A MUTATION AT THE FAD8 LOCUS OF ARABIDOPSIS IDENTIFIES A 2ND CHLOROPLAST OMEGA-3 DESATURASE

Two independently isolated mutations at the fad7 locus in Arabidopsis produced plants with a temperature-conditional phenotype. Leaves of fa d7 mutants grown at 28 degrees C contained less than 30% of wild-type levels of trienoic fatty acids (16:3 plus 18:3) compared with more tha n 70% of wild-type levels for plants grown at 15 degrees C. Screening of an M(2) population derived from the fad7-1 line led to the identifi cation of a line, SH1, in which the proportion of trienoic acids was m uch less than in fad7 plants. The segregation pattern of F-2 progeny f rom a cross between SH1 and wild type indicated that the additional fa tty acid mutation in SH1 is at a new locus, designated fads. In a gene tic background that was wild type at the FAD7 locus, the fad8 mutation had no detectable effect on overall leaf fatty acid composition irres pective of the temperature at which plants were grown. However, fatty acid analyses of individual leaf lipids revealed small decreases in th e levels of 18:3 in two chloroplast lipids. In fad8 plants grown at 22 degrees C, phosphatidylglycerol contained 22.5% 18:3 compared with 33 .5% in wildtype Arabidopsis. For sulfoquinovosyldiacylglycerol, the va lues were 31.4 and 44.5%, respectively. Together with information from studies of the cloned FADE gene (S. Gibson, V. Arondel, K. Iba, C. So merville [1994] Plant Physiol 106: 1615-1621), these results indicate that the FAD8 locus encodes a chloroplast-localized 16:2/18:2 desatura se that has a substrate specificity similar to the FAD7 gene product b ut that is induced by low temperature.