Light and chloroplast redox state modulate the progression of tobacco leaf infection by Pseudomonas syringae pv tabaci

Light influences plant stress responses, with chloroplasts playing a pivotal role as both energy providers and light sensors. They communicate with the nucleus through multiple retrograde signals, including secondary metabolites and reactive oxygen species (ROS). To investigate the contribution of chloroplast redox biochemistry during biotic interactions, we studied the response of tobacco leaves expressing the alternative electron shuttle flavodoxin to Pseudomonas syringae pathovars displaying different types of plant-pathogen interactions under light and dark conditions. Flavodoxin is reported to limit light-dependent ROS propagation and over-reduction of the photosynthetic electron transport system under stress. Light intensified localized cell death (LCD) in response to the incompatible pathovar tomato (Pto), but slowed disease progression caused by infective pathovar tabaci (Pta). Flavodoxin mitigated light responses during both interactions, including decreased ROS levels, reduced stromule occurrence, and lower phytoalexin production. Similar metabolic profiles were observed in the dark for both strains, with a general up-regulation of sugars, metabolic intermediates, and amino acids. In the light, instead, Pta increased hexoses and intermediates, while Pto decreased them. The results suggest that LCD-like lesions are elicited in the light even during virulent interactions, and that light effects are related to signals originating from the photosynthetic machinery.