Open maps of locales

I need help with respect to the following question. With the help of M.M. Clementino and J. Picado, I understood that an open morphism f: X --> Y in the category of locales (dual to the category of frames) has the property that taking inverse images (pullbacks) along f commutes with taking closures of sublocales: f*[cl(N)] = cl(f*[N]) for all sublocales N of Y. Conversely, does this property force f to be open (as it does for topological spaces)? If not, is the answer positive when f is the embedding of a sublocale? I appreciate any suggestions/answers that you may have. Thanks, Walter Tholen.