Conservation of Eukaryotic polIII promoters

[And'ys bionet]

Would anyone be able to tell me (or point me in the right direction) how
highly conserved pol III promoters are?  The background to this is as
follows.

I would like to use Ambion's pSilencer 1.0 -U6 vector which "Expresses siRNA
within mammalian cells using a U6 Polymerase III promoter".  However, I
would like to use it in a *plant* cell.  I have contacted Ambion, and they
think it should work, but, no disrespect intended to Ambion, I just wondered
if anyone could give me a bit more data to back this up?  It is no trivial
thing to produce and screen the transgenic plants (we don't work with
Arabidopsis) so I would need some reasonable level of assurance that the
thing is likely to work before embarking on this.

Many thanks,

--
Dr. Andy Bettany
Research Scientist
Institute of Grassland and Environmental Research
Plas Gogerddan
Wales, UK