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.

比较侵染不同宿主物种的寄生植物的基因表达谱，对于解析宿主-寄生植物互作机制具有重要意义。黑钝果寄生（Taxillus nigrans）是中国西南地区常见的半寄生植物，可侵染多种宿主物种。然而，目前缺乏其核苷酸序列数据，阻碍了针对该物种的转录组水平研究。在本研究中，我们对侵染4种典型宿主物种的黑钝果寄生（T. nigrans）个体进行转录组测序，这4种宿主分别为：构树（Bpap，阔叶树种）、柳杉（Cfor，针叶树种）、银木（Csep，常绿树种）以及银杏（Gbil，落叶针叶树种），并比较了不同宿主下的基因表达谱与代谢通路。本研究共构建9个cDNA文库，获得总计40.06 Gb的干净序列数据，经从头（de novo）组装得到293,823条转录本，其N50长度为1790 bp。通过比较侵染不同宿主的黑钝果寄生个体，共鉴定得到大量差异表达基因（DEGs）：Bpap与Cfor组间为713个DEGs，Bpap与Csep组间为1219个，Bpap与Gbil组间为1514个，Cfor与Csep组间为1639个，Cfor与Gbil组间为1722个，Csep与Gbil组间为1723个。经京都基因与基因组百科全书（KEGG）富集分析发现，6组两两比较共有的381个单基因簇（unigene）主要参与碳水化合物代谢与能量代谢过程。具体涉及的通路包括：三羧酸循环（TCA循环）、次生代谢产物生物合成、碳代谢、氨基酸生物合成以及乙醛酸/二羧酸代谢。此外，还鉴定得到251个特异性单基因簇，仅在Bpap vs. Cfor（249个）或Csep vs. Gbil（2个）比较组中存在；其中部分单基因簇与质膜、内源刺激响应、离子结合以及有机羟基化合物代谢过程相关。本研究结果为进一步解析植物寄生过程中的具体分子机制奠定了基础。