Targeted Gene Modification of HMGR Enhances Biosynthesis of Terpenoid and Phenylpropanoid Volatiles in Petunia and Lettuce

Terpenoids constitute the largest class of plant-specialized metabolites, playing essential roles throughout the plants' life cycle and having diverse applications for humans in nutrition, medicine, and flavor. 3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) is a rate-limiting enzyme of the mevalonate (MVA) pathway, producing sesquiterpenes, saponins, and other terpenoids. HMGR is post-translationally regulated by downstream MVA products through its N-terminal regulatory domain, limiting terpenoid production. To overcome this bottleneck, we employed a virus-based CRISPR/Cas9 system to genetically modify the N-terminal regulatory domain of <i>HMGR</i> in petunia (<i>Petunia × hybrida</i>) and lettuce (<i>Lactuca sativa</i> L.). In petunia, <i>HMGR1</i>-edited lines exhibited vigorous growth, larger flowers, and increased production of sesquiterpenes. Interestingly, they also showed enhanced production of phenylpropanoid volatiles, revealing a connection between these pathways. Transcript analysis revealed altered expression of genes involved in terpenoid biosynthesis, pyruvate metabolism, phenylpropanoid biosynthesis, and gibberellin- and auxin-related pathways, indicating enhanced carbon flux through these metabolic networks. In lettuce, <i>HMGR7</i>-edited plants displayed elevated emission of sesquiterpenes, apocarotenoids, and the phenylpropanoid benzaldehyde. Together, these results establish a transgene-free strategy to enhance the production of terpenoid and phenylpropanoid volatiles, and provide a framework for developing resilient, nutrient-enriched crops.