4 DISCUSSION
The infestation of aphids in plants can cause huge damage to crop
production. Currently, exogenous interventions such as chemical
pesticides are the most effective approach to control insect pests and
achieve high crop yields (Y.-F. Li et al., 2018). However, the long-term
use of pesticides could lead to insects developing resistance and cause
the ineffectiveness of these chemicals. Activation of the plant itself
anti-aphid system is considered a green protocol in plant protection and
sustainable agricultural production. In this study, we demonstrated that
the application of nano-Se could effectively reduce aphid populations in
wheat seedlings (Table 1). This could be due to the mechanism that
nano-Se could promote the synthesis of flavones, phenolic acids, plant
hormones and the expression of the related genes, which could further
influence the phenylpropane pathway and the metabolism of indole
alkaloids. The application of nano-Se also increased antioxidants and
VOCs concentrations, which promotes the resistance of wheat seedlings to
aphids (Figure 8).
Plant secondary metabolites play an important role in the resistance to
insect feedings. These chemicals commonly include phenolics (phenolic
acids and flavonoids) and terpenoids (diterpenes and monoterpenes),
which could attract specific insects and their natural enemies (Jamieson
et al., 2017). A previous study showed that Se hyperaccumulators could
promote plant growth by protecting Artemisia ludoviciana andSymphyotrichum ericoides against
grasshoppers (El Mehdawi, Quinn, &
Pilon-Smits, 2011). The effect of Se on secondary metabolites was mainly
based on abiotic stress, including salt, heavy metal, and drought
(Hawrylak-Nowak, Hasanuzzaman, & Matraszek-Gawron, 2018). However, few
studies were conducted to elucidate the effects of Se on the formation
of secondary metabolites (phenylpropane and indole alkaloid metabolism)
and their impact on insects. Our previous study reported that foliar
application of nano-Se (10 mg/L) could alleviate pesticide stress via
regulating the ascorbate-glutathione cycle in tea trees (D. Li et al.,
2021). In pepper plants, nano-Se (20 mg/L) could enhance phenylpropane
and fatty acid metabolism (D. Li et al., 2020), causing a significant
increase in the concentration of flavonoids (by 80%) and phenolic acids
(by 65%). In this study, the increased concentration of myricetin,
rutin, apigenin, kaempferol, quercetin, caffeic acid, hydroxy-3,
5-dimethoxycinnamic acid, 4-hydroxybenzoic acid, 4-hydroxycinnamic acid,
and chlorogenic acid was observed after the application of nano-Se
(5mg/L) to wheat seedlings with aphid infection (Figure 4). These
metabolites of apigenin and caffeic acid were then selected to evaluate
the growth and metabolism of wheat under aphid stress. This practice
could enhance wheat growth including plant height, length, weight,
flavonoids (luteolin and quercetin), and phenolic acids (caffeic acid,
4-hydroxybenzoic acid, and 4-hydroxycinnamic acid) in the seedlings
(Figures S1, S2, and S3). The application of plant strengthener
4-fluorophenoxyacetic acid was found to suppress pest populations (Wang
et al., 2020). The enhanced resistance to insects could be due to the
deposition of flavonoid polymers in plant cells. Czerniewicz’s et al.
found that the formation of luteolin, apigenin, caffeic acid, and
4-hydroxybenzoic acid compounds could impair insect feeding, thus
protecting wheat from aphid infection (Czerniewicz et al., 2017).
In general, plants can establish a dynamic equilibrium of ROS via
formation or removal processes depending on the growth conditions. Plant
secondary metabolism and antioxidant system can synergistically improve
plant resistance to biotic and abiotic stresses. In the present study,
the antioxidant enzymes (CAT, POD, SOD, LOX, GSH-Px, and APX) and
non-enzymes (VE, MDA, GSH, Pro, and LPO) system were found to change to
varying degrees with nano-Se treatment (Figure.3). Selenite present in
soil (0.5 mg/kg) enhanced the antioxidant enzymes (CAT, POD, and PPO)
and reduced the MDA concentration, forming a favorable situation in
oilseed against Sclerotinia sclerotiorum stress (J. Xu et al.,
2020). Li et al. used 200 µM of Se treatment to inhibit silkworms via
significantly decreasing their growth and reproduction (Jiang et al.,
2020). In addition, sodium selenate (6.5 µM) foliar application can
effectively protect rice against N. lugens infestation (Scheys,
De Schutter, Subramanyam, Van Damme, & Smagghe, 2020). Sodium selenate
has been shown to effectively inhibit the growth of insect pests in
crops, but without elucidating the underneath mechanism (Hanson,
Lindblom, Loeffler, & Pilon‐Smits, 2004; Z. Xu et al., 2021). Our study
suggests that nano-Se application could alleviate the effects of
herbivores by regulating the secondary metabolism and antioxidative
ability.
