Calliterpenone modulates AtGGPPS1 expression and terpenoid pathway flux to regulate growth in Arabidopsis

Calliterpenone (CT), a natural diterpenoid, has emerged as a potential plant growth regulator; however, though its molecular mechanism of action remains largely unclear. Given its structural resemblance to gibberellins (GAs), we hypothesized that CT influences growth by modulating GA biosynthesis and isoprenoid precursor pathways. Phenotypic analyses revealed that CT enhances root length, biomass, and chlorophyll content, similar to GA₃, with synergistic effects under combined treatment (CT + GA₃). Exogenous CT application promoted early bolting, larger rosettes, increased plant height, and higher reproductive yield. Gene expression analyses indicated dose-dependent regulation of GA homeostasis: low CT induced GA biosynthetic genes (AtCPS, AtKS, AtKO, AtKAO1, AtGA3OX1, AtGA3OX3) and suppressed catabolic genes (AtGA2OX2, AtGA2OX3), while high CT had the opposite effect. CT also differentially regulated terpenoid precursor pathways, enhancing plastidial MEP flux at low concentrations and activating cytosolic MVA pathway genes at high concentrations. Among Geranylgeranyl diphosphate synthase (GGPPS) isoforms, AtGGPPS1 showed maximal responsiveness, validated through promoter:GUS assays. Functional studies using overexpression (AtGGPPS1OX) and CRISPR/Cas9 knockout (atggpps1^CR) lines confirmed CT-mediated growth regulation and compensatory MEP pathway activation. These findings establish CT as a GA-analogous diterpenoid that promotes plant growth through transcriptional regulation of AtGGPPS1 and isoprenoid pathway modulation.