1 INTRODUCTION
Wheat (Triticum aestivum L.) is one of the most widely grown grain crops in the world. It could be vulnerably attacked by various herbivores during the process of growth. Aphids are common agricultural spines suck pests, which have a relatively short reproduction cycle hence large population. They directly ingest the nutrients present in plant phloem screen, and can act as the transmission vector of plant viral diseases (Zust & Agrawal, 2016). These activities can seriously affect the crop yields and the quality of grains (Y. Xu & Gray, 2020). Pesticides are the most widely used to control aphids; however, the overuse of chemical pesticides could lead to potential environmental contamination and damage ecosystem services (Y.-F. Li, An, Dang, Pan, & Gao, 2018). Integrated pest management has been strategically adopted to control aphids with biological controls including insect-resistant varieties, crop rotation, natural enemy (Ali, Desneux, Lu, & Wu, 2018), and RNA interference, in many cases they fail to achieve satisfactory performance due to short persistence and high costs (Jacques et al., 2020). Therefore, it is urgent to develop green, safe, and ecologically reasonable methods for the prevention and control of aphids.
Intervention factors such as plant hormones and nanomaterials play a vital role in plant defense against insect pests (He et al., 2020). Qi et al. showed jasmonic acid (JA) and ethylene contents were increased by Melon aphid infestation, which enhanced the resistance of cucumber plants to aphids (Qi et al., 2020). Many allelopathic substances such as 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) is commonly found in wheat, corn, and other gramineous crops in response to environmental changes (Varsani et al., 2019). DIMBOA and its degradation products manifested strong inhibitory effects on European corn borer in wild oat and ryegrass (NIEMEYER., 2009). Varsani et al. reported that DIMBOA, derived from benzoxazinoid (BX) in maize, is involved in callus accumulation with the effects to enhance the plant resistance to aphids (Varsani et al., 2019). Melatonin (MT) can regulate the levels of reactive oxygen species (ROS) in plants, thereby maintaining cell homeostasis and promoting plant growth (Chapman, Marchi-Werle, Hunt, Heng-Moss, & Louis, 2018). Recently, Bapat et al. found that silica nanoparticles (0.2mg/mL) application significantly reduced insect weight and inhibited the Helicoverpa armigera growth in tomato plants (Bapat, Zinjarde, & Tamhane, 2020). Selenium is the main component in the antioxidant enzyme glutathione peroxidase (GSH-Px) and plays an important role in oxidative defense (Ahmad, Abd Allah, Hashem, Sarwat, & Gucel, 2016). Zhang et al. found that sodium selenite could regulate the indole metabolism by alleviating imidacloprid-induced stress in garlic (X. Zhang et al., 2020). Compared with inorganic and organic Se, nano-Se demonstrated high bioavailability, low toxicity, and strong free radical scavenging ability (Y. Li et al., 2020). Foliar applied nano-Se can enhance flavonoids and phenolic acids concentration in strawberries and alleviate the associated salt stress (Zahedi, Abdelrahman, Hosseini, Hoveizeh, & Tran, 2019). Mechora et al. showed that the highly accumulated Se could protect Stanlega pinnata andAstragalus bisulcatus against flower thrips and two-spotted spider mite (Colin F Quinn1†, 2010). Interestingly, it was observed that multiple Se species could increase melatonin concentration in plants; Se and melatonin combination could alleviate biological and abiotic stress by regulating plant primary and secondary metabolism (M. Q. Li et al., 2016; Ulhassan et al., 2019). However, little is known about the mechanism of the increased plant resistance to insects induced by Se treatment and the related indoles alkaloids and phenylpropane pathways.
Volatile organic compounds (VOCs) are of great potential in plants to defense against insects. Plants respond to herbivores through multiple defensive modes, and the formation of VOCs is commonly an immediate and important response (War, Sharma, Paulraj, War, & Ignacimuthu, 2011). Mitra et al. found the formation of high levels of benzyl alcohol, 1,3‐diethylbenzene, acetophenone, 1‐nonanol, ethyl acetophenone, and1‐hexanol was associated with the decrease in aphids numbers (Mitra, Das, Debnath, Mobarak, & Barik, 2021). Accumulation of S‐linalool and (E)‐β‐caryophyllene could alter insect communities by attracting the natural enemies thus repelling the brown planthoppers from rice plants (Xiao et al., 2012). Element selenium can inhibit the growth and reproduction of the myzus persicae, weevils, cabbage root flies, beetles, caterpillars, and crickets by accumulating the VOCs contents (Mechora, 2019). Our two-year field trial study showed that the foliar application of nano-Se can effectively alleviate the pesticide-induced oxidative stress by improving the levels of secondary metabolites and aroma compounds in tea plants (D. Li et al., 2021). However, the role of nano-Se in the mechanism of increasing VOCs and wheat resistance to aphids remains largely unclear.
This study aimed to investigate the potential mechanism of the enhanced resistance of wheat to aphids after foliar application of nano-Se. Changes in antioxidant ability, flavonoids, phenolic compounds, phytohormone and VOCs were measured in the wheat seedlings. The optimal proportions of these metabolites, as well as the amount of nano-Se application, were examined based on the defense of wheat plants to Sitobion avenae . It is concluded that the combination of nano-Se and melatonin could be involved in the resistance mechanism mainly by regulating the VOCs, phenylpropane, and indoles alkaloid pathways.