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• 5-aminolevulinic acid (ALA) alleviates the effect of methyl viologen-induced photooxidative stress on pear leaves. • ALA trigger endogenous H 2 O 2 accumulation. • NADPH oxidase-mediated H 2 O 2 signaling , which results in activated antioxidant enzyme activities, may plays an important role in mediating ALA-induced tolerance to photooxidative stress. The beneficial effects of 5-aminolevulinic acid (ALA) on abiotic stress have been demonstrated in several plant species. However, little is known about the signal transduction pathways triggered by ALA in the photooxidative stress response. In this study, we investigated the effects of ALA on methyl viologen (MV)-induced photooxidative stress tolerance in detached pear leaves (Pyrus ussuriensis). Furthermore, we also examined the involvement of NADPH oxidase (RBOH)-mediated H 2 O 2 as a signaling molecule in ALA-induced photooxidative stress tolerance. Our results showed that exogenous ALA alleviated the observed decreases in chlorophyll content and maximum potential photosystem II efficiency under MV-induced stress. Exogenous ALA also effectively reduced MV-induced oxidative damage and enhanced the activities of antioxidative enzymes (SOD and CAT). In situ determination of reactive oxygen species revealed that ALA induced H 2 O 2 production in the pear leaves. Importantly, ALA-induced tolerance was abolished by an RBOH inhibitor. Further investigation revealed that the transient overexpression of RBOHF in tobacco leaves alleviated MV-induced damage by enhancing the activities of SOD and CAT. This study provides new insights into the mechanisms by which ALA improves photooxidative tolerance in pear leaves. [ABSTRACT FROM AUTHOR]