Allelo-Labdane Diterpenoids from Abies fabri Targeting CYP707A2 to Inhibit Arabidopsis Seed Germination

Gymnosperm plants are rich in allelochemicals that can be used as ideal eco-friendly alternatives to chemical herbicides. Petroleum ether and ethyl acetate crude extracts from the leaves of Abies fabri (Mast.) Craib (Pinaceae) inhibited seed germination of Arabidopsis thaliana. Twenty-four terpenoids were isolated and identified by chromatographic separation and NMR spectral analysis. Compounds 7 and 14–17 showed germination inhibitory activity, among which 7, namely 13-epitorreferol, was selected for mechanistic investigation. Transcriptomic analysis revealed that differentially expressed genes (DEGs) were mainly enriched in stress response and plant hormone signaling, especially abscisic acid (ABA) pathways. The results of the in vitro enzyme activity assay and mutant phenotypic analysis indicated that CYP707A2 (ABA 8′-hydroxylase) was one of the targets mediating the inhibitory activity of 7 on seed germination. This work expands allelochemical resources, offering a tool for investigating ABA signaling and providing leads for the development of eco-friendly herbicides.