Expansin message regulation in parasitic angiosperms: Marking time in development

Parasitic strategies are widely distributed across the angiosperms and are estimated to have evolved at least eight different times. Within the obliga te hemiparasitic and holoparasitic members, elaborate strategies for host s election have emerged. Here, we demonstrate that in the parasitic Scrophula riceae Striga asiatica, for which signal-mediated host detection is critica l, expansin mRNA provides a reliable and accurate downstream molecular mark er for the transition to the parasitic mode. Three different expansin genes , saExp1, saExp2, and saExp3, are regulated by xenognostic quinones. saExp3 appears to function as a seedling expansin, and its mRNA is depleted withi n minutes after induction of the host attachment organ, saExp1 and saExp2 s hare less homology with the known expansins, and their transcripts accumula te linearly over a critical induction period. The regulation of these genes suggests that the resources for developmental commitment must accumulate t o a defined threshold before commitment to organogenesis is terminal. When the induction signal is removed prematurely, the accumulated message decays with a time constant that correlates with the time required for additional signal exposures to reinduce parasitic development. These results suggest that sophisticated controls exist for the accumulation of the necessary com ponents for terminal commitment to the parasitic mode. Furthermore, buildin g on the redox dependence of the inducing signal, they suggest a model akin to a "molecular capacitor" for clocking organogenesis in S. asiatica.