A bHLH transcription factor AaMYC2-type positively regulates glandular trichome density and artemisinin biosynthesis in Artemisia annua

Abstract

Artemisinin-based combinational therapies (ACTs) constitute the first line of malaria treatment. However, due to its trichome-specific biosynthesis, low concentration, and poor understanding of regulatory mechanisms involved in artemisinin biosynthesis and trichome development, it becomes very difficult to meet the increased demand for ACTs. Here, we have reported that a bHLH transcription factor, AaMYC2-type, plays an important role in regulating GST development and artemisinin biosynthesis in Artemisia annua. AaMYC2-type encodes a protein that is transcriptionally active and localised to the nucleus. It is prominently expressed in aerial parts like leaves, stems, inflorescence and least expressed in roots. AaMYC2-type expression is significantly increased under different hormonal treatments. In transgenic overexpression lines, AaMYC2-type OE, a significant increase in the expression of trichome development and artemisinin biosynthesis genes was observed. While in knockdown lines, Aamyc2-type, expression of trichome development and artemisinin biosynthesis genes were significantly reduced. Yeast one-hybrid assay clearly shows that the AaMYC2-type directly binds to the E-boxes in the promoter regions of ADS and CYP71AVI. The SEM microscopy depicted the number of trichomes elevated from 11 mm^-2 in AaMYC2-type OE lines to 6.1 mm^-2 in Aamyc2-type. The final effect of the alteration in biosynthetic and trichome developmental genes was observed in the accumulation of artemisinin. In the AaMYC2-type OE, the artemisinin content was 12 mg g^-1DW, which was reduced to 3.2 mg g^-1DW in the Aamyc2-type. Altogether, the above findings suggest that the AaMYC2-type play a dual regulating role in controlling both trichome developmental and artemisinin biosynthetic genes.