Haustorium development in the parasitic plant, Triphysaria (Orobanchaceae): A genetic and molecular analysis

In the Orobanchaceae, a single origin of root parasitism followed by multiple losses of photosynthetic capacity characterizes the evolution of parasitic plants. Increasing heterotrophy or reliance on host nutrients is accompanied by loss of genetic material from the chloroplast, gain of parasite-specific traits, and increasing host specificity via recognition of common plant secondary metabolites, such as phenolics and quinones.;Triphysaria, a hemiparasitic plant belonging to the Orobanchaceae, was used as a model to investigate the genetic and molecular mechanisms governing haustorium development. Haustoria are the "organs of parasitism" and form at the root tips of parasitic Orobanchaceae in response to host-derived haustorium inducing factors (HIF's). Variation in natural populations of Triphysaria was observed for haustorium development in response to the HIF, 2,6-dimethoxy-p-benzoquinone (DMBQ). DMBQ responsiveness was shown to be heritable and influenced by maternal effects.;Development of autohaustoria, haustoria that form in the absence of host-derived factors, was monitored in Triphysaria pusilla. Triphysaria rarely form haustoria when grown alone or with conspecific plants, suggestive of a mechanism for self-recognition and avoidance of self-parasitism. Propensity to form autohaustoria showed a strong positive correlation with degree of anthocyanin pigmentation. GA3 pre-treatment of seeds obtained from high and low anthocyanin parents lead to a significant and unexpected increase in autohaustoria formation for both groups. Results suggest that high anthocyanin plants may be self-inducing via exuded flavonoids and that plant hormones, particularly auxin and gibberellin, may be involved in regulating self-recognition and autohaustorium development in root parasitic plants.;Transcript accumulation of three genes was assayed in variant Triphysaria populations. Two genes, TvQR1 and TvQR2, encode putative quinone oxidoreductases and one, TvPirin, encodes a nuclear transcription factor involved in cell cycle regulation. TvQRI performs a one-electron reduction of quinone to semiquinone, and was positively correlated to haustorium development. TvQR2 performs a two-electron reduction of quinone to phenolic acid and was correlated to DMBQ induction, though not haustorium development. TvPirin was also correlated to DMBQ induction, but not haustorium development. Results support the proposed redox cycling model of semiquinone-induced haustorium development in the Orobanchaceae.