PSI photoinhibition alters iron homeostasis in chloroplasts and initiates retrograde signaling regulating nuclear gene expression
Photosystem I (PSI) is a critical component of the photosynthetic machinery in plants. Under conditions of environmental stress, PSI becomes photoinhibited, leading to redox imbalance in the chloroplast. PSI photoinhibition is caused by an increase in electron pressure within PSI, which damages the iron-sulfur centers. In this study, we investigated the effect of PSI electron acceptors on the susceptibility of PSI to photoinhibition at different CO2 concentrations in the plant environment. We also analyzed the global gene expression in plants exposed to PSI photoinhibition. PSI was photoinhibited using a specific illumination technique that inhibited PSI with minimal effect on PSII. CO2 levels neither increased nor decreased the likelihood of PSI photodamage. PSI photoinhibition, independent of CO2 levels, upregulated the genes involved in the response to iron excess in plants and downregulated the genes involved in iron deficiency. It also induced the genes of photosynthetic proteins that act as electron acceptors for PSI. We propose that PSI photoinhibition causes a release of iron from iron-sulfur centers, which initiates a retrograde signal from the chloroplast to the nucleus to modify gene expression. In addition, deprivation of CO2 from the air initiated a signal that induced flavonoid biosynthesis genes, probably via jasmonate production. Overall design: Arabidopsis thaliana plants were exposed to  growth light (GL; control) and PSI photoinhibition treatment (PSI-PI) under 0, 100, 400 and 1000 ppm CO2 to study gene expression in response to PSI photoinhibiton.