Effect of temperature on delta-9 stearoyl-ACP and microsomal omega-6 desaturase gene expression and fatty acid content in developing soybean seeds

Delta-9 stearoyl-ACP (SAD) and microsomal omega-6 oleate (FAD2) desaturases contribute to the maintenance of lipid fluidity in membranes and the fatty acid composition of storage lipids in seed. Since these enzymes must operate at varying environmental temperatures, they are under constitutive control, but they may also be subject to fine regulation both transcriptionally and posttranscriptionally. We measured transcript accumulation of the seed-expressed SAD-A and SAD-B and FAD2-1A and FAD2-1B genes in the seeds of three soybean varieties grown at cool (22/180 degrees C), normal (26/22 degrees C), or warm (30/26 degrees C) temperatures during pod fill. At the cool temperature, transcript accumulation of both the SAD and FAD2-1 genes was significantly elevated, with FAD2-1B 2- to 10-fold or greater than FAD2-1A at 35 d after flowering. Expression of both SAD and FAD2-1 were significantly decreased in seed that developed at the warm temperature. Decreased FAD2-1 transcript accumulation at the warm temperature was positively associated with significantly increased oleic and decreased linoleic acid content in the three varieties examined. Decreased SAD transcript accumulation at the warm temperature was positively associated with a significantly increased level of stearic acid but only in the high-stearate mutant line, A6. We conclude that environmental temperature modulates oleic and linoleic acid in developing seed through regulated FAD2-1 gene expression, but temperature modulation of stearic acid content in wildtype soybean may be more complex, involving in addition to SAD-A and -B, plastid thioesterase genes FATA and FATB.