FIGURE 2.

More complex scenarios for retrograde signaling. (A) The signaling molecule generated in and exported from the organelle (filled circles), and the signaling molecule that enters the nucleus (open circles) might not be identical. ABA, which is synthesized in the cytosol from a chloroplast precursor, is a possible example. (B) The signaling molecule generated in the organelle might not even leave the organelle. ¹O₂ serves here as an example, as it could, in principle, generate volatile oxidation products of carotenoids that serve as “downstream” messengers. (C) Direct delivery of the signaling molecule to the nucleus via stromules.