The Chloroplast Singlet Oxygen-Triggered Biosynthesis of Salicylic Acid and Jasmonic Acid is Mediated by EX1 and GUN1 in Arabidopsis

Reactive oxygen species (ROS) and defence hormones like salicylic acid (SA) and jasmonic acid (JA) play pivotal roles in triggering cell death. However, the precise mechanism governing the interaction between ROS and SA/JA remains elusive. Recently, our research revealed that RNAi mutants with suppressed expression of PROGRAMMED CELL DEATH8 (PCD8) exhibit an overabundance of tetrapyrrole intermediates, particularly uroporphyrinogen III (Uro III), leading to the accumulation of singlet oxygen (1O2) during the transition from darkness to light, thereby instigating leaf necrosis. In this investigation, we uncovered that 1O2 stimulates biosynthesis of SA and JA, activating SA/JA signalling and the expression of responsive genes in PCD8 RNAi (pcd8) mutants. Introducing NahG or knocking out PAD4 or NPR1 significantly alleviates the cell death phenotype of pcd8 mutants, while coi1 partially mitigates the pcd8 phenotype. Further exploration revealed that EX1 and GUN1 can partially rescue the pcd8 phenotype by reducing the levels of Uro III and 1O2. Notably, mutations in EX1 mutations but not GUN1, substantially diminish SA content in pcd8 mutants compared to the wild type, implying that EX1 acts as the primary mediator of 1O2 signalling-mediated SA biosynthesis. Moreover, the triple ex1 gun1 pcd8 displays a phenotype similar to ex1. Overall, our findings underscore that the 1O2-induced cell death phenotype requires EX1/GUN1-mediated retrograde signalling in pcd8 mutants, providing novel insights into the interplay between ROS and SA/JA. We uncovered that 1O2 stimulates biosynthesis of SA and JA, activating SA/JA signalling in Arabidopsis PCD8 RNAi mutants; and 1O2 burst-triggered PCD requires SA and JA; interestingly, EX1 senses 1O2 and initiates the signalling of 1O2 to SA/JA, while GUN1 acts upstream of EX1 and mediates the signal from 1O2 to JA. Overall, it establishes the relationship between chlorophyll metabolism, hormone biosynthesis, and retrograde signalling pathways and provides a new perspective for the interaction between ROS and hormone.