Stromules: An Incident or Formalized Pathway for Molecules Transfer Between Organelles?

The stromules are tubular extensions of chloroplasts, or broader plastids, formed by the organelle envelope and filled with the stroma, the internal content of organelle. The formation of stromules is related to the cytoskeleton. Stromules occur in photosynthetic tissues under illumination and are therefore proposed to be important for retrograde signaling, which is essential for adaptation to stress factors. Stromules are also observed after pathogen attack. Some groups propose that stromules are a resemblance to endoplasmic reticulum dynamics, without having actual significance in molecules transport within a cell. However, there is no consensus among researchers regarding the actual function and significance of stromules, which can be the result of different models used to study stromules, and the necessity of using fluorescent labels, with all advantages and limitations of fluorescence-based methodology. Here I briefly discuss current knowledge on the subject from a perspective of stromule origin-the chloroplast envelope, and the potential advantages of having a conduit within organelles instead of relying on diffusion through cytosol.