Abstract
Lupeol, a natural lupane-type pentacyclic triterpene, possesses various pharmacological properties, and its production attracts attention. Significant quantities of lupeol are deposited on the castor aerial organ surface and are easily extractable as a predominant wax constituent. Thus, castor might be considered as a potential bioreactor for the production of lupeol. The lupeol biosynthesis pathway is well known, but how it is regulated remains largely unknown. Among large numbers of castor cultivars, we targeted one accession line (337) with high levels of lupeol on its stem surface and low levels thereof on its hypocotyl surface, implicating that lupeol synthesis is differentially regulated in the two organs. To explore the underlying mechanisms, we did comparative transcriptome analysis of the first internode of 337 stem and the upper hypocotyl. Our results show that large amounts of auxin-related genes are differentially expressed in both parts, implying some possible interactions between auxin and lupeol production. We also found that several auxin-responsive cis-elements are present in promoter regions of HMGR and LUS genes encoding two key enzymes involved in lupeol production. Furthermore, auxin treatments apparently induced the expression levels of RcHMGR and RcLUS. Furthermore, we observed that auxin treatment significantly increased lupeol contents, whereas inhibiting auxin transport led to an opposite phenotype. Our study reveals some relationships between hormone activity and lupeol synthesis and might provide a promising way for improving lupeol yields in castor.
Highlights
Lupeol is a naturally occurring pentacyclic triterpene and has been studied for its interesting pharmacological properties, for instance as anti-cancer, anti-diabetes, and antiinflammation agents [1]
We found that auxin-responsive cis-elements are present in the promoter regions of RcHMGR and RcLUS, which are two key genes involved in lupeol synthesis
Lupeol was previously identified as a predominant wax component in the castor epidermis, and its content in total stem wax is correlated with the formation of epicuticular wax crystals on stems of glaucous individuals [7]
Summary
Lupeol is a naturally occurring pentacyclic triterpene and has been studied for its interesting pharmacological properties, for instance as anti-cancer, anti-diabetes, and antiinflammation agents [1]. HMGR catalyzing is a rate-limiting step in the MVA pathway and is encoded by three genes in castor, among which only one, HMGR, is epidermal-specific [5,6]. LUS is responsible for lupeol synthesis and is encoded by two genes in castor, both of which are highly expressed in the epidermis [4,6,7]. HMGS, SQS, and one of SQEs, are highly expressed in the epidermis [6] These genes showing epidermis-specific expression patterns are supposed to be closely related to the lupeol biosynthesis pathway. Large amounts of auxin-related genes are differentially expressed in the hypocotyl and the adjacent stem internode, implying that auxin might regulate lupeol biosynthesis. Our results proved that auxin acts as a positive regulator of lupeol biosynthesis
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