Discovery of key regulators of dark gland development and hypericin biosynthesis in St. John's Wort (Hypericum perforatum).

Paride Rizzo,André Gündel,Twan Rutten,Lothar Altschmied,Helmut Bäumlein,Pauline Stark,Katrin Franke,Ljudmilla Borisjuk,Ludger Wessjohann,Timothy F Sharbel,Marcus Koch,Sarah Scharfenberg

doi:10.1111/pbi.13141

Abstract

SummaryHypericin is a molecule of high pharmaceutical importance that is synthesized and stored in dark glands (DGs) of St. John's Wort (Hypericum perforatum). Understanding which genes are involved in dark gland development and hypericin biosynthesis is important for the development of new Hypericum extracts that are highly demanded for medical applications. We identified two transcription factors whose expression is strictly synchronized with the differentiation of DGs. We correlated the content of hypericin, pseudohypericin, endocrocin, skyrin glycosides and several flavonoids with gene expression and DG development to obtain a revised model for hypericin biosynthesis. Here, we report for the first time genotypes which are polymorphic for the presence/total absence (G+/G−) of DGs in their placental tissues (PTs). DG development was characterized in PTs using several microscopy techniques. Fourier transform infrared microscopy was established as a novel method to precisely locate polyaromatic compounds, such as hypericin, in plant tissues. In addition, we obtained transcriptome and metabolome profiles of unprecedented resolution in Hypericum. This study addresses for the first time the development of dark glands and identifies genes that constitute strong building blocks for the further elucidation of hypericin synthesis, its manipulation in plants, its engineering in microbial systems and its applications in medical research.

Highlights

Whereas hyperforin is contained in translucent glands (Ciccarelli et al, 2001; Soelberg et al, 2007) which give the leaf a perforated appearance and the species its scientific name, hypericin on the other hand accumulates in so-called dark glands (DGs) which differentiate in leaves (Zobayed et al, 2006) and other tissues (Figure S1)
Dark glands occur in most organs of H. perforatum (Figure S1), except roots under natural conditions, hormoneinduced dark glands in lateral root cultures are reported by Murthy et al (2014)
Twenty-one genotypes with on average >40 of dark glands per placental tissues (PTs) were classified as carrying heavily glanded placentas (G++ PT; Figure S3), while 40 genotypes lacking DGs completely were classified as glandless placental tissue phenotype (G- PT)

Summary

Introduction

The genus Hypericum has a worldwide distribution and includes more than 460 species occupying very diverse habitats (Crockett and Robson, 2011). The most known representative, Hypericum perforatum, better known as St. John’s Wort, is a model organism for the study of apomixis (asexual reproduction) and an ancient medicinal plant mainly used for the treatment of depressions (Galla et al, 2011; Rizzo, 2016; Schallau et al, 2010). Hypericin and hyperforin, are among the most intensively studied bioactive compounds (Kusari et al, 2015; Zobayed et al, 2006). Whereas hyperforin is contained in translucent (or pale) glands (Ciccarelli et al, 2001; Soelberg et al, 2007) which give the leaf a perforated appearance and the species its scientific name, hypericin on the other hand accumulates in so-called dark glands (DGs) which differentiate in leaves (Zobayed et al, 2006) and other tissues (Figure S1). It has been shown that crude extracts of H. perforatum can reduce the memory impairment in amyloid precursor protein (APP)-transgenic mice (Hofrichter et al, 2013)

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