Transcriptome Profiling of a Common Mistletoe Species Parasitizing Four Typical Host Species in Urban Southwest China

Comparing gene expression among parasitic plants infecting different host species can have significant implications for understanding host-parasite interactions. <i>Taxillus nigrans</i> is a common hemiparasitic species in Southwest China that parasitizes a variety of host species. However, a lack of nucleotide sequence data to date has hindered transcriptome-level research on <i>T</i>.<i> nigrans</i>. In this study, the transcriptomes of <i>T. nigrans</i> individuals parasitizing four typical host species (<i>Broussonetia papyrifera</i> [Bpap] a broad-leaved tree species; <i>Cryptomeria fortunei </i>[Cfor] a coniferous tree species; <i>Cinnamomum septentrionale </i>[Csep] an evergreen tree species; and<i> Ginkgo biloba</i> [Gbil] a deciduous-coniferous tree species) were sequenced, and the expression profiles and metabolic pathways compared among hosts. A total of 40.06 Gb of clean sequence data were generated in nine cDNA libraries. These were <i>de novo</i> assembled into 293,823 transcripts with an N50 value of 1,790 bp. A large number of differentially expressed genes (DEGs) were identified when comparing <i>T. nigrans</i> individuals on different host species: Bpap vs. Cfor (713 DEGs), Bpap vs. Csep (1,219), Bpap vs. Gbil (1,514), Cfor vs. Csep (1,639), Cfor vs. Gbil (1,722), and Csep vs. Gbil (1,723). Three hundred and eighty-one unigenes were common to all six pairwise comparisons; these were primarily associated with carbohydrate metabolism and energy metabolism, as determined in a KEGG enrichment analysis. Specific involvements included: the TCA cycle, biosynthesis of secondary metabolites, carbon metabolism, biosynthesis of amino acids and glyoxylate/dicarboxylate metabolism. A total of 251 unique unigenes were also identified, specific to either the Bpap vs. Cfor (249 unigenes) or Csep vs. Gbil (two unigenes) comparisons; partial unigenes were associated with the plasma membrane, response to endogenous stimuli, ion binding, and organic hydroxy compound metabolic processes. These results provide a foundation for further explorations of the detailed molecular mechanisms involved in plant parasitism.