Trienoic fatty acids are required to maintain chloroplast function at low temperatures

The chloroplast membranes of all higher plants contain very high proportions of trienoic fatty acids. To investigate how these lipid structures are important in photosynthesis, we have generated a triple mutant line of Arabidopsis that contains negligible levels of trienoic fatty acids. For mutant plants grown at 22 degreesC, photosynthetic fluorescence parameters were indistinguishable from wild type at 25 degreesC. Lowering the measurement temperature led to a small decrease in photosynthetic quantum yield, Phi (II), in the mutant relative to wild-type controls. These and other results indicate that low temperature has only a small effect on photosynthesis in the short term. However, long-term growth of plants at 4 degreesC resulted in decreases in fluorescence parameters, chlorophyll content, and thylakoid membrane content in triple-mutant plants relative to wild type. Comparisons among different mutant lines indicated that these detrimental effects of growth at 4 degreesC are strongly correlated with trienoic fatty acid content with levels of 16:3 + 18:3, approximately one-third of wild type being sufficient to sustain normal photosynthetic function. In total, our results indicate that trienoic fatty acids are important to ensure the correct biogenesis and maintenance of chloroplasts during growth of plants at low temperatures.