SUMOylation of different targets fine-tunes phytochrome signaling

Plants monitor their surrounding ambient light environment by specialized photoreceptor proteins. Among them, phytochromes monitor red and far-red light. These molecules perceive photons, undergo a conformational change, and regulate diverse light signaling pathways, resulting in the mediation of key developmental and growth responses throughout the whole life of plants. Posttranslational modifications of the photoreceptors and their signaling partners may modify their function. For example, the regulatory role of phosphorylation has been investigated for decades by using different methodological approaches. In the past few years, a set of studies revealed that ubiquitin-like short protein molecules, called small ubiquitin-like modifiers (SUMOs) are attached reversibly to different members of phytochrome signaling pathways, including phytochrome B, the dominant receptor of red light signaling. Furthermore, SUMO attachment modifies the action of the target proteins, leading to altered light signaling and photomorphogenesis. This review summarizes recent results regarding SUMOylation of various target proteins, the regulation of their SUMOylation level, and the physiological consequences of SUMO attachment. Potential future research directions are also discussed.