The RNA world hypothesis states that during an early stage of life, RNA molecules functioned as genome and as the only genome-encoded catalyst. This hypothesis is supported by several lines of evidence, one of which is the in vitro selection of catalytic RNAs (ribozymes) in the laboratory for a wide range of reactions that might have been used by RNA world organisms. This review focuses on three types of ribozymes that could have been involved in the synthesis of RNA, the core activity in the self-replication of RNA world organisms. These ribozyme classes catalyze nucleoside synthesis, triphosphorylation, and the polymerization of nucleoside triphosphates. The strengths and weaknesses regarding each ribozyme’s possible function in a self-replicating RNA network are described, together with the obstacles that need to be overcome before an RNA world organism can be generated in the laboratory.
Fungal communities are established in A. americanum stems as well as in fruits and seeds, suggesting vertical transmission. These internally derived fungi act antagonistically toward fungi with pathogenic tendencies. As such, native mistletoe endophytes might protect A. americanum against fungal pathogens in nature. In the future, manipulation of endophytes might be a component of mistletoe control programs.
The lodgepole pine dwarf mistletoe Arceuthobium americanum is a severe pest in coniferous forests of western North America. To facilitate laboratory studies of this obligate parasite, a modified White's medium was used for in vitro culture; the concentrations of IAA and Kn were varied to determine optimal IAA/Kn ratios. It was found that explant health was related to the concentration of IAA (p = 0.008), but not Kn (p = 0.937), and that explants were healthiest at an IAA/Kn ratio of 0.1. Radicular apices were generated at IAA/Kn ratios of 1.0 or greater, but no shoot organogenesis was observed. These data suggest that A. americanum is more sensitive to auxins than cytokinins. Parasites of plants are known to secrete high levels of cytokinins, which stimulate the host to shuttle nutrients to the infected area, and so we suspect that A. americanum has reduced cytokinin sensitivity.
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