More ‘Friendly’
Selectable Markers: the Positive Selection Method
In
contrast to the traditional selection where the transgenic cells acquire the
ability to survive on selective media while the non-transgenic cells are killed
(negative selection), the positive selection method, first developed by Joersbo
and Okkels (1996) favors regeneration and growth of the transgenic cells while
the non-transgenic cells are starved but not killed. The positive selection
method exploits the fact that cytokinin must be added to plant explants in
order to obtain optimal shoot regeneration rates. By adding cytokinin as an
inactive glucuronide derivate, cells which have acquired the GUS gene by transformation are able to
convert the cytokinin glucuronide to active cytokinin while untransformed cells
are arrested in development. In this system, GUS serves the dual purpose of being both a selectable and
screenable marker gene. Another interesting system of positive selection uses
the xylose isomerase gene from Thermoanaerobacterium
thermosulforogenas as a selectable gene,which expression allows
effectiveselection of transgenic plan cells using D-xylose as the selection
agent (Haldrup et al., 1998). The
transformation frequencies obtained by positive selection appear to be higher
than using the negative selection method. This could be related to the fact
that during negative selection the majority of the cells in the explants die.
Such dying cells may release toxic substances which in turn may impair
regeneration of the transformed cells. In addition, dying cells may form a
barrier between the medium and the transgenic cells preventing uptake of
essential nutrients.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.