PROGRAMMED CELL DEATH8 interacts with tetrapyrrole biosynthesis enzymes and ClpC1 to maintain homeostasis of tetrapyrrole metabolites in Arabidopsis.

Tetrapyrrole biosynthesis is a dynamically and strictly regulated process. Disruptions in tetrapyrrole metabolism influence many aspects of plant physiology, including photosynthesis, programmed cell death (PCD) and retrograde signaling, thus affecting plant growth and development at multiple levels. However, the genetic and molecular basis of tetrapyrrole biosynthesis is not fully understood. We report here PCD8, a newly identified thylakoid-localized protein encoded by an essential gene in Arabidopsis. PCD8 knock-down causes a necrotic phenotype due to excessive chloroplast damage. A burst of singlet oxygen that results from over-accumulated tetrapyrrole intermediates upon illumination is suggested to be responsible for cell death in the knock-down mutants. Genetic and biochemical analyses revealed that PCD8 interacts with ClpC1 and a number of tetrapyrrole biosynthesis (TBS) enzymes, such as HEMC, CHLD and PORC of TBS. Taken together, our findings uncover the function of chloroplast-localized PCD8 and provide a new perspective to elucidate molecular mechanism how TBS is finely regulated in plants.