New insights into the retrograde signaling pathway between the plastids and the nucleus.

In plant cells, the genetic information required for biological activity is divided into three organelles—the nucleus, plastids and mitochondria. These organelles require tightly coordinated gene expression to accomplish the appropriate biological processes. Chloroplasts harness light energy and use it for carbon fixation in photosynthesis. However, majority of the proteins involved in photosynthesis is encoded by the nucleus genome. Thus, nuclearencoded photosynthesis-related proteins are targeted to plastids after their synthesis in the cytosol. Therefore, it is critical to regulate nuclear gene expression in response to the functional or metabolic state of the plastids; this process relies on signals from the plastids to the nucleus that are known as retrograde signals. Our genetic studies revealed that GENOMES UNCOUPLED 1 (GUN1) and Golden2-like1 (GLK1) mediate the retrograde signal that coordinates plastid protein import and nuclear gene expression. In this study, we propose a novel signaling pathway that regulates nuclear gene expression according to the rate of protein import into the plastid.