Localized hormone fluxes and early haustorium development in the hemiparasitic plant Triphysaria versicolor

Perhaps the most obvious phenotypes associated with chemical signaling between plants are manifested by parasitic species of Orobanchaceae. The development of haustoria, invasive root structures that allow hemiparasitic plants to transition from autotrophic to heterotrophic growth, is rapid, highly synchronous, and readily observed in vitro. Haustorium development is initiated in aseptic roots of the facultative parasite Triphysaria versicolor when exposed to phenolic molecules associated with host root exudates and rhizosphere bioactivity. Morphological features of early haustorium ontogeny include rapid cessation of root elongation, expansion, and differentiation of epidermal cells into haustorial hairs, and cortical cell expansion. These developmental processes were stimulated in aseptic T. versicolor seedlings by the application of exogenous phytohormones and inhibited by the application of hormone antagonists. Surgically dissected root tips formed haustoria if the root was exposed to haustorial-inducing factors prior to dissection. In contrast, root tips that were dissected prior to inducing-factor treatment were unable to form haustoria unless supplemented with indole-3-acetic acid. A transient transformation assay demonstrated that auxin and ethylene-responsive promoters were up-regulated when T. versicolor was exposed to either exogenous hormones or purified haustoria-inducing factors. These experiments demonstrate that localized auxin and ethylene accumulation are early events in haustorium development and that parasitic plants recruit established plant developmental mechanisms to realize parasite-specific functions.