Silicon-based Induced Resistance In Maize Against Fall Armyworm [Spodoptera Frugiperda (Lepidoptera: Noctuidae)]

The Fall armyworm（Spodoptera frugiperda）is a significant economic pest in the United States and has recently become a substantial concern in African and Asian countries;and has become an important pest on maize in some areas of China.It is successful because it is very polyphagous and has the capacity to migrate large distances,has intimidating adaptability to adversity,and has insecticide resistance.In China,maize is one of the most significant crops.It plays a significant role in food security.Mostly,chemical pesticides are used for the management of S.frugiperda,but it has developed resistance against pesticides and other commonly used control methods.Mostly farmers use heavy amount of chemical pesticides for its quick control,which can affect non-target organisms and beneficial insects and can also cause environmental pollution.Therefore,alternative control methods have become more necessary for the sustainable management of S.frugiperda.One of these methods is to make plants more resistant to pests and diseases.Plant resistance to insect pests can also be obtained by induction,where a plant becomes less suitable for the insect due to a particular condition that affects its physiology.Currently,silicon（Si）is being used in many pest control systems due to its ability to increase plant resistance to biotic and abiotic stress and promote plant growth.Therefore,current experiment was carried out to test the effect of soluble sources of Si on plant as well as on the fitness of S.frugiperda feeding on those treated plants.The current study focused on:（1）the effect of exogenous application of Si O₂ and K₂Si O₃ applied with two application methods（Foliar application and soil drenching）on Si accumulation,antioxidant enzymes activity,and vegetative parameters of maize plants,（2）the effect of both Si sources on the leaf consumption rate,conventional life table,and age-stage two-sex life table parameters of S.frugiperda and（3）application of transcriptomic and biochemical analysis to comprehensively investigate the gene expression changes of S.frugiperda in response to Si O₂ and K₂Si O₃.（1）The results showed that the Si sources and application methods positively affected the plant vegetative parameters（Plant height,shoot fresh weight,shoot dry weight,root fresh weight,root dry weight）,physiological parameters（Superoxide dismutase（SOD）activity,Peroxidase（POD）activity,Catalase（CAT）activity,and chlorophyll content）,and Si accumulation in maize leaves compared to the control.Results revealed a significant（P?0.05）increase in Si accumulation in maize leaves in the foliar application of Si O₂ and other Si treatments compared to control.Similarly,antioxidant enzyme activities were also positively affected by all Si applications compared to the control.The activity of SOD,POD,and CAT in the plants treated with foliar application of Si O₂ was significantly（P?0.05）higher than the plants in the control treatment,where no Si was applied.The activities of SOD,POD,and CAT in control treatment were(0.0553g^-1 FW),(0.0646 g^-1 FW),and(0.0748 g^-1 FW),respectively.Furthermore,significantly（P?0.05）taller plants with more chlorophyll content in leaves were observed in the foliar application of Si O₂compared to K₂Si O₃ and control.Moreover,a significant（P?0.05）effect of foliar application of Si O₂ and K₂Si O₃ was observed on shoot fresh weight and shoot dry weight compared to those plants which did not receive Si application.In contrast,root fresh weight and root dry weight was significantly（P?0.05）more in the plants receiving soil drenching of both Si sources compared to foliar application and control treatment.Among application methods of Si,the effect of Si O₂was significantly more prominent on all parameters of maize plants compared to K₂Si O₃ and control.Foliar applications of both Si sources significantly affected all parameters of maize except root fresh and dry weight,which was more affected by soil drenching of Si.The importance of foliar treatment is further highlighted by the fact that plant roots are not well developed during the early development stages to receive mineral nutrients from the soil.Furthermore,results revealed that Si accumulation in maize leaves was positively correlated to antioxidant enzymes（SOD,POD,and CAT）activity and other vegetative parameters of maize plants like stem length,fresh and dry weight of roots and shoots,and chlorophyll content,which shows that plants can become more resistant against biotic and abiotic factors of the environment.（2）Results of the current study revealed that Si applications significantly affected the fitness of S.frugiperda.When S.frugiperda larvae were fed with Si-treated leaves,a significant effect of Si accumulation in leaves was observed on the leaf consumption rate,and age-stage two-sex life table parameters of S.frugiperda.Results revealed that a significantly negative effect of Si applications was observed on the leaf consumption of larvae.Because minimum leaf consumption was observed in the larvae feeding on the plant leaves treated with foliar application of Si O₂ and K₂Si O₃ compared to control.The foliar applications of Si O₂and K₂Si O₃ resulted in significantly（P?0.05）higher mortality percentage and developmental duration and decreased larval and pupal biomass of S.frugiperda.The foliar application of Si O₂ caused maximum larval mortality compared to K₂Si O₃.The maximum effect of Si treatments was observed in the early stages of larvae as insect mortality was significantly higher in the 1^st and 2^nd instar of larval growth.Similarly,considerably higher mortality was recorded in one-day and two-day-old larvae.Meanwhile,larval mortality was significantly more affected by Si’s foliar application than soil drenching.A significantly negative effect of all the Si applications was observed on the reproductive values of S.frugiperda.Values of Intrinsic rate of increase,Finite rate of increase,Net reproduction rate,Mean generation time,Gross reproduction rate,Fecundity per female,pre-adult survival rate and adult pre-ovipositional time were recorded in foliar application of Si O₂,which is significantly from all other treatments.These impacts of silicon applications on the biology and behavior of S.frugiperda could help minimize its population and decrease crop damage.（3）Results of the current study showed a significant effect of Si applications in maize on gene expression changes of S.frugiperda larvae feeding on it.Application of K₂Si O₃ caused 1266differentially expressed genes（DEGs）,of which 578 were up-regulated and 688 down-regulated.Exposure to Si O₂caused differential expression of 3637 DEGs（1545 up-,2092 down-regulated）.Furthermore,the treatment consisting of Si O₂ application led to a significant increase in the number of DEGs compared to that of the K₂Si O₃,indicating a Si source effect.Moreover,GO and KEGG enrichment analysis identified important DEGs belonging to specific categories,such as amino acid,carbohydrate,lipid,energy,xenobiotics metabolisms,signal transduction,and posttranslational modification pathways significantly altered at the K₂Si O₃ and Si O₂.Similarly,a significant effect of both Si sources was observed on enzyme activities in enriched pathways compared to the control.The identification and validation of DEGs encoding detoxification-related genes using q RT-PCR,which correlated with the RNA-seq data,provide the first toxicity mechanisms for a better understanding of Si applications action in S.frugiperda and other insect pests at the molecular level.The study highlights the significant impact of Si O₂ exposure on various metabolic pathways and enzyme activities.The findings suggest that the Si source plays a crucial role in modulating the expression of genes and metabolic pathways,which could have implications for understanding molecular mechanisms.These findings could have significant implications for the development of novel pest control strategies based on Si application.