How to apply the attribute "rel=nofollow" to links in the comments? I configured the Text Format "Plain text" (admin/config/content/formats) to "Convert URLs into links" so I would like to add the attribute rel="nofollow" to all the links in the comments.

This way I would minimize the effects of spammy links in the comments.

Is there an easier way than using regular expressions to insert the attribute?
Go to Configuration -> Content authoring -> Text formats -> Plain text (your format), then tick the Limit allowed HTML tags option.

In the filter settings at the bottom of the page is the Add rel="nofollow" to all links checkbox. Tick that... Since this is plain text, you will probably want to remove the contents of the Allowed HTML Tags field, too.