Contents lists available at ScienceDirect Crop Protection journal homepage: www.elsevier.com/locate/cropro Silicon mitigates biotic stresses in crop plants: A review Hafiz Faiq Bakhat a,* , Najma Bibi a , Zahida Zia a , Sunaina Abbas a , Hafiz Mohkum Hammad a , Shah Fahad b,e,** , Muhammad Rizwan Ashraf c , Ghulam Mustafa Shah a , Faiz Rabbani a , Shafqat Saeed d a Department of Environmental Sciences, COMSATS Institute of Information Technology, Vehari 61100, Pakistan b Department of Agriculture, University of Swabi, Khyber Pakhtunkhwa, Pakistan c Department of Entomology, University of Agriculture Faisalabad (Burewala/Vehari-sub Campus), Pakistan d Department of Entomology, Muhammad Nawaz Shareef University of Agriculture, Multan, Pakistan e College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, Hubei, China ARTICLE INFO Keywords: Silicon Pest Plants Physical defense Chemical defense Pesticide Integrated strategy ABSTRACT Silicon (Si) is the second most abundant element in the lithosphere. Soils commonly contain as much as 30% Si, the majority of which is found in minerals and rocks. In plants, the element Si is recognized as a “beneficial quasi-essential” mineral nutrient. It is taken up by the plant roots and trans-located to aerial parts through transpiration streams. Naturally, its accumulation in aerial parts augments its polymerization in the intercellular spaces and beneath the cuticles creates a barrier against pathogen attack. Moreover, soluble Si in the cytosol triggers various metabolic pathways that result in the production of jasmonic acid and herbivore induced plant organic compounds. Combination of these Si-mediated physical and biochemical processes enhances plant de- fenses against biotic stresses (insects, fungus and bacteria). In addition, soluble Si in the plant system attracts natural predators and parasitoids during pest attack and consequently increases biological control. Although, a large set of data shows that Si provides natural defense against pest attack, application of Si as a pest control agent has not gained much attention from the scientists, policy makers and farming communities. Here, current knowledge regarding Si-mediated plant defense to pest attack is reviewed. Si-application tends to reduce pest infestations and may provide a sustainable environment friendly integrated strategy as an alternative to ex- tensive pesticide use. 1. Introduction Plant growth depends on various mineral nutrient elements present in the soil. These can be categorized into essential, beneficial and toxic elements (Bienert et al., 2008). Essential elements are critical for all plants in different growth conditions while, toxic elements disrupt various metabolic processes and negatively affect plant growth. Bene- ficial elements are vital for some specific plant species growing under certain environmental conditions. Although each and every plant con- tains silicon (Si), its essentiality is not proven yet because concrete evidences are lacking on the biochemical and physiological role of Si in plant biology. Various studies have shown that plants fertilized with Si have higher biomass production compared with non-Si-fertilized plants. The useful effects of Si on different plant species are well documented (Ma and Takahashi, 2002; Datnoff et al., 2001). Conversely, many re- searchers have also documented the beneficial effects of exogenous Si under various biotic and abiotic conditions on plant growth (Zia et al., 2017; Liang et al., 2007). In plants, a relatively higher quantity of Si is found than many other essential macronutrients such as calcium, magnesium and phosphorus. Grasses even may contain higher levels of Si than any of the other inorganic mineral nutrients. The concentrations of Si in various plants differ depending on genotype and species of a plant owing to differences in the Si uptake mechanism of the plant. Furthermore, Si transportation follows passive as well energy-depen- dent pathways. Low temperatures and some metabolic processes limit Si transportation (Sahebi et al., 2015). Although plants are able to survive with very low Si availability under greenhouse or some con- trolled laboratory conditions, Si-deprived plants are often structurally weaker than Si-rich plants. They show some abnormalities in growth, development, and reproduction (Rafi et al., 1997). Plants deficient in Si are more prone to abiotic stresses such as drought, metal toxicities, salinity, and nutrient deficiency (Bakhat et al., 2017; Datnoff et al., http://dx.doi.org/10.1016/j.cropro.2017.10.008 Received 16 March 2017; Received in revised form 7 October 2017; Accepted 11 October 2017 * Corresponding author. ** Corresponding author. Department of Agriculture, University of Swabi, Khyber Pakhtunkhwa, Pakistan. E-mail addresses: faiqsiddique@ciitvehari.edu.pk (H.F. Bakhat), shah.fahad@mail.hzau.edu.cn, shah_fahad80@yahoo.com, shahfahad@uoswabi.edu.pk (S. Fahad). Crop Protection 104 (2018) 21–34 0261-2194/ © 2017 Elsevier Ltd. All rights reserved. MARK