antioxidants Article Antioxidative Responses of Duckweed (Lemna minor L.) to Phenol and Rhizosphere-Associated Bacterial Strain Hafnia paralvei C32-106/3 Olga Radulovi´ c 1, *, Slaviša Stankovi´ c 2 , Olja Stanojevi´ c 2 , Zoran Vujˇ ci´ c 3 , Biljana Dojnov 4 , Milana Trifunovi´ c-Momˇ cilov 1 and Marija Markovi´ c 1   Citation: Radulovi´ c, O.; Stankovi´ c, S.; Stanojevi´ c, O.; Vujˇ ci´ c, Z.; Dojnov, B.; Trifunovi´ c-Momˇ cilov, M.; Markovi´ c, M. Antioxidative Responses of Duckweed (Lemna minor L.) to Phenol and Rhizosphere-Associated Bacterial Strain Hafnia paralvei C32-106/3. Antioxidants 2021, 10, 1719. https:// doi.org/10.3390/antiox10111719 Academic Editors: Fernanda Fidalgo, Anket Sharma and Cristiano Soares Received: 24 September 2021 Accepted: 26 October 2021 Published: 28 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Plant Physiology, Institute for Biological Research “Siniša Stankovi´ c”, National Institute of the Republic of Serbia, University of Belgrade, 142 Bulevar Despota Stefana, 11060 Belgrade, Serbia; milanag@ibiss.bg.ac.rs (M.T.-M.); marija.nikolic@ibiss.bg.ac.rs (M.M.) 2 Faculty of Biology, University of Belgrade, 16 Studentski Trg, 11000 Belgrade, Serbia; slavisas@bio.bg.ac.rs (S.S.); olja.stanojevic@bio.bg.ac.rs (O.S.) 3 Department of Biochemistry, Faculty of Chemistry, University of Belgrade, 12-16 Studentski Trg, 11000 Belgrade, Serbia; zvujcic@chem.bg.ac.rs 4 Department of Chemistry, Institute of Chemistry, Technology and Metallurgy, National Institute of the Republic of Serbia, University of Belgrade, 12 Njegoševa, 11000 Belgrade, Serbia; bdojnov@chem.bg.ac.rs * Correspondence: olga.radulovic@ibiss.bg.ac.rs Abstract: Duckweed (L. minor) is a cosmopolitan aquatic plant of simplified morphology and rapid vegetative reproduction. In this study, an H. paralvei bacterial strain and its influence on the antioxida- tive response of the duckweeds to phenol, a recalcitrant environmental pollutant, were investigated. Sterile duckweed cultures were inoculated with H. paralvei in vitro and cultivated in the presence or absence of phenol (500 mg L −1 ), in order to investigate bacterial effects on plant oxidative stress during 5 days. Total soluble proteins, guaiacol peroxidase expression, concentration of hydrogen per- oxide and malondialdehyde as well as the total ascorbic acid of the plants were monitored. Moreover, bacterial production of indole-3-acetic acid (IAA) was measured in order to investigate H. paralvei’s influence on plant growth. In general, the addition of phenol elevated all biochemical parameters in L. minor except AsA and total soluble proteins. Phenol as well as bacteria influenced the expression of guaiacol peroxidase. Different isoforms were associated with phenol compared to isoforms expressed in phenol-free medium. Considering that duckweeds showed increased antioxidative parameters in the presence of phenol, it can be assumed that the measured parameters might be involved in the plant’s defense system. H. paralvei is an IAA producer and its presence in the rhizosphere of duckweeds decreased the oxidative stress of the plants, which can be taken as evidence that this bacterial strain acts protectively on the plants during phenol exposure. Keywords: phenol; bacteria; duckweed; antioxidative; stress 1. Introduction The common duckweed (Lemna minor, L.) is a rapidly reproducing vascular plant of simplified morphology, with remarkable tolerance to various pollutants. These characteris- tics make L. minor an optimal model organism for toxicity testing as well as wastewater treatment [1–3]. In agriculture, due to its rapidly increasing low-starch biomass and abil- ity to thrive under very different conditions, duckweeds are used as a cheap source of protein [4]. Industrial growth, especially in developing countries, results in the constant influx of phenol into aquatic ecosystems, where it causes lethal or toxic damage to all living organisms in a wide range of concentrations from as low as 0.26 to 1204.6 mg L −1 [5]. Plants very often grow under toxic conditions, resulting in oxidative stress and accu- mulation of reactive oxygen species (ROS) which are harmful to cells if they exceed the natural defense mechanisms of plants [6–8]. The efficiency of the antioxidative response Antioxidants 2021, 10, 1719. https://doi.org/10.3390/antiox10111719 https://www.mdpi.com/journal/antioxidants