ORCID ID: 0000-0001-7912-7220 (F.M.).
Studies of the singlet oxygen (1 O 2 )-overproducing flu and chlorina1 (ch1) mutants of Arabidopsis (Arabidopsis thaliana) have shown that 1 O 2 -induced changes in gene expression can lead to either programmed cell death (PCD) or acclimation. A transcriptomic analysis of the ch1 mutant has allowed the identification of genes whose expression is specifically affected by each phenomenon. One such gene is OXIDATIVE SIGNAL INDUCIBLE1 (OXI1) encoding an AGC kinase that was noticeably induced by excess light energy and 1 O 2 stress conditions leading to cell death. Photo-induced oxidative damage and cell death were drastically reduced in the OXI1 null mutant (oxi1) and in the double mutant ch1*oxi1 compared with the wild type and the ch1 single mutant, respectively. This occurred without any changes in the production rate of 1 O 2 but was cancelled by exogenous applications of the phytohormone jasmonate. OXI1-mediated 1 O 2 signaling appeared to operate through a different pathway from the previously characterized OXI1-dependent response to pathogens and H 2 O 2 and was found to be independent of the EXECUTER proteins. In high-light-stressed plants, the oxi1 mutation was associated with reduced jasmonate levels and with the up-regulation of genes encoding negative regulators of jasmonate signaling and PCD. Our results show that OXI1 is a new regulator of 1 O 2 -induced PCD, likely acting upstream of jasmonate.
Singlet oxygen ( O ) signalling in plants is essential to trigger both acclimatory mechanisms and programmed cell death under high light stress. However, because of its chemical features, O requires mediators, and the players involved in this pathway are largely unknown. The β-carotene oxidation product, β-cyclocitral, is one such mediator. Produced in the chloroplast, β-cyclocitral induces changes in nuclear gene expression leading to photoacclimation. Recently, the METHYLENE BLUE SENSITIVITY protein MBS has been identified as a key player in O signalling leading to tolerance to high light. Here, we provide evidence that MBS1 is essential for acclimation to O and cross-talks with β-cyclocitral to mediate transfer of the O signal to the nucleus, leading to photoacclimation. The presented results position MBS1 downstream of β-cyclocitral in O signalling and suggest an additional role for MBS1 in the regulation of plant growth and development under chronic O production.
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