METHYLENE BLUE SENSITIVITY 1 (MBS1) is required for acclimation of Arabidopsis to singlet oxygen and acts downstream of β-cyclocitral

Singlet oxygen (1O2) signalling in plants is essential to trigger both acclimatory mechanisms and programmed cell death under high light stress. However, because of its chemical features, 1O2 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 1O2 signalling leading to tolerance to high light. Here, we provide evidence that MBS1 is essential for acclimation to 1O2 and cross-talks with β-cyclocitral to mediate transfer of the 1O2 signal to the nucleus, leading to photoacclimation. The presented results position MBS1 downstream of β-cyclocitral in 1O2 signalling and suggest an additional role for MBS1 in the regulation of plant growth and development under chronic 1O2 production.