Together with EX2, enables higher plants to perceive singlet oxygen as a stress signal in plastid that activates a genetically determined nuclear stress response program which triggers a programmed cell death (PCD). This transfer of singlet oxygen-induced stress-related signals from the plastid to the nucleus that triggers genetically controlled PCD pathway is unique to photosynthetic eukaryotes and operates under mild stress conditions, impeding photosystem II (PSII) without causing photooxidative damage of the plant.