Alternative to gene silencing for functional assay?? much help needed :) - (Aug/02/2012 )

Hi,
I have question regarding functional analysis. I basically have a plant receptor and we'd like to know whether its activity is dependant on a co-receptor in the same plant. To do this our main strategy was to perform transient gene silencing of the co-receptor by siRNA (using hairpin RNA). But unfortunately, the qPCR analysis consistently shows that the co-receptor gene is not being silenced ..

Is there an alternative technique that I can use to show that the receptor is dependant on this co-receptor for activity other than silencing?

Is there any potential for using a drug that might target the co-receptor? Do you think the interaction is direct? If so, you may be able to do something like a co-immunoprecipitation. Reporter gene assay for a particular down stream target?