Massive Exchange of mRNA between a Parasitic Plant and its Hosts
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Cuscuta pentagona is an obligate parasitic plant that hinders production of crops throughout the world. Parasitic plants have unique morphological and physiological features, the most prominent being the haustorium, a specialized organ that functions to connect them with their host's vascular system. The Cuscuta haustorium is remarkable in that it enables mRNA movement to occur between hosts and parasite, but little is known about the mechanisms regulating cross-species mRNA transfer or its biological significance to the parasite. These questions were addressed with genomics approaches that used high throughput sequencing to assess the presence of host mRNAs in the parasite as well as parasite mRNAs in the host. For the main experiment Cuscuta was grown on stems of Arabidopsis thaliana and tomato (Solanum lycopersicon) hosts because the completely sequenced genomes of these plants facilitates identification of host and parasite transcripts in mixed mRNA samples. Tissues sequenced included the Cuscuta stem alone, the region of Cuscuta-host attachment, and the host stem adjacent to the attachment site. The sequences generated from each tissue were mapped to host reference genes to distinguish host sequences, and the remaining sequences were used in a de novo assembly of a Cuscuta transcriptome. This analysis revealed that thousands of different Arabidopsis transcripts, representing nearly half of the expressed transcriptome of Arabidopsis, were represented in the attached Cuscuta. RNA movement was also found to be bidirectional, with a substantial proportion of expressed Cuscuta transcripts found in host tissue. The mechanism underlying the exchange remains unknown, as well as the function of mobile RNAs in either the parasite or host. An approach was developed to assay potential translation of host mRNAs by detecting them in the Cuscuta translatome as revealed by sequencing polysomal RNA and ribosome-protected RNA. This work highlights RNA trafficking as a potentially important new form of interaction between hosts and Cuscuta.
- Doctoral Dissertations