The genomic RNA ofpotyviruses has a characteristic 5' non-translated region (5'NTR) to which a viral protein, VPg, is covalently attached. This suggests that the viral RNA lacks a conventional cap structure and thus its translation may not proceed in the same way as most cellular mRNAs. To investigate the role of the 5'NTR during translation, various derivatives of the turnip mosaic potyvirus (TuMV) leader were fused to the reporter gene fl-glucuronidase (GUS). These constructs were used to monitor the efficiency of translation in vitro in a rabbit reticulocyte lysate and in planta following microprojectile DNA delivery into tobacco cell suspensions. GUS transcripts fused with the TuMV 5'NTR, whether they were capped or not, were efficiently translated, whereas GUS transcripts without the viral leader needed to be capped for expression. When transcripts of the viral leader were supplied in excess over functional transcripts, translation was inhibited in a dose-dependent manner. Similarly, transcripts synthesized from the reverse complement of the 5'NTR inhibited translation to the same extent as the wild-type sequence, indicating that cap independence was not conferred by a specific sequence within the viral leader. A stable hairpin loop was placed in front or after the viral sequence. This hairpin loop normally prevented translation of control GUS transcripts but when the viral leader was positioned after it a significant level of GUS activity was measured, whether the transcripts were capped or not. On the other hand, when the hairpin loop was positioned after the viral leader, no GUS activity was measured. These results suggested that ribosomes bound to an internal site within the TuMV 5'NTR and then presumably scanned the sequence for the initiator AUG.

Somatic embryogenesis was induced from full-sib immature zygotic embryos of hybrid larch (Larix × leptoeuropaea) that were collected at three different dates. Analysis of variance showed interaction between the collection date and the induction medium. The highest response (55%) was observed from embryos that were at the precotyledonary stage. Twelve media containing various concentrations of abscisic acid and sucrose were used to promote the development of 'high quality' mature somatic embryos that would undergo a period of developmental arrest. Only media supplemented with abscisic acid (20, 40, and 60txM), indolebutyric acid (llxM), and 0.1 or 0.2M sucrose supported such a development. The number of mature somatic embryos produced per gram flesh weight of embryonal mass was significantly affected by the three factors tested: embryogenic line, sucrose concentration, and abscisic acid concentration. Moreover, strong interaction effects among these factors existed, complicating the formulation of a universal maturation medium that would be optimal for all embryogenic lines.Abbreviations: ABA-abscisic acid, BA-benzyladenine, IBA-indolebutyric acid, 2,4-D-2,4-dichlorophenoxyacetic acid, E M -embryonal mass, E P o t -embryogenic potential

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