Quinone oxidoreductase message levels are differentially regulated in parasitic and non-parasitic plants exposed to allelopathic quinones

Allelopathic chemicals released by plants into the rhizosphere have effects on neighboring plants ranging from phytoxicity to inducing organogenesis. The allelopathic activity of naturally occurring quinones and phenols is pr imarily a function of reactive radicals generated during redox cycling betw een quinone and hydroquinone states. We isolated cDNAs encoding two distinc t quinone oxidoreductases from roots of the parasitic plant Triphysaria tre ated with the allelopathic quinone 2,6-dimethoxybenzoquinone (DMBQ). TvQR1 is a member of the zeta -crystallin quinone oxidoreductase family that cata lyzes one-electron quinone reductions, generating free radical semiquinones . TvQR2 belongs to a family of detoxifying quinone oxidoreductases that cat alyze bivalent redox reactions which avoid the radical intermediate. TvQR1 and TvQR2 message levels are rapidly upregulated in Triphysaria roots as a primary response to treatment with various allelopathic quinones. Inhibitio n of quinone oxidoreductase enzymatic activity with dicumarol prior to quin one treatment resulted in increased transcript levels. While TvQR2 homologs were upregulated by DMBQ in roots of all plants examined, TvQR1 homologs w ere upregulated only in roots of parasitic plants. Phylogenetic trees const ructed of TvQR1 and TvQR2 protein homologs in Archea, Eubacteria and Eukary otes indicated that both gene families are ancient, yet the families have d issimilar evolutionary histories in angiosperms. We hypothesize that TvQR2- like proteins function to detoxify allelopathic quinones in the rhizosphere , while TvQR1 has specific functions associated with haustorium development in parasitic plants.