Chloroplasts of Arabidopsis Are the Source and a Primary Target of a Plant-Specific Programmed Cell Death Signaling Pathway

Enhanced levels of singlet oxygen ( 1 O 2 ) in chloroplasts trigger programmed cell death. The impact of 1 O 2 production in chloroplasts was monitored first in the conditional fluorescent ( flu ) mutant of Arabidopsis thaliana that accumulates 1 O 2 upon a dark/light shift. The onset of 1 O 2 production is rapidly followed by a loss of chloroplast integrity that precedes the rupture of the central vacuole and the final collapse of the cell. Inactivation of the two plastid proteins EXECUTER (EX1) and EX2 in the flu mutant abrogates these responses, indicating that disintegration of chloroplasts is due to EX-dependent signaling rather than 1 O 2 directly. In flu seedlings, 1 O 2 -mediated cell death signaling operates as a default pathway that results in seedlings committing suicide. By contrast, EX-dependent signaling in the wild type induces the formation of microlesions without decreasing the viability of seedlings. 1 O 2 -mediated and EX-dependent loss of plastid integrity and cell death in these plants occurs only in cells containing fully developed chloroplasts. Our findings support an as yet unreported signaling role of 1 O 2 in the wild type exposed to mild light stress that invokes photoinhibition of photosystem II without causing photooxidative damage of the plant.