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Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2021)

Parasitic plants have evolved the ability to recognize neighboring plants and invade their tissues to acquire resources. We hypothesize that the mechanisms used by parasitic plant roots to recognize neighboring plant roots are a specialized adaptation of a general phenomenon that happens in most plants. In Chapter 2, I analyzed the transcriptional regulation of host recognition and prehaustorium development in Triphysaria versicolor using RNASeq. Promoter elements enriched in host-responsive genes were identified and evaluated using a fluorescent reporting construct. Seven elements showed clear tissue specific regulation in Triphysaria. The spatial regulation of five elements was strongly conserved in the two non-parasitic plants Arabidopsis and Mimulus. One element enhanced transcription of our fluorescent reporter in response to the host derived compound DMBQ. In Chapter 3, I show the TvQR1 promoter contains DMBQ responsive and tissue specific elements with conserved activities in two non-parasitic plants. The lack of up-regulation in the endogenous QR1 genes of these non-parasites suggests Triphysaria has co-opted the expression of QR1 for haustorium formation using conserved cis-elements. Lastly, as described in Chapter 4, I tested a Host Induced Gene Silencing approach to control the parasitic weed Phelipanche aegyptiaca. This method yielded a modest reduction in transcripts for four lipid biosynthesis genes but did not sufficiently suppress parasite growth for agricultural application. Together, this work contributes to the understanding of plant-plant interactions through studying the transcriptional regulation of host recognition in a parasitic plant and its conserved aspects in non-parasitic plants. It also yields possible applications for engineering patterns of gene expression in plants and gives insights into controlling parasitic weeds.

    Cover page: The Transcriptional Regulation of Host Recognition and Prehaustorium Development in Triphysaria versicolor
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2020)
    Triphysaria is a facultative parasitic plant in the Orobanchaceae that parasitizes the roots of a wide range of host plants including Arabidopsis, Medicago, rice and maize. The important exception to this broad host range is that Triphysaria rarely parasitize other Triphysaria. We explored self and kin recognition in Triphysaria versicolor and showed that exudates collected from roots of host species, Arabidopsis thaliana and Medicago truncatula, induced haustorium development when applied to the roots of Triphysaria seedlings in vitro while those collected from Triphysaria did not. In mixed exudate experiments, Triphysaria exudates did not inhibit the haustorium-inducing activity of those from host roots. Interestingly, when roots of Triphysaria seedlings were treated with either horseradish peroxidase or fungal laccase, the extracts showed haustorium-inducing factor (HIF) activity, suggesting that Triphysaria roots contain the proper substrates for producing HIFs. Transgenic Triphysaria roots overexpressing a fungal laccase gene TvLCC1 showed an increased responsiveness to a known HIF, 2,6-dimethoxy benzoquinone (DMBQ), in developing haustoria. Our results indicate kin recognition in Triphysaria is associated with the lack of active HIFs in root exudates. Treatment of Triphysaria roots with enzymatic oxidases activates or releases molecules that are HIFs. This study shows that exogenously applied oxidases can activate HIFs in Triphysaria roots that had no previous HIF activity. Further studies are necessary to determine if differential oxidase activities in host and parasite roots account for the kin recognition in haustorium development.
      Cover page: Kin Recognition in the Parasitic Plant Triphysaria versicolor Is Mediated Through Root Exudates
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2019)
      Broomrapes (Phelipanche aegyptiaca and Orobanche spp.) are obligate plant parasites that cause extreme damage to crop plants. The parasite seeds have strict requirements for germination, involving preconditioning and exposure to specific chemicals strigolactones [SLs] exuded by the host roots. SLs are plant hormones derived from plant carotenoids via a pathway involving the Carotenoid Cleavage Dioxygenase 8 (CCD8). Having no effective means to control parasitic weeds in most crops, and with CRISPR/Cas9 being an effective gene-editing tool, here we demonstrate that CRISPR/Cas9-mediated mutagenesis of the CCD8 gene can be used to develop host resistance to the parasitic weed P. aegyptiaca. Cas9/single guide (sg) RNA constructs were targeted to the second exon of CCD8 in tomato (Solanum lycopersicum L.) plants. Several CCD8Cas9 mutated tomato lines with variable insertions or deletions in CCD8 were obtained with no identified off-targets. Genotype analysis of T1 plants showed that the introduced CCD8 mutations are inherited. Compared to control tomato plants, the CCD8Cas9 mutant had morphological changes that included dwarfing, excessive shoot branching and adventitious root formation. In addition, SL-deficient CCD8Cas9 mutants showed a significant reduction in parasite infestation compared to non-mutated tomato plants. In the CCD8Cas9 mutated lines, orobanchol (SL) content was significantly reduced but total carotenoids level and expression of genes related to carotenoid biosynthesis were increased, as compared to control plants. Taking into account, the impact of plant parasitic weeds on agriculture and difficulty to constitute efficient control methods, the current study offers insights into the development of a new, efficient method that could be combined with various collections of resistant tomato rootstocks.
        Cover page: CRISPR/Cas9-mediated mutagenesis of CAROTENOID CLEAVAGE DIOXYGENASE8 in tomato provides resistance against the parasitic weed Phelipanche aegyptiaca
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