Regulation of artemisinin biosynthesis in Artemisia annua L
Abstract: Artemisinin and its derivatives are the most powerful medicines against malaria. However,the low annual yield of artemisinin from industry restricts the usage of this effective drug inendemic areas such as Africa. Nowadays the artemisinin industry mainly depends onextractions from Artemisia annua L. plants, which unfortunately accumulate very littleartemisinin for most varieties. In order to increase artemisinin content in these plants, wemust understand deeply and clearly how the biosynthetic pathway of artemisinin in plantsworks. Many key enzymes have been found and we need to study the genes that encodethem, as well as how these genes are regulated.Amorpha-4,11-diene 12-hydroxylase (CYP71AV1) is one of the key enzymes of theartemisinin biosynthetic pathway. This gene is specifically expressed in glandular trichomes,where artemisinin is synthesized and stored. We cloned the promoter of this gene and withthe GUS fusion assay we managed to confirm that the promoter of this gene is trichomespecific. A few putative cis-acting regulatory elements may control the promoter activity.Artemisinic aldehyde 11(13) reductase (DBR2) is also an important enzyme of thepathway. It allows one of the precursors of artemisinin, i.e. artemisinic aldehyde, to gotowards artemisinin rather than arteannuin B. The gene expression level also differsbetween high artemisinin producer varieties and low artemisinin producer varieties. Theformer group expresses DBR2 much more than the latter group. According to our results,the promoter of the DBR2 gene appears to be the key reason for the different expressionlevel of DBR2 in high and low producers.How the artemisinin biosynthetic pathway is regulated has not been revealed to any greatextent. We used wounding treatment on A. annua plants and found that when the plantsresponded to wounding stress, their artemisinin content increased. Some key genes of thepathway also showed an increased expression. This suggests that artemisinin may be relatedto wounding stress responses and that the regulation factors of the pathway are upregulatedupon wounding stress.The most common regulation factors for plant secondary metabolites biosynthesis aretranscription factors. We cloned an AaMYB1 gene from the MYB family. The geneticallymodified plants, which over-expressed AaMYB1 were tested for their artemisinin content,as well as the expression level of some key genes of the pathway. The artemisinin contentincreased in these transgenic plants and the key genes were also up-regulated by AaMYB1.In addition, over-expressing AaMYB1 in Arbidopsis thaliana shows that the gene is able toregulate trichome initiation and development as well as gibberellic acid biosynthesis, whichis related to early flowering.
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