Photosystem II in plants is a complex structure that helps with photosynthesis. Scientists studied a barley mutant lacking Photosystem I to see how Photosystem II reacts to extreme light exposure. They found that the mutant had a smaller antenna size around Photosystem II, which forms arrays to distribute energy efficiently and protect against damage. The mutant also showed two pathways activated by excess light, one regulating antenna size based on electron transport chain redox state, and the other controlling carotenoid production in response to stress.