| Abstract |
Plant heme oxygenase-1 (HO-1), an essential enzyme for heme degradation, is induced under various abiotic stresses and is generally regarded as a protective factor against oxidative stress. We generated transgenic rice plants overexpressing human HO-1 (hHO1-OE) to examine the role of HO-1 in the plant stress response to photodynamic damage induced by the peroxidizing herbicide oxyfluorfen (OF). Compared to wild-type (WT) plants, the hHO1-OE lines exhibited more severe leaf necrosis following OF treatment. The transgenic lines also showed greater electrolyte leakage and malondialdehyde levels, reduced photosynthetic efficiency, and higher H2O2 accumulation, as well as greater accumulation of the photosensitizer protoporphyrin IX (Proto IX) relative to WT plants. After 30 h of OF exposure, the transgenic plants displayed larger reductions in Mg-Proto IX, chlorophyll, and heme contents than WT plants, accompanied by a stronger downregulation of most genes involved in porphyrin biosynthesis. In addition, the hHO1-OE lines exhibited a more pronounced decline in the ascorbate redox state and a weaker induction of heme-dependent H2O2-scavenging enzymes. Collectively, these results indicate that hHO-1 overexpression increases sensitivity to OF-induced photodynamic stress by dysregulating porphyrin metabolism and redox homeostasis. Our findings demonstrate that, under OF-induced photodynamic stress, excessive HO-1 expression can exert pro-oxidant effects, highlighting the importance of maintaining optimal HO-1 levels for stress tolerance.
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