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Plants respond to environmental changes by acclimation that activates defence mechanisms and enhances the plant's resistance against a subsequent more severe stress. Chloroplasts play an important role as a sensor of environmental stress factors that interfere with the photosynthetic electron transport and enhance the production of reactive oxygen species (ROS). One of these ROS, singlet oxygen (1O2), activates a signalling pathway within chloroplasts that depends on the two plastid-localized proteins EXECUTER 1 and 2. Moderate light stress induces acclimation protecting photosynthetic membranes against a subsequent more severe high light stress and at the same time activates 1O2- mediated and EXECUTER-dependent signalling. Pre-treatment of Arabidopsis seedlings with moderate light stress confers cross-protection against a virulent Pseudomonas syringae strain. While non-pre-acclimated seedlings are highly susceptible to the pathogen regardless of whether 1O2- and EXECUTER-dependent signalling is active or not, pre-stressed acclimated seedlings without this signalling pathway lose part of their pathogen resistance. These results implicate 1O2- and EXECUTER-dependent signalling in inducing acclimation but suggest also a contribution by other yet unknown signalling pathways during this response of plants to light stress. [ABSTRACT FROM AUTHOR]