The chloroplast singlet oxygen‐triggered biosynthesis of salicylic acid and jasmonic acid is mediated by EX1 and GUN1 in Arabidopsis

Abstract 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 ( 1 O 2 ) during the transition from darkness to light, thereby instigating leaf necrosis. In this investigation, we uncovered that 1 O 2 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 1 O 2 . 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 1 O 2 signalling‐mediated SA biosynthesis. Moreover, the triple ex1 gun1 pcd8 displays a phenotype similar to ex1 . Overall, our findings underscore that the 1 O 2 ‐induced cell death phenotype requires EX1/GUN1‐mediated retrograde signalling in pcd8 mutants, providing novel insights into the interplay between ROS and SA/JA.