A better Google+ notification experience in email | Official Gmail Blog

Google goes contextual for communication + collaboration

"Notification emails are a great way to keep up with what's happening in the Google+ stream: whether someone mentions you, comments on your post, or shares with you directly. It's not always enough to just read these updates, however; sometimes you want to respond right away, right from your inbox. The good news is that starting today, you can reply to Google+ notifications from Gmail.
On the desktop, in addition to adding people to circles, and viewing recent Google+ content in the people widget, we're now making Google+ notification emails in Gmail completely interactive:

• You can view, comment on, and +1 posts from inside your inbox

• Your comments appear in the Google+ stream in real-time, and

• Responses from others instantly appear in Gmail, as part of the notification message"

A better Google+ notification experience in email | Official Gmail Blog