Transcriptome analysis reveals that blocking the ethylene signal transduction pathway is a key point for 2,4-d inhibited shikonin biosynthesis in lithospermum erythrorhizon (boraginaceae)

The synthetic growth hormone2,4-dichlorophenoxyacetic acid (2,4-D) significantly inhibits shikonin and its derivatives biosynthesis in the medicinal plant Lithospermum erythrorhizon. However, the molecular mechanism of this regulation remains unclear.In this study, we attempt to address this issue by comparing the transcriptome of 2,4-D treated cell cultures with that of the control (CK). A total of 216 up-regulated and 269 down-regulated genes by 2,4-D were discovered. Gene ontology (GO) enrichment analysis revealed that differentially expressed genes (DEGs) were statistically significantly related to the metabolic process. Pathway classification enrichment analysis further confirmed that the DEGs were mainly related to the secondary metabolism. More importantly, 2,4-D has no effect on the two key enzyme genes ACS and ACO for ethylene biosynthesis. However,2,4-D significantly down-regulated the genes directly involved in ethylene signal transduction pathway (ESTP), including HMA5, EIN3, and ORCA3. 2,4-D also down-regulated two indirect genes, LOX and GA2ox, thereby conferring the jasmonate (JA) and GA biosynthesis, which regulates ESTP by targeting EIN3. By blocking ESTP, 2,4-D finally down-regulated the phenylpropanoid formation-related genes, thereby conferring shikonin and its derivatives biosynthesis. Our findings provide new hints for the deep understanding of the molecular mechanism of shikonin formation. (author)