MicroRNA Profiling in Melon During Aphid Herbivory in Susceptible and Resistant Interactions

Aphis gossypii resistance in melon is due to the presence of Vat, a single dominant gene belonging to the NBS-LRR family of resistance genes. Significant transcriptional reprogramming occurs in Vat⁺ plants during aphid infestation as observed from gene expression studies. MicroRNAs (miRNAs) are involved in the regulation of many biotic stress responses. The role of miRNAs was investigated in response to aphid herbivory, during both resistant and susceptible interactions. SmallRNA (smRNA) libraries were constructed from bulked leaf tissues of Vat⁺ melon line following early and late aphid infestations. Sequence analysis indicated the expression profiles of conserved and newly identified miRNAs were altered during different stages of aphid herbivory. These results were verified by qPCR experiments, in both resistant Vat⁺ and susceptible Vat^- interactions. The comparative analyses revealed that the expression of most of the conserved miRNA families were up-regulated during the early stages of aphid infestation in the resistant interaction, whereas the susceptible interaction showed the opposite trend. Along with the conserved miRNA families, eighteen cucurbit-specific miRNAs and five novel candidate miRNAs were expressed during the different stages of aphid herbivory.  Target genes were predicted for expressed miRNAs and they were verified by 5’ RACE studies. The comparison of the miRNA profiles in the resistant and susceptible interactions along with analysis of their target genes, provides insight into the miRNA-dependent post-transcriptional gene regulation in Vat-mediated resistance.