Photosynthesis-dependent H(2)O(2) 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 (H(2)O(2)) produced by chloroplasts in a light-dependent manner. Using HyPer2, a genetically encoded fluorescent H(2)O(2) sensor, we show that in photosynthetic Nicotiana benthamiana epidermal cells, exposure to high light increases H(2)O(2) production in chloroplast stroma, cytosol and nuclei. Critically, over-expression of stromal ascorbate peroxidase (H(2)O(2) scavenger) or treatment with DCMU (photosynthesis inhibitor) attenuates nuclear H(2)O(2) accumulation and high light-responsive gene expression. Cytosolic ascorbate peroxidase over-expression has little effect on nuclear H(2)O(2) accumulation and high light-responsive gene expression. This is because the H(2)O(2) derives from a sub-population of chloroplasts closely associated with nuclei. Therefore, direct H(2)O(2) transfer from chloroplasts to nuclei, avoiding the cytosol, enables photosynthetic control over gene expression.