© 2006 Heron Publishing—Victoria, Canada
Endogenous ascorbic acid modulates meristem reactivation in white spruce somatic embryos and affects thymidine and uridine
metabolism
Claudio Stasolla (1, 3) and Edward C. Yeung (2)
1. Department of Plant Science, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2 / 2. Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada, T2N 1N4 / 3. Corresponding author (stasolla@ms.umanitoba.ca) / Received October 19, 2005; accepted December 19, 2005; published online June 1, 2006
Summary
Previously, we demonstrated that, in some cell lines, exogenous applications of ascorbic acid (ASC) enhance the conversion
frequency of white spruce (Picea glauca (Moench) Voss) somatic embryos, by stimulating mitotic activity in the apical meristems. To examine this event in more detail,
we investigated the effects of ASC on de novo, salvage and degradation pathways of pyrimidine metabolism by following the
metabolic fate of 14C-labeled orotic acid, thymidine, uridine and uracil in shoot and root poles of germinating embryos, after altering the cellular
ASC content of the embryos. Alterations in endogenous ASC content did not affect the utilization of either orotic acid or
uracil, but affected the metabolism of thymidine and uridine. Specifically, a lowering of endogenous ASC content by applications
of lycorine (L), an inhibitor of the last enzyme of the ASC de novo biosynthetic pathway, resulted in a lower embryo conversion
frequency, as well as a reduced percentage of thymidine and uridine incorporated into nucleotides and nucleic acids. The reduction
in thymidine and uridine anabolism was mainly ascribed to the decreased activities of thymidine kinase (TRK) and uridine kinase
(URK), the respective salvage enzymes of thymidine and uridine, measured in L-treated embryos. These effects were solely a
result of a decrease in endogenous ASC content because applications of ascorbic acid plus lycorine (ASC + L) increased embryo
conversion frequency, thymidine and uridine salvage activities, and TRK and URK activities to near control values. Inclusion
of exogenous ASC in the germination medium did not affect the percentage of embryos able to convert to viable plantlets, although
it increased thymidine and uridine utilization for nucleic acid synthesis in the shoot and root poles of the embryos. Taken
together, these findings confirm that cellular ASC plays a key role in the reactivation of the apical meristems of germinating
white spruce somatic embryos.
Keywords:
autoradiography, cell division, degradation pathway, DNA, lycorine, meristem reactivation, orotic acid, Picea glauca, salvage pathway, uracil.
