• cold stress
• pigment content
• proteomic

Low temperature is one of the major abiotic stresses limiting the productivity and the geographical distribution of many species. The effect of cold acclimatization is evident at chloroplast level because one of the main target of damages causes by low temperatures is the photosynthetic apparatus. For this reason our aim is to analyze the change in level of expression of chloroplast proteins during stress. 13-days old plants were acclimatized at 4°C for 1 week and treated at -10°C for 12 h and then recovered for 24 h. Freezing treatment produced stress phenotypes of rolling leaves, decrease in pigment content. At low temperature wild-type plants exhibited symptoms of severe oxidative stress: lipid peroxidation, chlorophyll bleaching, and photoinhibition. In acclimatated plants, which accumulate over twice as much zeaxanthin as the control, these symptoms were significantly ameliorated.
The changes of total proteins in chloroplasts were examined using two-dimensional electrophoresis. Among 200 protein spots reproducibly detected on each gel, we found up- and down-regulated spots. Mass spectrometry analysis allowed the identification of 30 differentially expressed proteins, including well know cold-responsive proteins. Several proteins showed enhanced degradation during freezing stress, especially the photosynthetic proteins such as Rubisco activase (RcbA) and Rubisco large subunit (RcbL) of which 4 fragments were detected. The identified proteins are involved in several processes: photosynthesis, RNA processing, protein translation and processing, metabolism of carbon, nitrogen end energy. These proteins might work cooperatively to reach an homeostatic equilibrium to overcome stress conditions.