Plant Materials and Manipulations
The resistant line K1-6 and sensitive line K2-7 melon cultivars are preserved in Xinjiang Academy of Agricultural Sciences (Fig. 1). Broomrape seeds, identified as Phelipanche aegyptiaca Pers, were collected in the Kashgar region of Xinjiang. The pots (Diameter:16 cm, Height:12.5 cm) were filled with a vermiculite-sand mixture (1:1) and infused with 0.02 g of broomrape seeds. Each pot was planted with 1 ~ 2 melon seeds, and this setup was repeated 10 times for each treatment. The pots were placed randomly in a greenhouse (Temperature 28℃, light/darkness = 16:8), receiving regular watering. Root samples from the parasitized melon were collected at 0 days (non-infectious treatment) and 25 days after infection for the resistant line K1-6 (R0 and R25) and sensitive line K2-7 (F0 and F25). Parasitic broomrape samples were taken for degradome sequencing to find target genes. Each sample was repeated 3 times, rapidly frozen in liquid nitrogen and stored at -80℃. The blank control was induced by strigolactone in Petri dishes and sampled after broomrape germination.
Construction of Small RNA Library
The broomrape samples were sent to Genepioneer Biotechnologies for small RNA sequencing. Total RNA was extracted using the RNA Easy Fast (DP452), and small RNA libraries were prepared using the TruSeq Small RNA Sample Prep Kits (Illumina, San Diego, USA) according to standard procedures. After library preparation, the constructed libraries were sequenced using Illumina Hiseq 2000/2500 with a sequence reading length of 1×50 bp.
miRNA Sequencing and Data Processing
The initial sequencing data was aligned with the reference genome, with subsequent filtration to compare with the broomrape genomes. The remaining data were analyzed using the miRNA data analysis software ACGT101-miR (LC Sciences, Houston, Texas, USA). The analysis process of the software is as follows: elimination of 3' junction and irrelevant sequences to get clean data; Length screening, filtering out data with baseline length at 18–25 nt in the clean data; assessment against various RNA databases (mRNA, RFam, Repbase) to exclude non-miRNA data, retaining valid sequences for further analysis.
miRNA Identification
miRNA identification includes both known and novel miRNAs. We compared effective data with plant miRNA precursors and maturing sequences in the miRbase database. This comparison involved BLAST analysis of unique sequences spanning 18–25 nucleotides against a specific species precursor in miRBase (miRBase 21.0; http://www.mirbase.org/), in order to identify known miRNA as well as novel miRNA.
Degradome Sequencing Identifies Cross-Border miRNA Target Genes
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The samples were sent to Genepioneer Biotechnologies for degradome sequencing. The proposed library steps are as follows: miRNA capture by magnetic beads, adaptor ligation; mixed reverse transcription of Biotinylated Random Primers and mRNA; PCR amplification, after completing the whole library preparation, the constructed library was sequenced with Illumina Hiseq 2000/ 2500, and the sequencing read length was 1×50 bp at the single end.
Degradome Data Analysis and Target Gene Prediction
The raw data obtained from sequencing were processed through a series of data processing to obtain comparable sequenced pairs for subsequent analysis. The comparable pair sequences were compared with the melon cDNA database sequences to generate the degradome density files. The target gene mRNA sequences paired with the small RNA sequences of sequenced species were predicted by the Target finder target gene prediction software. The Allen Score was used to evaluate the degree of base complementarity between miRNA and target gene binding sites, indicating the binding site pairing penalty (where lower values indicate better pairing).
Target Gene Prediction
The target genes corresponding to melon miRNAs predicted by Target finder software based on the principle of base complementary pairing. The mRNAs in the generated degradome density files were combined and analyzed to identify the the target genes of the miRNAs. Functional annotation of the target genes was performed by GO enrichment analysis and KEGG enrichment analysis.
qRT-PCR
Total RNA was digested by DNase I and cDNA were synthesized using Rrans Script Green miRNA Two-Step qRT-PCR SuperMix. Differential miRNAs were randomly chosen for validation, and primers were designed accordingly. Each sample underwent triplicate repetitions to determine relative expression of the target miRNA. The primers used in RT-PCR are listed in Supplementary Table 1.