Abstract
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.
Highlights
Kin- and self-recognition is a common theme in biology that occurs in bacteria, plants and animals (Keenan et al, 2001; Dudley and File, 2007)
Our results suggest that the mechanism of Triphysaria kin recognition may be associated with the restricted release of haustoriuminducing factor (HIF) in their root exudates, which can be activated by peroxidases and laccases
Triphysaria versicolor is a generalist capable of parasitizing both monocot and dicot host species. When their roots contact those of the host plant Medicago truncatula, Triphysaria developed haustoria, globular root swellings with abundant localized hairs (Figure 1A and Supplemental Video 1)
Summary
Kin- and self-recognition is a common theme in biology that occurs in bacteria, plants and animals (Keenan et al, 2001; Dudley and File, 2007). Animals recognize kin as an important factor in social behaviors as well as at the molecular levels: the animal immune system is highly sophisticated and regulates self-recognition system (Keenan et al, 2001; Chaplin, 2010). Plants are capable of recognizing self or closely related plants at multiple levels. Sexual-incompatibility occurs widely in flowering plants to prevent fertilization by self or genetically related pollens which promotes out-. Deschampsia caespitosa are capable of detecting root exudates from plants of different genetic identities and change root morphology rather than root biomass in response to conspecific root exudates (Semchenko et al, 2014)
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