Phytohormone was considered as the first line to defense against the
pests (Erb, Meldau, & Howe, 2012). The JA, SA, and MT signaling was
intensively involved in herbivore defense in plants. In the present
study, the application of nano-Se (5, 10, and 20 mg/L) could lead to the
increase in JA, MT, and DIMBOA concentration, while the SA concentration
was reduced (Figure.2). The remarkable effects were observed on MT and
SA concentration changes, these two phytohormones were selected to apply
directly on wheat seedlings to evaluate the plant metabolism. The growth
parameters (height, length, and weight) and phytohormones (JA, SA,
DIMOBA and MT) were measured in wheat seedlings (Figure. S1 and S4). JA
and other signaling pathways interact to regular and enhance plant
resistance to insects (Lortzing & Steppuhn, 2016). It has been
demonstrated that the ROS and hormonal signaling were integrated to
respond to insect herbivores (Kerchev, Fenton, Foyer, & Hancock, 2012).
Silicon could also regulate the biosynthesis of phytohormones (JA, SA
and IAA) and reduce the concentration of ROS; these two processes
together enhance plant resistance to herbivorous insects (Tripathi et
al., 2021). In our study, nano-Se
could activate the indole alkaloid pathway and the related genes, which
correspondingly increased the MT and DIMBOA contents.
Extensive research demonstrated that
MT is involved in regulating circadian rhythms in aphids (Barbera et
al., 2020). The signal alternatively activates the molecular pathway
that leads to the reproduction and migration responses. MT application
(50µM) could induce the antioxidant enzymes activities (PPO, CHI, and
PAL), thus regulating the ROS concentration in plants (Liu et al.,
2019). Furthermore, MT treatment increased MeJA concentration and
up-regulated the JA biosynthesis genes, as a result, it enhanced the
tomato defense against disease (Arnao & Hernandez-Ruiz, 2019). MT is
found to regulate flavonoids and carotenoid biosynthesis as well,
promote the synthesis of amino acids, organic acids and sugars, and
enhance the plant resilience under biotic (fungi, bacteria, and insects)
and abiotic (heat, cold, drought, and salt) stresses (Tiwari et al.,
2020). DIMBOA is an allelochemical and can react with glucose to enhance
resistance to aphids by promoting the callose deposition in maize
(Meihls et al., 2013). DIMBOA (0.1
mM) could reduce the Sitobion avenae F numbers to levels below
the economic damage threshold (HANSEN, 2006). However, the anti-aphid
mechanism related to nano-Se and DIMBOA was not elucidated. In this
study, the combination of nano-Se and MT application increased DIMBOA
and MT, as well as the levels of the related genes which could be
associated with the indole alkaloid pathway, thereby could decrease the
aphid numbers in the wheat seedlings.
In present study, the release number of VOCs increased in the wheat
seedlings treated by nano-Se. GC-IMS analysis revealed the increased
concentration of ethanol, 1-popanol, Isopentyl alcohol, 2-butanone,
2-heptanone, acetoin, acetone, benzaldehyde, ethyl 2-methylpropanoate,
and ethyl acetate dimer in the presence of nano-Se bio-fortification.
However, 2-pentenal (E) dimer, hexanal monomer, ethyl hexanoate, and
phenylacetaldehyde concentrations were reduced. Plants can produce a
wide array of volatile metabolites, including terpenes, fatty acid
derivatives, aromatic compounds, and
amino acid derivatives (Clavijo McCormick, Unsicker, & Gershenzon,
2012). The increased concentration of benzaldehyde, acetoin, and
1-propanol facilitated the repelling response to herbivorous insects in
sorghum seedlings (Park et al., 2020). Zhuang et al. found that the
volatile indoles increased with insect feeding; these chemicals could
improve the resistance in rice plants (Zhuang et al., 2012). Zhang et
al. showed that VOCs including acetone, hexanal, and
pentalamide play a vital role in
impacting insect behaviors (Y. Zhang, Teng, Wang, & Jiang, 2021).
For example, VOCs could function as
either attractants or antifeedants, and effectively control insect
activities. It is commonly accepted that VOCs and insect pheromones
could be combined to regulate insect behaviors. Our present study found
that the combinations of nano-Se with MT can markedly reduce the insect
numbers most likely due to the increasing VOCs in wheat seedlings.
In summary, foliar application of nano-Se could effectively decrease the
aphid number compared with the control. The corresponding
insect-resistant mechanism could be associated with the increasing
levels of secondary metabolites (API and caffeic acid) and plant
hormones (MT and DIMBOA) in the indole alkaloid and phenylpropane
metabolisms in wheat seedlings. Different ratios of nano-Se and these
metabolite combinations were tested the defense-related compounds toSitobion avenae in wheat
plants. The combination of nano-Se and MT achieved the best controls in
reducing the S. avenaenumber.
The concentrations of antioxidants, phenolic acids, flavonoids, and
plant hormones were found to be markedly enhanced by acting
synergistically of nano-Se and MT combination. The VOCs could function
as insect attractants and repellents, which included ethanol, 1-popanol,
isopentyl alcohol, 2-butanone, 2-heptanone, acetoin, acetone,
benzaldehyde, ethyl-2-methylpropanoate, and ethyl acetate dimer.
Therefore, the application of the optimal ratio of nano-Se and MT could
be a promising strategy to protect wheat seedlings from S. avenaeinfection by collective actions from the secondary metabolic pathways
and contributions of the increasing antioxidants and VOCs levels.