Malondialdehyde generated from peroxidized linolenic acid causes protein modification in heat-stressed plants

When polyunsaturated fatty acids (PUFAs) in biomembrane are peroxidized, a great diversity of aldehydes is formed, and some of which are highly reactive. Thus they are thought to have biological impacts in stressed plants; however, the detailed mechanism of generation and biochemical effects are unknown. In this study, we show that chloroplasts are major organelles in which malondialdehyde (MDA) generated from peroxidized linolenic acid modifies proteins in heat-stressed plants. First, to clarify the biochemical process of MDA generation from PUFAs and its attachment to proteins, we carried out in vitro experiments using model proteins (BSA and Rubisco) and methylesters of C18 PUFAs that are major components of plant biomembrane. Protein modification was detected by Western blotting using monoclonal antibodies that recognize MDA binding to proteins. Results showed that peroxidation of linolenic acid methylester by reactive oxygen species was essential for protein modification by MDA, and the MDA modification was highly dependent on temperature, leading to a loss of Rubisco activity. When isolated spinach thylakoid membrane was peroxidized at 37 °C, oxygen-evolving complex 33 kDa protein (OEC33) was modified by MDA. These model experiments suggest that protein modification by MDA preferentially occurs under higher temperatures and oxidative conditions, thus we examined protein modification in heat-stressed plants. Spinach plants were heat-stressed at 40 °C under illumination, and modification of OEC33 protein by MDA was detected. In heat-stressed Arabidopsis plants, light-harvesting complex protein was modified by MDA under illumination. This modification was not observed in linolenic acid-deficient mutants (fad3fad7fad8 triple mutant), suggesting that linolenic acid is a major source of protein modification by MDA in heat-stressed plants.