Semagenesis and the parasitic angiosperm Striga asiatica.

Over the last several years, intermediates in the reduction of dioxygen have been attributed diverse functional roles ranging from protection against pathogen attack to the regulation of cellular development. Evidence now suggests that parasitic angiosperms, which naturally commit to virulence through the growth of new organs, depend on reduced oxygen intermediates, or reactive oxygen species (ROS), for signal generation. Clearly, the role of ROS in both plant defense and other physiological responses complicates any models that employ these intermediates in host plant recognition. Here we exploit the transparent young Striga asiatica seedling to (i) localize the site of H(2)O(2) accumulation to the surface cells of the primary root meristem, (ii) demonstrate the accumulation of H(2)O(2) within cytoplasmic and apoplastic compartments, and (iii) document precise regulation of H(2)O(2) accumulation during development of the host attachment organ, the haustorium. These studies reveal a new active process for signal generation, host detection and commitment that is capable of ensuring the correct spatial and temporal positioning for attachment.