Toxicon 193 (2021) 21–27 Available online 26 January 2021 0041-0101/© 2021 Elsevier Ltd. All rights reserved. Review Saponin toxicity as key player in plant defense against pathogens Madiha Zaynab a, b, c , Yasir Sharif d , Safdar Abbas e , Muhammad Zohaib Afzal f , Muhammad Qasim g , Ahlam Khalofah h, i , Mohammad Javed Ansari j , Khalid Ali Khan h, i, k , Li Tao l , Shuangfei Li b, * a College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China b Shenzhen Key Laboratory of Marine Bioresource & Eco-environmental Sciences, College of Life Sciences and Oceanography, Shenzhen University, Shenzhen, Guangdong, 518071, China c Shenzhen Environmental Monitoring Center, Shenzhen, 518049, Guangdong, China d College of Plant Protection, Fujian Agriculture and Forestry University, Fuzhou, 350002, Fujian, China e Department of Biochemistry, Faculty of Biological Sciences, Quaid-i-Azam University, Islamabad, Pakistan f Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, 38000, Pakistan g Ministry of Agriculture Key Lab of Molecular Biology of Crop Pathogens and Insects,Institute of Insect Science,Zhejiang University, Hangzhou, 310058, China h Department of Biology, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, 61413, Saudi Arabia i Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha, 61413, Saudi Arabia j Department of Botany, Hindu College Moradabad (MJP Rohilkhand University Bareilly), 244001, India k Unit of Bee Research and Honey Production, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, 61413, Saudi Arabia l Shenzhen Base of South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shenzhen, China A R T I C L E INFO Handling Editor: Kevin Welch Keywords: Bacteria Insects Secondary metabolites Natural products Triterpenoid ABSTRACT Microbial pathogens attack every plant tissue, including leaves, roots, shoots, and fowers during all growth stages. Thus, they cause several diseases resulting in a plant’s failure or loss of the whole crop in severe cases. To combat the pathogens attack, plants produce some biologically active toxic compounds known as saponins. The saponins are secondary metabolic compounds produced in healthy plants with potential anti-pathogenic activity and serve as potential chemical barriers against pathogens. Saponins are classifed into two major groups the steroidal and terpenoid saponins. Here, we reported the signifcance of saponin toxins in the war against insect pests, fungal, and bacterial pathogens. Saponins are present in both cultivated (chilies, spinach, soybean, quinoa, onion, oat, tea, etc.) and wild plant species. As they are natural toxic constituents of plant defense, breeders and plant researchers aiming to boost plant imm unity should focus on transferring these compounds in cash crops. 1. Introduction Almost one-third of the 450,000 discovered plant species of this world are at the risk of extinction (Pimm and Joppa, 2015). The approximate cumulative amount of plant metabolites in a species ex- ceeds 10,000, but presently only fewer than 20% of these metabolites can be analyzed using metabolome methods (Abdelrahman et al., 2018; Lei et al., 2011). Throughout human history, people have often been using natural plant extracts for centuries as traditional medicines. Generally speaking, plants synthesize various primary and secondary metabolites with varying structures (Arbona et al., 2013). The primary metabolites are essential for plant development and growth, and secondary metabolites have major roles in reacting to particular stress for plants (Abdelrahman et al., 2019; Fujii et al., 2015). The secondary metabolites’ defensive functions have been confrmed by an in vitro examination of plants (Mes et al., 2000). The production of secondary metabolites results from millions of years of interaction between plants and pathogens and is thought to include more than 100,000 metabolites, but their interaction needs more clarifcation (Wink, 2008). Although plants with a high concentration of secondary metabolites are regarded as more resistant to biotic and abiotic stresses, their development has been deemed to be costly for plant growth and reproduction (Karban et al., 1997; Simms, 1992). The structure and function of secondary metabolites indicate why the induced defense has evolved from plants, * Corresponding author. E-mail address: szu_sfi@163.com (S. Li). Contents lists available at ScienceDirect Toxicon journal homepage: http://www.elsevier.com/locate/toxicon https://doi.org/10.1016/j.toxicon.2021.01.009 Received 4 November 2020; Received in revised form 24 December 2020; Accepted 20 January 2021