Elucidating diterpene biosynthesis in Setaria italica (foxtail millet) towards enhancing stress tolerance in food crops
Authors: Karunanithi Prema Sambandaswami , Berrios David , Wang Sadira , Zerbe Philipp 
Published: June 2021

My research focuses on elucidating the biosynthesis and biological activities of diterpenes in the model food crop Setaria italica (foxtail millet) with the goal of improving resistance against biotic and abiotic stresses in food crops. To increase global food production of agricultural crops by 70% for feeding an additional 2.3 billion people by 2050, knowledge of plant chemical defenses is necessary for limiting crop yield losses due to drought and disease. Plant terpenes are small molecule metabolites with critical roles in plant development (gibberellin phytohormones) and the defense against pests, pathogens and other environmental stresses such as drought and heavy metal stress. Key enzyme classes in forming the vast diversity of more than 70,000 plant terpenes are terpene synthases (TPS) and cytochrome P450‐dependent monooxygenases. TPS catalyze various cyclization and rearrangement reactions of prenyl‐diphosphates to produce the initial structural scaffolds, which are then further functionally modified (e.g. through hydroxylations) by P450 enzymes. Using our transcriptomics‐enabled gene discovery approach combined with plant‐based (Nicotiana benthamiana) and microbial (E. coli) co‐expression enzyme activity assays, and structure elucidation using GC‐MS and NMR analysis, we characterized the diterpene (20‐carbon terpene) synthase family of S. italica (SiDTPS) and one P450 (SiP450‐1) involved in C‐19 hydroxylation of diterpenes produced by SiDTPSs. S. italica may provide a unique insight into diterpene metabolism, since it is so far, the only species to contain all currently known diterpene precursors (copalyl diphosphate (CPP) of normal, ent‐, and syn‐ stereochemistry), whereas other major crops feature no more than two of the required enzymes. While (ent‐ and syn)‐CPP derived diterpenoids have been well characterized in rice (Oryza sativa) and maize (Zea mays) to confer quantitative pathogen defense, little is known about the role of (+)‐CPP‐derived diterpenes in crop stress resistance. Gene expression studies using qPCR show drought‐ and fungal elicitor (mycoprotein)‐inducible patterns of gene expression for select SiDTPSs and SiP450‐1 while diterpene products exhibit anti‐fungal activity in vitro against the major fungal pathogen, Fusarium verticilloides, suggesting a potential role in mediating fungal‐stress tolerance in planta. In collaboration with the DOE Joint Genome Institute, we will further work to develop LC‐MS protocols to detect the various identified diterpenoids from SiDTPS and SiP450‐1 diterpene enzyme products in planta and enable downstream genetic analysis of their biological activities.Support or Funding InformationDepartment of Plant Biology, UC Davis‐College of Biological SciencesJoint Genome Institute (DOE‐JGI), Grant CSP: DNA Synthesis #2568NIH‐NIGMS T32 GM007377, UC Davis Molecular and Cell Biology Training Grant ProgramDean's Mentorship Award 2017, UC Davis‐College of Biological ScienesThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.