Fit unequal variance model in R

I'am trying to develop some code if R, which would correspond to what I
did in SAS.
The data look like:

Treatment    Replicate    group1      GSI

..
The SAS code is:
proc mixed data=data_name order=data method=ml; *scoring=10;
      classes group1;
      model GSI=group1/residual influence solution;
      repeated /group=group1;

run;

Basically, I need different variance for each treatment group. I want to
do the similar thing in R.

Here is what I get so far:
lm1<-lme(response~treatment,data=o,random=~1|as.factor(dummy),weights=varIdent(form=~1|treatment),method="ML")

There should no random term in the model. However If I don't specify one,
lme won't work, so I made a dummy variable, which equals to 1 for every
observation.