There are several ways to use PlantUML and Emacs together :
You will find the needed macro at the following address http://www.emacswiki.org/emacs/IanYang
Once installed, embed PlantUML code in Emacs org-mode is used like this:
While the org file is exported to HTML or other formats, PlantUML is called to generate image and replace the block in place.
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response
Thanks to Ian Yang for the work done.
This method, however, is considered obsolete:
;; OBSOLETED, use ob-plantuml.el bundled in org instead.( A citation from https://www.emacswiki.org/emacs/org-export-blocks-format-plantuml.el )