Date of Award
Doctor of Philosophy
Biochemistry, Biophysics and Molecular Biology
Reuben J. Peters
Secondary metabolites play a significant role in mediating plant-microbe interactions. Rice (Oryza sativa) is one of the most important staple crops. Once rice is attacked by microbial pathogens, it will produce phytoalexins antibiotics to defend against enemies such as the bacterial pathogen Xanthomonas oryzae. Diterpenoids provide a rich source of metabolites mediating rice-X. oryzae interaction. First, gibberellin (GA), as complex diterpenoid phytohormone, besides the profound effects on plant growth and development, it has been shown to negatively regulate rice defense. Interestingly, we found a putative GA biosynthetic gene operon in rice bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc). Based on this, we biochemically characterized Xoc for its capacity to produce the precursor of GA. With genome mining, we found three other genera of rhizobia contained homologous GA biosynthetic gene operon and shared the identical biochemical function for production of ent-kaurene. As for Xoc, we detected the putative production of GA was relevant to virulence ability in Xoc through antagonism to jasmonic acid. The role of putative GA diterpenoid regulated Xoc's association with rice. Second, rice produces an arsenal of phytoalexins as responses to fungal and bacterial infection. Most of the known rice phytoalexins are diterpenoids. The relevance of these diterpenoids to defense was evaluated based on their antibiotic activity in vitro; the physiological role in planta remains undefined. Rice diterpenoids phytoalexins biosynthesis proceeds ent-copalyl diphosphate synthase (OsCPS2) or syn-copalyl diphosphate synthase (OsCPS4) from diterpenoid precursor geranylgeranyl diphosphate (GGPP). With reverse genetics approach we found the relevance of rice diterpenoids to defense depends on OsCPS2 pathway instead of OsCPS4. Furthermore, with application of CRISPR/Cas9 genome editing technology, I not only explored the relevance of the specific diterpenoid pathway downstream of OsCPS2 to rice plant defense but identified the redirection of metabolites levels in rice specialized metabolism as well. These results will not only elucidate some insight into the metabolites balance in rice-X. oryzae interaction but also provide the potential agricultural application for molecular breeding.
Lu, Xuan, "Investigating the roles of diterpenoids in rice-Xanthomonas oryzae interactions" (2015). Graduate Theses and Dissertations. 14912.