Phyton-International Journal of Experimental Botany DOI: 10.32604/phyton.2020.07180 This work is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article Stress Ameliorative Effects of Indole Acetic Acid on Hordeum vulgare L. Seedlings Subjected to Zinc Toxicity Monika Sood 1 , Shanti S. Sharma 1 , Joginder Singh 1 , Ram Prasad 2,3,* and Dhriti Kapoor 1,* 1 School of Bioengineering and Biosciences, Lovely Professional University Phagwara, Punjab, 144411, India 2 School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510006, China 3 Department of Botany, Mahatma Gandhi Central University, Motihari, Bihar, 845401, India * Corresponding Authors: Ram Prasad. Email: ramprasad@mgcub.ac.in; rpjnu2001@gmail.com; Dhriti Kapoor. Email: dhriti405@gmail.com Received: 05 May 2019; Accepted: 9 September 2019. Abstract: The heavy metals present in the environment accumulate in the plants and affect their productivity and yield. By entering the food chain, metals cause several serious health problems in human beings as well as in other organisms. Indole acetic acid (IAA) is known to act as a signaling molecule between symbiotic association of metal accumulating plants and plant growth promoting rhizobacteria (PGPR). Present study demonstrated a protective role of IAA against surplus Zinc (Zn)-induced toxicity to Hordeum vulgare seedlings. Elevated Zn concentrations suppressed the plant growth, caused a reduction in leaf relative water contents (RWC) and elevated free proline and non-protein thiols (NPT) accumulation. Zinc treatment also led to enhanced lipid peroxidation (MDA contents) as well as the activity of ascorbate peroxidase (APX), showing the involvement of antioxidative defense mechanism to reduce Zn induced toxicity. IAA oxidase activity was also observed to increase due to Zn treatment. IAA pretreatment of H. vulgare caryopsis could partly revert the Zn-induced toxicity in seedlings. Keywords: Heavy metals; phytohormone; lipid peroxidation; non-protein thiols (NPT); relative water contents (RWC); free proline; ascorbate peroxidase (APX); IAA oxidase 1 Introduction In recent years, due to industrialization, ecosystems are exposed to a variety of pollutants with the risk of environmental pollution and human issues [1,2]. Pollution of biosphere by toxic metals has accelerated dramatically since the beginning of industrial revolution [3]. Following their release from the various anthropogenic or natural sources heavy metals [HMs] a class of toxic metals accumulate in plants and other living organisms and thus enter the food chain; ultimately, the human health is at risk. Heavy metals are defined as elements having density greater than 5 g·cm -3 . In contrast to the organic pollutants, which can undergo biodegradation, HMs are non-biodegradable. They can be easily adsorbed by soil particles and remain within the ecosystem for a long period [4]. Although, some of them are important as micronutrients e.g., Fe, Mn, Mo, Zn, Ni, Cu, Cr and are essential in trace amounts for cell metabolism. In contrast, there are some heavy metals that are toxic for plants and microorganisms e.g., Hg, Ag, Cd, Pb and U [5]. Heavy metals of biological significance can be divided into two groups namely, redox active and redox inactive metals. Autoxidation of redox active metals such as Fe 2+ or Cu 2+ results in successive reduction of molecular O2 to H2O2 and yield the intermediates O2 • , HO . , and H2O2 which are potentially toxic as compared to molecular O2 [6]. Another mechanism by which HMs affects the cellular metabolism is displacing the essential metal from the active site of enzyme and then reducing its catalytic activity. Due to the above described interferences, the membrane functioning in plant cells is influenced