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

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