phylogenetic and functional studies on parasitism genes in Orobanchaceae and Convolulaceae

Open Access
- Author:
- Zhang, Huiting
- Graduate Program:
- Plant Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 16, 2020
- Committee Members:
- Claude Walker Depamphilis, Dissertation Advisor/Co-Advisor
Claude Walker Depamphilis, Committee Chair/Co-Chair
Michael Axtell, Committee Member
Charles T Anderson, Committee Member
Yinong Yang, Outside Member
Teh-Hui Kao, Program Head/Chair - Keywords:
- Parasitic plants
cell wall
phylogenomics
transcriptome
molecular biology
host-induced-gene-silencing - Abstract:
- Parasitic plants establish an intimate interaction with their hosts by forming a feeding structure, the haustorium, which penetrates host tissues and withdraws nutrients from the host conducting systems. To understand this unique phenomenon, several studies over the last decade used comparative genomic approaches and identified a number of “parasitism genes” that were hypothesized to play essential roles in the origin of plant parasitism and were recognized as the key targets for disruption of parasite-host interactions. One such gene, the pectate lyase-like (PLL) gene, was of special interest. To investigate the evolutionary history of this gene family, a land plant wide phylogenetic analysis was performed, which classified this gene family into seven major clades. One haustorial specific PLL gene in Triphysaria versicolor (TrVePLL1) was selected for functional studies. Knocking down the transcript level of TrVePLL1 by integrating a hairpin RNAi construct into T. versicolor roots, or via host-induced-gene-silencing (HIGS) reduced the success rate of mature haustorium formation. To expand the functional assays to more parasitism genes, a workflow, which used transcriptomic data and gene family phylogenetic analysis, was developed to select a number of potential candidates. Eleven orthogroups were selected and six T. versicolor genes were tested with gene knockdown assays. Phenotypes such as lower mature haustorium formation rate and excessive xylem bridge development were observed in TrVeLBD1 and TrVePrx1 knock down lines, respectively. Upregulation of cell wall related enzymes in haustorial development is one of the shared findings in multiple studies across different parasite plant lineages. To test the hypothesis that parasitic plants are cell wall engineers and to identify parasite genes involved in host cell wall modification as well as in the remodeling of their own walls during haustorial penetration, a phylogenomic study was performed that was complemented by gene expression analysis and molecular evolutionary analyses. Several lines of evidence were gathered supporting this hypothesis - (i) Four horizontally acquired cell wall genes were discovered in Cuscuta and one probable chimeric xylanase gene were identified in Orobanchaceae; (ii) Approximately 10% of the cell wall genes investigated in Orobanchaceae have gained haustorial specific expression; (iii) High sequence divergence and signatures of adaptive evolution were observed in some cell wall genes in both Orobanchaceae and Cuscuta. In addition, the results also provided evidence of tandem duplication as one of the driving forces for parasite adaptation. The research described in this dissertation has increased our understanding of plant parasitism and will assist the development of novel control strategies for parasitic weeds in agriculture.