Root cytosolic enzyme assays

[Kirk Schnorr]

I am writing on behalf of a friend of mine that is having difficulty with
the gus assays he is performing on transgenic plumbaginifolia roots.  He
would like to know of there is a simple, quantitative colorometric test
for a housekeeping root enzyme so that he can make an adjusted value for 
his gus activities.  Does anyone know of some good enzymes and their
assays?

Thanks in advance

Kirk Schnorr
Laboratoire de Biologie Cellulaire
INRA Versailles
France.