Photosynthesis-dependent H₂O₂ transfer from chloroplasts to nuclei provides a high-light signalling mechanism

Chloroplasts communicate information by signalling to nuclei during acclimation to fluctuating light. Several potential operating signals originating from chloroplasts have been proposed, but none have been shown to move to nuclei to modulate gene expression. One proposed signal is hydrogen peroxide (H₂O₂) produced by chloroplasts in a light-dependent manner. Using HyPer2, a genetically encoded fluorescent H₂O₂ sensor, we show that in photosynthetic Nicotiana benthamiana epidermal cells, exposure to high light increases H₂O₂ production in chloroplast stroma, cytosol and nuclei. Critically, over-expression of stromal ascorbate peroxidase (H₂O₂ scavenger) or treatment with DCMU (photosynthesis inhibitor) attenuates nuclear H₂O₂ accumulation and high light-responsive gene expression. Cytosolic ascorbate peroxidase over-expression has little effect on nuclear H₂O₂ accumulation and high light-responsive gene expression. This is because the H₂O₂ derives from a sub-population of chloroplasts closely associated with nuclei. Therefore, direct H₂O₂ transfer from chloroplasts to nuclei, avoiding the cytosol, enables photosynthetic control over gene expression.Multiple plastid-derived signals have been proposed but not shown to move to the nucleus to promote plant acclimation to fluctuating light. Here the authors use a fluorescent hydrogen peroxide sensor to provide evidence that H₂O₂ is transferred directly from chloroplasts to nuclei to control nuclear gene expression.