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
