On February, 3rd, Al Mattes wrote:

Phytochealtors are naturally occurring in some plants and seem to be the mechanism by which these plants are able to handle what otherwise would be toxic metals.  Some plants are able to produce more of the chelators than others and there is evidence that there are several different forms of phytochelators that exist.  Production is often triggered by the presence of Cadmium in the soil although there is also evidence that the phytochelators are produced by some plants when Arsenic is found in the soil as well.  There are many attempts to introduce the genetic basis for the production of phytochelators into other plants currently underway in labs around the world.

Dr. W. Rauser of the University of Guelph is a noted authority on this subject and has a review that was pubished many years ago that covered the state of knowledge to that point.  He has also authored a new review that is not yet published.

A search of the literature should reveal the original article as well as many others that have been published on the subject.  It is a complex area.
 There is some indication that the phytochelator works by binding the metal to sulphur and thereby producing an insoluble sulphide but I have only one reference that suggests this.

Hope this helps

Al Mattes