Chloroplasts as mediators of plant biotic interactions over short and long distances

In nature, plants interact with numerous other organisms. Some interactions benefit both the plant and the other organism(s), while others lead to disease or even death of the plant hosts. The traditional focus of research into plant biotic interactions has been on the negative effects on plants and the strategies plants use to mitigate or prevent these. Over the last several years there has been increasing appreciation for the diversity and importance of plant biotic interactions in plant success as well as the evolution and stabilization of ecosystems. With this new perspective, it is also becoming clear that the metabolic output of chloroplasts in plants is critical to establishing and maintaining these interactions. Here we highlight the roles of chloroplasts in diverse biotic interactions. Photosynthetic chloroplasts are the source of hormones, small molecules and a prodigious number of secondary metabolites, a significant portion of which influence plant biotic interactions. Importantly, the effects of chloroplasts on these interactions are not limited to sites of direct association or contact but also act at a distance in systemic leaves and roots, in the rhizosphere, in the air surrounding a plant and in neighboring plants, and they can persist over several years.