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.