| The parasitic angiosperms depend on specific sensory pathways to regulate commitment to host plants. In Striga asiatica, contact between the parasitic seedling and the host surface is known to result in the release of xenognostic quinones from the host's cell wall; however, the mechanisms that mediate the release and the controls on the process are unknown. Image analysis of the S. asiatica seedling with laser scanning confocal microscopy and electron microscopy have now revealed a constitutive production of H2O2 radiating from the surface cells of the root meristem. Specific inhibitor studies suggest that the source of the H2O2 is a NAD(P)H oxidase, and that the oxidant is maintained at a threshold below that necessary for the activation of cell defense and programmed cell death usually associated with H2 O2 production. Moreover, the xenognostic quinones down-regulate H2O2 production. The timing and concentration dependence for this down-regulation suggest that sophisticated feed-back controls exist to fine-tune the H2O2 concentration so as to avoid host defenses. |
