Fig. 4

Model for H₂O₂ sensing. A Balanced H₂O₂ contents in wild type plants (WT) in normal situations without exogenous stress. B Imbalanced H₂O₂ contents between chloroplast and cytoplasm in two different situations: (1) wild type versus HyPER expression in chloroplasts (left) or (2) wild type under control conditions versus wild type under stress conditions (right). Total intracellular H₂O₂ contents were measured by H2DCFDA and expressed relative to wild type or wild type control conditions. In both cases, H₂O₂ content is imbalanced in the same way and high stromule numbers as well as expression of genes induced by oxidative stress are observed, indicating that the ratio between the contents of the cytoplasm and the chloroplasts might be the parameter which is sensed by the cells. This is still a hypothesis and has to be further analyzed, e.g. in crossings between plants expressing HyPER in the cytoplasm and in the chloroplasts