Characterization of a rice (Oryza sativa L.) gene encoding a temperature-dependent chloroplast omega-3 fatty acid desaturase.

A cDNA, designated Osfad8, encoding a chloroplast omega-3 fatty acid desaturase responsible for trienoic fatty acid formation, was isolated from the leaves of Oryza sativa L. by RT-PCR. Southern blot hybridization indicated that a small gene family composed of two copies or closely linked genes exists. RNA in situ hybridization showed that the accumulation of Osfad8 mRNA was abundant in leaves but hardly detectable in roots. The Osfad8 transcript level in leaves was much higher at 15 degrees C than at normal temperature (25 degrees C). In situ hybridization also showed particularly prominent expression of Osfad8 in the palisade layer and spongy parenchyma cells of leaves when exposed to 15 degrees C conditions for 5 days and 10 days. Two transgenic lines (8S-52 and 8S-101) harboring the Osfad8 ORF in sense orientation under the control of the CaMV 35S promoter contained increased levels of hexadecatrienoic (16:3) and linolenic (18:3) fatty acids. When exposed to 2 degrees C for 7 days, the damage observed to the control plants was significantly alleviated in the 8S-52 and 8S-101 lines. The amounts of trienoic fatty acids in an Osfad8 antisense line (8A-35) declined 40.2% compared to the control plants. The 8A-35 plants survived after growth at 44 degrees C for 3 days while the control plants died. These data suggest that Osfad8 encodes a temperature-dependent chloroplast omega-3 fatty acid desaturase.