Chloroplast signalling in the light induction of nuclear HSP70 genes requires the accumulation of chlorophyll precursors and their accessibility to cytoplasm/nucleus

Summary Chlorophyll precursors Mg‐protoporphyrin IX and its monomethylester are candidates for plastid‐derived molecules involved in light signalling from the chloroplast to the nucleus. The pool sizes of these two Mg2+‐containing porphyrins and of protoporphyrin IX transiently increased upon a shift of Chlamydomonas cultures from dark to light. This increase coincided with the accumulation of mRNAs encoded by the nuclear genes HSP70A and HSP70B. Analysis of a mutant (brs‐1), previously shown to be defective in the light induction of these genes, revealed high levels of protoporphyrin IX but no light‐induced increase in the levels of Mg2+‐containing porphyrins. Inhibitors of cytoplasmic protein synthesis prevented both the light‐induced rise in pool levels and induction of the HSP70 genes. Similarly, pre‐gametes, intermediates of sexual differentiation, lacked both responses to light. The block in light induction of the HSP70 genes in inhibitor‐treated cells and in pre‐gametes could be circumvented by the exogenous addition of Mg‐protoporphyrin IX in the dark. This suggests an essential role for light‐induced Mg‐protoporphyrin IX accumulation in this chloroplast‐to‐nucleus signalling pathway. However, accumulation of this porphyrin in the dark – presumably in the chloroplast – did not result in induction. A second crucial role for light in this signalling pathway is postulated which makes this plastidic compound accessible to the cytoplasm/nucleus where the downstream signalling pathway may be activated.