Gibberellin promotes xylem expansion and cell lignification by regulating sugar accumulation and the expression of JcMYB43 and JcMYB63 in the woody plant Jatropha curcas

Gibberellins (GAs) are a group of diterpene plant hormones that regulates various plant developmental processes, including wood formation. Nevertheless, the regulatory pattern and the downstream targets of GA in the regulation of xylem expansion and cell lignification in woody plants remains unclear. In transgenic Jatropha curcas with significantly increased or decreased bioactive GA content via separate overexpression of JcGA20ox1 or JcGA2ox6, comparative transcriptomic, metabolomic and physiological investigations were conducted on the young stems. Lignin quantification and ultrastructural investigations of the young stems at different development stages revealed that JcGA20ox1 plants presented much faster lignin deposition and xylem expansion even at early development stages. The transcriptomic results revealed that the majority of the differentially expressed genes (DEGs) in the JcGA20ox1 and JcGA2ox6 plants were mainly related to metabolic pathways. Analysis of the DEGs and the gene regulatory network revealed that the increased lignification was due to the activated expression of several key transcription factors and the structural genes involved in the lignin biosynthesis pathway, which was confirmed by the significantly increased precursors of lignin identified via metabolomic analysis. Interestingly, a total of 15 sugar-related metabolites were identified, most of which were increased in the xylem of JcGA20ox1, but decreased in JcGA2ox6 plants. Importantly, two key GA-responsive transcription factors JcMYB43 and JcMYB63 were identified to play dual roles in promoting both xylem expansion and cell lignification. Conclusively, this study provides novel insights into the molecular mechanism of GA-regulated xylem development in the woody plant.