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Microalgae are the preferred species for producing astaxanthin because they pose a low toxicity risk than chemical synthesis. Astaxanthin has multiple health benefits and is being used in: medicines, nutraceuticals, cosmetics, and functional foods. Haematococcus pluvialis is a model microalga for astaxanthin biosynthesis; however, its natural astaxanthin content is low. Therefore, it is necessary to develop methods to improve the biosynthesis of astaxanthin to meet industrial demands, making its commercialization cost-effective. Several strategies related to cultivation conditions are employed to enhance the biosynthesis of astaxanthin in H. pluvialis. However, the mechanism of its regulation by transcription factors is unknown. For the first time, this study critically reviewed the studies on identifying transcription factors, progress in H. pluvialis genetic transformation, and use of phytohormones that increase the gene expression related to astaxanthin biosynthesis. In addition, we propose future approaches, including (i) Cloning and characterization of transcription factors, (ii) Transcriptional engineering through overexpression of positive regulators or downregulation/silencing of negative regulators, (iii) Gene editing for enrichment or deletion of transcription factors binding sites, (iv) Hormonal modulation of transcription factors. This review provides considerable knowledge about the molecular regulation of astaxanthin biosynthesis and the existing research gap. Besides, it provides the basis for transcription factors mediated metabolic engineering of astaxanthin biosynthesis in H. pluvialis. [ABSTRACT FROM AUTHOR]

Artemisinin, a sesquiterpene lactone obtained from Artemisia annua , is an essential therapeutic against malaria. YABBY family transcription factor AaYABBY5 is an activator of AaCYP71AV1 (cytochrome P450–dependent hydroxylase) and AaDBR2 (double-bond reductase 2); however, the protein–protein interactions of AaYABBY5, as well as the mechanism of its regulation, have not yet been elucidated. AaWRKY9 protein is a positive regulator of artemisinin biosynthesis that activates AaGSW1 (glandular trichome–specific WRKY1) and AaDBR2 (double-bond reductase 2). In this study, YABBY–WRKY interactions are revealed to indirectly regulate artemisinin production. AaYABBY5 significantly increased the activity of the luciferase (LUC) gene fused to the promoter of AaGSW1. Toward the molecular basis of this regulation, AaYABBY5 interaction with AaWRKY9 protein was found. The combined effectors AaYABBY5 + AaWRKY9 showed synergistic effects toward the activities of AaGSW1 and AaDBR2 promoters, respectively. In AaYABBY5 overexpression plants, the expression of GSW1 was found to be significantly increased when compared to that of AaYABBY5 antisense or control plants. In addition, AaGSW1 was identified as an upstream activator of AaYABBY5. Further, it was found that AaJAZ8, a transcriptional repressor of jasmonate signaling, interacted with AaYABBY5 and attenuated its activity. Co-expression of AaYABBY5 and anti- AaJAZ8 in A. annua increased the activity of AaYABBY5 toward artemisinin biosynthesis. This current study provides the first indication of the molecular basis of regulation of artemisinin biosynthesis through YABBY–WRKY interactions, which are regulated through AaJAZ8. This knowledge presents AaYABBY5 overexpression plants as a powerful genetic resource for artemisinin biosynthesis. [ABSTRACT FROM AUTHOR]

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]