Abstract
BACKGROUND: In response to environmental stresses, chloroplasts generate reactive oxygen species, including singlet oxygen ((1)O(2)), an excited state of oxygen that regulates chloroplast-to-nucleus (retrograde) signaling, chloroplast turnover, and programmed cell death (PCD). Yet, the central signaling mechanisms and downstream responses remain poorly understood. The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant conditionally accumulates (1)O(2), and Plant U-Box 4 (PUB4), a cytoplasmic E3 ubiquitin ligase, is involved in propagating (1)O(2) signals for chloroplast turnover and cellular degradation. Thus, the fc2 and fc2 pub4 mutants are useful genetic tools to elucidate these signaling pathways. Previous studies have focused on the role of (1)O(2) in promoting cellular degradation in fc2 mutants, but its impact on retrograde signaling from mature chloroplasts (the major site of (1)O(2) production) is poorly understood. RESULTS: To gain mechanistic insights into (1)O(2) signaling pathways, we compared transcriptomes of adult wt, fc2, and fc2 pub4 plants. The accumulation of (1)O(2) in fc2 plants broadly repressed genes involved in chloroplast function and photosynthesis, while inducing genes and transcription factors involved in abiotic and biotic stress, the biosynthesis of jasmonic acid (JA) and salicylic acid (SA), microautophagy, and senescence. Elevated JA and SA levels were observed in (1)O(2)-stressed fc2 plants. pub4 reversed most of this (1)O(2)-induced gene expression and reduced the JA content in fc2 plants. The pub4 mutation also blocked JA-induced senescence pathways in the dark. However, fc2 pub4 plants maintained constitutively elevated levels of SA even in the absence of bulk (1)O(2) accumulation. CONCLUSIONS: Together, this work demonstrates that in fc2 plants, (1)O(2) leads to a robust retrograde signal that may protect cells by downregulating photosynthesis and ROS production while simultaneously mounting a stress response involving SA and JA. The induction of microautophagy and senescence pathways indicate that (1)O(2)-induced cellular degradation is a genetic response to this stress, and the bulk of this transcriptional response is modulated by the PUB4 protein. However, the effect of pub4 on hormone synthesis and signaling is complex and indicates that an intricate interplay of SA and JA are involved in promoting stress responses and programmed cell death during photo-oxidative damage.