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Artemisia annua is the only natural source of the sesquiterpenoid artemisinin, which is widely used to treat malaria. The phytohormone jasmonic acid (JA) can significantly promote artemisinin biosynthesis in A. annua. AabHLH1 can bind and activate artemisinin biosynthetic genes, such as AaADS and AaCYP71AV1. In this study, we proved that AabHLH1 was responsive to MeJA treatment and highly expressed in glandular trichome-enriched tissues, and that its expression profile was similar to that of AaADS. Yeast two-hybrid assays showed that AabHLH1 interacted with all nine AaJAZ proteins in A. annua. Functional analysis with transgenic plants showed that several artemisinin biosynthetic genes were upregulated in AabHLH1-OE transgenic A. annua lines and downregulated in AabHLH1-EAR lines; furthermore, the artemisinin content was increased in the AabHLH1-OE lines and decreased in the AabHLH1-EAR lines. These results demonstrate that the JA-induced AabHLH1 positively regulates artemisinin biosynthesis by regulating the biosynthetic genes, and thus provide new insight into the regulatory mechanism of JA-induced artemisinin biosynthesis in A. annua., (© 2019 International Union of Biochemistry and Molecular Biology, Inc.)

Summary: Plant glandular secretory trichomes (GSTs) produce various specialized metabolites. Increasing GST density represents a strategy to enhance the yield of these chemicals; however, the gene regulatory network that controls GST initiation remains unclear.In a previous study of Artemisiaannua L., we found that a HD‐ZIP IV transcription factor, AaHD1, promotes GST initiation by directly regulating AaGSW2. Here, we identified two AaHD1‐interacting transcription factors, namely AaMIXTA‐like 2 (AaMYB16) and AaMYB5.Through the generation and characterization of transgenic plants, we found that AaMYB16 is a positive regulator of GST initiation, whereas AaMYB5 has the opposite effect. Notably, neither of them regulates GST formation independently. Rather, they act competitively, by interacting and modulating AaHD1 promoter binding activity. Additionally, the phytohormone jasmonic acid (JA) was shown to be associated with the AaHD1‐AaMYB16/AaMYB5 regulatory network through transcriptional regulation via a JASMONATE‐ZIM DOMAIN (JAZ) protein repressor.These results bring new insights into the mechanism of GST initiation through regulatory complexes, which appear to have similar functions in a range of vascular plant taxa. [ABSTRACT FROM AUTHOR]

Summary: Artemisinin, a sesquiterpene lactone widely used in malaria treatment, was discovered in the medicinal plant Artemisia annua. The biosynthesis of artemisinin is efficiently regulated by jasmonate (JA) and abscisic acid (ABA) via regulatory factors. However, the mechanisms linking JA and ABA signalling with artemisinin biosynthesis through an associated regulatory network of downstream transcription factors (TFs) remain enigmatic. Here we report AaTCP15, a JA and ABA dual‐responsive teosinte branched1/cycloidea/proliferating (TCP) TF, which is essential for JA and ABA‐induced artemisinin biosynthesis by directly binding to and activating the promoters of DBR2 and ALDH1, two genes encoding enzymes for artemisinin biosynthesis. Furthermore, AaORA, another positive regulator of artemisinin biosynthesis responds to JA and ABA, interacts with and enhances the transactivation activity of AaTCP15 and simultaneously activates AaTCP15 transcripts. Hence, they form an AaORA‐AaTCP15 module to synergistically activate DBR2, a crucial gene for artemisinin biosynthesis. More importantly, AaTCP15 expression is activated by the multiple reported JA and ABA‐responsive TFs that promote artemisinin biosynthesis. Among them, AaGSW1 acts at the nexus of JA and ABA signalling to activate the artemisinin biosynthetic pathway and directly binds to and activates the AaTCP15 promoter apart from the AaORA promoter, which further facilitates formation of the AaGSW1‐AaTCP15/AaORA regulatory module to integrate JA and ABA‐mediated artemisinin biosynthesis. Our results establish a multilayer regulatory network of the AaGSW1‐AaTCP15/AaORA module to regulate artemisinin biosynthesis through JA and ABA signalling, and provide an interesting avenue for future research exploring the special transcriptional regulation module of TCP genes associated with specialized metabolites in plants. [ABSTRACT FROM AUTHOR]

Six transcription factors of APETALA2/ethylene-response factor ( AP2/ ERF) family were cloned and analyzed in Artemisia annua. Real-time quantitative polymerase chain reaction ( RT- Q- PCR) showed that Aa ORA exhibited similar expression patterns to those of amorpha-4,11-diene synthase gene ( ADS), cytochrome P450-dependent hydroxylase gene ( CYP71AV1) and double bond reductase 2 gene ( DBR2) in different tissues of A. annua., Aa ORA is a trichome-specific transcription factor, which is expressed in both glandular secretory trichomes ( GSTs) and nonglandular T-shaped trichomes ( TSTs) of A. annua. The result of subcellular localization shows that Aa ORA is targeted to the nuclei and the cytoplasm., Overexpression and RNA interference ( RNAi) of Aa ORA in A. annua regulated, positively and significantly, the expression levels of ADS, CYP71 AV1, DBR2 and Aa ERF1. The up-regulated or down-regulated expression levels of these genes resulted in a significant increase or decrease in artemisinin and dihydroartemisinic acid. The results demonstrate that Aa ORA is a positive regulator in the biosynthesis of artemisinin., Overexpression of Aa ORA in Arabidopsis thaliana increased greatly the transcript levels of the defense marker genes PLANT DEFENSIN1.2 ( PDF1.2), HEVEIN-LIKE PROTEIN ( HEL) and BASIC CHITINASE ( B-CHI). After inoculation with Botrytis cinerea, the phenotypes of Aa ORA overexpression in A. thaliana and Aa ORA RNAi in A. annua demonstrate that Aa ORA is a positive regulator of disease resistance to B. cinerea. [ABSTRACT FROM AUTHOR]