Chloroplast redox control of nuclear gene expression -: A new class of plastid signals in interorganellar communication

Chloroplasts are genetically semiautonomous organelles that contain their own subset of 100-120 genes coding for chloroplast proteins, tRNAs, and rRNAs. However, the great majority of the chloroplast proteins are encoded in the nucleus and must be imported into the organelle after their translation in the cytosol. This arrangement requires a high degree of coordination between the gene expression machineries in chloroplasts and nucleus, which is achieved by a permanent exchange of information between both compartments. The existence of such coordinating signals has long been known; however, the underlying molecular mechanisms and signaling routes are not understood. The present data indicate that the expression of nuclear-encoded chloroplast proteins is coupled to the functional state of the chloroplasts. Photosynthesis, which is the major function of chloroplasts, plays a crucial role in this context. Changes in the reduction/oxidation (redox) state of components of the photosynthetic machinery act as signals, which regulate the expression of chloroplast proteins in both chloroplasts and nucleus and help to coordinate the expression both in compartments. Recent advances in understanding chloroplast redox regulation of nuclear gene expression are summarized, and the importance for intracellular signaling is discussed.