Plant U-Box 4 regulates chloroplast stress signaling and programmed cell death via Salicylic acid modulation.

In response to environmental stress, chloroplasts generate reactive oxygen species, including singlet oxygen ( 1 O 2 ), which regulates nuclear gene expression (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 involves Plant U-Box 4 (PUB4), a cytoplasmic E3 ubiquitin ligase, in propagating these signals. To gain insights into 1 O 2 signaling pathways, we compared transcriptomes of fc2 and fc2 pub4 mutants. The accumulation of 1 O 2 in fc2 plants broadly repressed genes involved in chloroplast function and photosynthesis, while 1 O 2 induced genes and transcription factors involved in abiotic and biotic stress, the biosynthesis of jasmonic acid (JA), and Salicylic acid (SA). Elevated JA and SA levels were observed in stressed fc2 plants, but were not responsible for PCD. pub4 reversed the majority of 1 O 2 -induced gene expression in fc2 and reduced the JA content, but maintained elevated levels of SA even in the absence of 1 O 2 stress. Reducing SA levels in fc2 pub4 restored 1 O 2 signaling and light sensitivity. Together, this work demonstrates that SA plays a protective role during photo-oxidative stress and that PUB4 mediates 1 O 2 signaling by modulating its levels.