Specialized diterpenoid metabolism in monocot crops: Biosynthesis and chemical diversity

Among the myriad specialized metabolites that plants employ to mediate interactions with their environment, diterpenoids form a chemically diverse group with vital biological functions. A few broadly abundant diterpenoids serve as core pathway intermediates in plant general metabolism. The majority of plant diterpenoids, however, function in specialized metabolism as often species-specific chemical defenses against herbivores and microbial diseases, in below-ground allelopathic interactions, as well as abiotic stress responses. Dynamic networks of anti-microbial diterpenoids were first demonstrated in rice (Oryza sativa) over four decades ago, and more recently, unique diterpenoid blends with demonstrated antibiotic bioactivities were also discovered in maize (Zea mays). Enabled by advances in -omics and biochemical approaches, species-specific diterpenoid-diversifying enzymes have been identified in these and other Poaceous species, including wheat (Triticum aestivum) and switchgrass (Panicum virgatum), and are discussed in this article with an emphasis on the critical diterpene synthase and cytochrome P450 monooxygenase families and their products. The continued investigation of the biosynthesis, diversity, and function of terpenoid-mediated crop defenses provides foundational knowledge to enable the development of strategies for improving crop resistance traits in the face of impeding pest, pathogen, and climate pressures impacting global agricultural production.