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cau for ferredoxin decline and restores electron delivery to productive routes, resulting in enhanced stress tolerance. We have designed an in vivo system to optimize flavodoxin reduction and NADP+ regeneration under stress using a version of cyanobacterial ferredoxin–NADP+ reductase without nsport regulatory and dissipative routes of the stroma [3]. In cyanobacteria and some marine algae, Fd can be replaced by flavodoxin (Fld), an isofunctional electron shuttle not present in plants which contains mechanisms have been invoked to explain this phenomenon. First, stress conditions inhibit CO2 assimilation and hence, NADPH oxi-dation in the regenerative stages of the Calvin cycle [7]. Second, many environmental insults cause down-regulation of Fd expres-sion in plants and cyanobacteria [9–11]. Both effects contribute to decrease the levels of the major physiological PETC acceptors (oxidized Fd and NADP+). Under these conditions, the PETC should become over-reduced and the ability of O2 to subtract electrons from the chain is expected to increase, leading to runaway ROS propagation [7]