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

The chloroplast membranes of all higher plants contain very high proportion s of trienoic fatty acids. To investigate how these lipid structures are im portant in photosynthesis, we have generated a triple mutant line of Arabid opsis that contains negligible levels of trienoic fatty acids. For mutant p lants grown at 22 degreesC, photosynthetic fluorescence parameters were ind istinguishable from wild type at 25 degreesC. Lowering the measurement temp erature led to a small decrease in photosynthetic quantum yield, Phi (II), in the mutant relative to wild-type controls. These and other results indic ate that low temperature has only a small effect on photosynthesis in the s hort term. However, long-term growth of plants at 4 degreesC resulted in de creases in fluorescence parameters, chlorophyll content, and thylakoid memb rane content in triple-mutant plants relative to wild type. Comparisons amo ng different mutant lines indicated that these detrimental effects of growt h at 4 degreesC are strongly correlated with trienoic fatty acid content wi th levels of 16:3 + 18:3, approximately one-third of wild type being suffic ient to sustain normal photosynthetic function. In total, our results indic ate that trienoic fatty acids are important to ensure the correct biogenesi s and maintenance of chloroplasts during growth of plants at low temperatur es.