Plant innate immunity – sunny side up?

• Reactive oxygen species (ROS) and calcium signaling are central to plant immunity.
• Surprisingly, ROS and calcium signaling branch via the chloroplast during immunity.
• Light influences the chloroplast branch to alter immunity.
• The chloroplast branch provides environmental feedback on the immune reaction.

Reactive oxygen species (ROS)- and calcium- dependent signaling pathways play well-established roles during plant innate immunity. Chloroplasts host major biosynthetic pathways and have central roles in energy production, redox homeostasis, and retrograde signaling. However, the organelle's importance in immunity has been somehow overlooked. Recent findings suggest that the chloroplast also has an unanticipated function as a hub for ROS- and calcium-signaling that affects immunity responses at an early stage after pathogen attack. In this opinion article, we discuss a chloroplastic calcium-ROS signaling branch of plant innate immunity. We propose that this chloroplastic branch acts as a light-dependent rheostat that, through the production of ROS, influences the severity of the immune response.