Prepare plan in plpgsql

Hi,
i'm wondering if there is a way to prepare and execute a plan in a
plpgsql function.

The prepare seems to work as expected but obviously the execute
plan_name(...) statement is interpreted as the plpgsql execute keyword
and says me that the plan_name function don't exists

is there a way to use prepared queries in plpgsql?
if not what is the best way to pass a parameter directly to a query?
(im using postGIS and it would be nice if I could use another way that
unparsing and parsing a geometry parameter in an execute dynamic
query)

thank you and have all a nice day!