Different Light Wavelengths Differentially Influence the Progression of the Hypersensitive Response Induced by Pathogen Infection in Tobacco

Using light-emitting diodes (LEDs), we examined how different light wavelengths influence the hypersensitive response (HR) in tobacco plants infected with <i>Pseudomonas syringae</i> pv. <i>tomato</i> (Pst). Pst-infiltrated plants exhibited greater resistance to Pst infection under green and blue light compared to white and red light, as indicated by reduced HR-associated programmed cell death, lower H₂O₂ production, and up to 64% reduction in membrane damage. During the late stage of HR, catalase and ascorbate peroxidase activities peaked under green and blue LEDs, with 5- and 10-fold increases, respectively, while superoxide dismutase activity was higher under white and red LEDs. Defense-related genes <i>CHS1</i>, <i>PALa</i>, <i>PR1</i>, and <i>PR2</i> were more strongly induced by white and red light. The plants treated with green or blue LEDs during Pst infection prompted faster degradation of phototoxic Mg-porphyrins and exhibited smaller declines in <i>F</i>_v/<i>F</i>_m, electron transport rate, chlorophyll content, and <i>LHCB</i> expression compared to those treated with white or red LEDs. By contrast, the induction of the chlorophyll catabolic gene <i>SGR</i> was 54% and 77% lower in green and blue LEDs, respectively, compared to white LEDs. This study demonstrates that light quality differentially affects Pst-mediated HR, with green and blue light more effectively suppressing HR progression, mainly by reducing oxidative stress through enhanced antioxidative capacity and mitigation of photosynthetic impairments.