Comparative biochemical and transcriptomic analyses reveal the bases of Rhopalosiphum padi L. resistance in Aegilops tauschii Coss.

Rhopalosiphum padi L. is one of the most devastating cereal aphids in the world. Its feeding does not induce a clear phytotoxic response in plants. Little information is available on defence response triggered by R. padi feeding on wild wheats, frequently used to improve hexaploid wheat. An attempt has been made to understand the differential biochemical and molecular responses of R. padi feeding on moderately resistant and susceptible Aegilops tauschii genotypes. RNA-Seq based transcriptomic analysis was conducted on 14 day-old leaf tissues of moderately aphid resistant (R) genotype (Ae. tauschii pau14232) and susceptible (S) genotype (Ae. tauschii pau14138) infested with R. padi for 12, 24 and 48 h. Plant response to aphid infestation was comparatively faster in R genotype. A total of 10,589 genes were altered in R genotype in comparison to only 8068 differentially expressed genes (DEGs) in S genotypes. There was a gradual increase in significant DEGs from 12 to 48 h after R. padi infestation in R (4931-6582) and S genotype (4860-5736). Kyoto Encyclopedia of Genes and Genomes enrichment analysis revealed that 18 pathway genes were differentially expressed in R and S genotypes at different feeding time intervals. Gene ontology enrichment analysis of DEGs after R. padi feeding indicated up-regulation of genes for secondary metabolite synthesis, reactive oxygen species (ROS)-scavenging, transcription factors (ethylene responsive transcript factor [ERF], NAC, WRKY, MADS-box and Myb) and salicylic acid and ethylene signalling pathways in R genotype while down-regulation in S genotype. However, photosynthesis and light-harvesting DEGs were down-regulated in both the genotypes resulting in decreased chlorophyll content. Biochemical analysis showed that R. padi feeding induced substantial hydrogen peroxide accumulation in R genotype unlike in S genotype. Also, activities of peroxidase and catalase were comparatively higher in R than S genotype, confirming efficient ROS-scavenging in R genotype. Quantitative polymerase chain reaction expression validation studies of 11 DEGs followed same trend as in RNA-Seq data. Our study concluded up-regulation of stress response genes, salicylate and ethylene signalling pathways and efficient ROS-scavenging imparted a fitness advantage to R genotype during R. padi attack, resulting in reduced nymphiposition and nymphal survival. These findings will help in understanding the mechanism of host plant resistance and the development of aphid resistant wheat varieties.