Elsevier

Phytochemistry Letters

Volume 37, June 2020, Pages 51-58
Phytochemistry Letters

Sesquiterpenoids from culture of the fungus Stereum complicatum (Steraceae): structural diversity, antifungal and phytotoxic activities

https://doi.org/10.1016/j.phytol.2020.03.012Get rights and content

Highlights

  • Four new and three known sesquiterpenes were isolated from Stereum complicatum.

  • New compounds were identified as sterostreins X and Y, hirsutenol G and sterpurol C while known compounds were sterostreins H, P and Q.

  • Sterostrein H showed phytotoxic activity against Lemna paucicostata (IC50 46.2 μM)

Abstract

Herein, we describe the isolation and identification of four new and three known sesquiterpenes from the culture broth of the fungus Stereum complicatum. Chemical structures were elucidated based on HR-DART-MS, 1D and 2D NMR experiments. Compounds were identified as sterostrein X (1), sterostrein Y (2), hirsutenol G (3), sterpurol C (4), sterostrein H (5), sterostrein P (6) and sterostrein Q (7). The phytotoxicity of 1-7 was assayed against Lactuca sativa, Agrostis stolonifera and Lemna paucicostata. Compounds showed slight or no activity against L. sativa, whereas 5 and 6 were the most active against A. stolonifera. Moreover, 5 was the only compound that was phytotoxic to L. paucicostata (IC50 46.2 μM). Compounds 2 and 5 slightly inhibited growth of B. cinerea and C. fragariae at 100 mM and C. acutatum at 50 mM, while 5 inhibited C. acutatum at 20 mM. Compound 6 was slightly active against all fungi at 20 mM.

Introduction

The Steraceae family (Basidiomycete) contains about 22 genera and 125 species. Among these genera, Stereum comprises 27 species with a widespread geographical distribution (Kirk et al., 2008). Species of Stereum are found to thrive in several types of dead wood, dead leaves, and sometimes on living leaves, thus, they are considered as saprobic organisms. The isolation and purification of secondary metabolites have been described only for a few Stereum species, with Stereum hirsutum and its sesquiterpenoids the most studied species and common metabolites found, respectively. Stereum species comprise an important source of structurally diverse sesquiterpenoids of the hirsutane, sterpurane, cadinane, stereumane, isolactarane, drimane, illudalane and norilludalane types (Liermann et al., 2010; Jinlun et al., 1992; Zheng et al., 2013; Li et al., 2011; Till et al., 2008; Young-Hee et al., 2006; Masahiko et al., 2012). Additionally, other classes of metabolites such as benzoate and benzofuran derivatives, triterpenes, sterols, and isoindolinone alkaloids have also been described in Stereum species (Ke et al., 2014; Rong et al., 2011; Jian-Neng et al., 2017; Meng-Qing et al., 2017; Ito-Kobayashi et al., 2008). Compounds isolated from Stereum have been reported for their cytotoxic activity, nematicidal, antifungal, human pancreatic lipase inhibition and antibacterial activities (Jian-Neng et al., 2017; Jun-Mei et al., 2017; Aqueveque et al., 2017; Hahk-Soo and Jong-Pyung, 2016; Yuan-Chang et al., 2018).

Stereum complicatum was found to infect living turnip (Brassica rapa var. rapa L.) leaves in Oxford MS, causing necrotic lesions (Fig. 1A). Most plant pathogens produce phytotoxins. Hence, we hypothesized that this fungus could be secreting phytotoxins that induce tissue necrosis. Very little information is available on the secondary metabolites of Stereum complicatum (Mellows et al., 1973). There is a robust literature on phytotoxins isolated from fungi; however, we have found no studies on the phytotoxic activity of compounds isolated from any species of Stereum (Martínez-Luisa et al., 2005; Molitor et al., 2012; Meepagala et al., 2015). Terpenoids are among the different phytotoxins reported from fungi (Favaretto et al., 2019; Yoo et al., 2006). In this paper we describe the isolation and structure elucidation of seven sesquiterpenoids, including four new compounds with illudane, hirsutane and sterpurane-like backbones from the ethyl acetate extract of the potato dextrose broth (PDB) culture of S. complicatum. All purified compounds were evaluated for their phytotoxic, antifungal and antibacterial activities.

Section snippets

Isolation and structure elucidation

The ethyl acetate extract from the culture broth filtrate of S. complicatum showed both antifungal and phytotoxic activity. Therefore, this medium polarity extract was further fractionated in a silica gel cartridge using flash chromatography. The most active fractions from ethyl acetate were further screened as sources of phytotoxins and antifungal compounds. Compounds from active fractions were finally purify either using silica gel flash chromatography or reversed-phase high-performance

General Experimental Procedures

Optical rotations were determined on an Autopol IV Automatic Polarimeter model 589-546 (Rudolph Research Analytical, Hackettstown, NJ, USA).

1D and 2D NMR spectra were recorded on a Bruker NMR spectrometer (Billerica, MA, USA) operating at 400 or 500 MHz (1H) and 100 or 125 MHz (13C) at 30 °C, using standard Bruker pulse programs. Samples were run in MeOH-d4 and the chemical shifts are given in δ (ppm) referenced to residual solvent signals.

Direct analysis in real time - high resolution mass

Declaration of Competing Interest

None

Acknowledgments

Authors thank Jessie L. Robertson and Robert D. Johnson from USDA ARS-NPURU for technical assistance in antifungal and phytotoxin bioassays. This research was supported in part by an appointment to the ARS Research Participation Program administered by ORISE through an interagency between the DOE and USDA. ORISE is managed by ORAU under DOE contract number DE-SC0014664. All opinions expressed in this paper are the authors' and do not necessarily reflect the policies and views of USDA, ARS, DOE

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