Effects Of Nitrogen Application Rate On Aphid Resistance Of Different Resistant Wheat Cultivars

As one of the major threats to the yield and quality of Triticum aestivum L in China,Sitobion miscanthi is the predominant species of wheat aphid in China,the feeding damage is continuous throughout the whole growth period of wheat.Promoting wheat breeding for aphid-resistant cultivars is an efficient and economical way to control aphids.As a key nutrient for wheat growth,nitrogen has potential to manipulate the aphid resistance of wheat.However,the effect of nitrogen application on aphid resistance of wheat and the underlying mechanism,is still largely unknown.In this study,we examined the effects of three nitrogen application rates(no nitrogen application,the nitrogen suitable for crops(160 kg/hm~2),and the nitrogen application rate used by farmers(320 kg/hm~2)on the aphid resistance of six wheat varieties with different aphid resistance.We analyzed the effects of nitrogen application rate on the physical structure and main secondary metabolites of wheat leaf surface,and explored the transcriptome landscape of wheat with different resistance to nitrogen application rate,in order to provide a theoretical basis for a comprehensive understanding of the effects of nitrogen application rate on different resistant wheat varieties.Findings were as follows:1.The effect of nitrogen application rate on aphid resistance of wheat varieties with different aphid resistance was clarifiedUnder the condition of high nitrogen fertilizer,the development period of the six wheat plants was significantly shorter than that under the condition of no nitrogen fertilizer,and most of the aphid instars were shortened with the increase of nitrogen application rate.The development period of the two high resistance wheat varieties was much shorter than that of the medium resistance and aphid sensitive wheat varieties with increasing nitrogen application rate.The aphid fertility significantly increased with the increases of nitrogen application rate,and the increase of aphid yield of the high and medium resistance varieties was higher than that of the aphid sensitive varieties.The body weight of aphids feeding on the wheat plants with nitrogen fertilizer was significantly higher than that without nitrogen fertilizer,and the body weight gain of the high and medium resistance varieties was higher than that of the aphid sensitive varieties.The survival rate of aphids increased with increasing nitrogen fertilizer.The feeding wave form of aphids on wheat plants under different nitrogen application rates was analyzed by insect penetrating potentiometer(EPG).The results showed that,compared with the treatment without nitrogen fertilizer,the duration of np wave decreased under nitrogen fertilizer,the duration of C wave decreased with increasing nitrogen application rate,the frequency and duration of pd wave decreased with increasing nitrogen application rate,the duration of E1 wave decreased with increasing nitrogen application rate,and the duration of E2 wave increased with increasing nitrogen application.2.The effect of nitrogen on the physical structure of the fed wheat was clarifiedThe effects of nitrogen application rates on the physical structure and surface wax content of leaf epidermis of wheat plants with different resistance levels were analyzed by light microscopy and chloroform extraction analysis.The results showed that leaf coat length of medium-resistance wheat varieties decreased significantly at medium nitrogen level compared with the treatment without nitrogen fertilizer.The coat length of high resistance wheat varieties and medium-resistance wheat varieties decreased significantly at high nitrogen level compared with the treatment without nitrogen fertilizer.Leaf surface hair density of the six tested wheat cultivars decreased with increasing nitrogen application rates,but there was no significant effect on leaf surface wax content.3.The effect of nitrogen on the chemical composition of the fed wheat was clarifiedThe results showed that the higher the resistance of wheat varieties to aphid,the higher the contents of flavonoids and total phenols.The contents of flavonoids and total phenols in all wheat varieties increased significantly after feeding with aphid for 48hours.The contents of flavonoids and total phenols in the six wheat plants tested decreased significantly with the increases of nitrogen application rate,regardless of whether they were induced by aphid feeding or not.The LC-MS metabolome analysis of wheat metabolites clarified the metabolic pathways of wheat plants before and after aphids feeding and in response to different nitrogen application rates.The differential metabolites of wheat varieties with different aphid resistance before and after aphid feeding induction and under different nitrogen application rates were identified by metabolome analysis,and 13 key metabolites and 5 key metabolic pathways were screened out.4.The effects of nitrogen application rate on transcriptional regulation of wheat cultivars with different aphid resistance were clarifiedRNAseq indicated that no significant difference in gene expression level between middle resistance cultivar and sensitive cultivar,while distinguished with high resistance cultivar.Compared with under high nitrogen level,each of cultivar has more differentially expressed genes responding to aphid feeding under low nitrogen level.Compared with wheat plants without aphid feeding,nitrogen application induced more upregulated DEGs in wheat plants after aphid feeding.Especially,in high resistance wheat plants,aphid feeding or increasing nitrogen application can induce same downregulated DEGs enriched in some carbon metabolism pathways,including some secondary metabolites involved in insect resistant process such as flavonoid synthesis,phenylpropanoid biosynthesis fatty acid metabolism.In conclusion,the aphid resistance of wheat was affected by nitrogen application rate.The aphid resistance of wheat varieties decreased with the increases of nitrogen application rate.Nitrogen application rate changed wheat leaf structure and induced differential metabolites and differentially expressed genes related to insect resistance in wheat plants.However,the nitrogen application rate had different effects on different aphid resistance wheat varieties.The more aphid resistance wheat varieties were,the more sensitive to nitrogen fertilizer.Our results provided a theoretical basis for understanding the mechanism of nitrogen participation in aphid resistance in wheat.