A gene for the delta9 desaturase specific to stearoyl-ACP (acyl carrier protein) was identified from yellow lupine (Lupinus luteus) cDNA and genomic libraries through the differential display method. The desaturase transcript appears in plants infected with Bradyrhizobium sp. (Lupinus) as revealed by Northern hybridization, RT-PCR and expression of beta-glucuronidase under the desaturase promoter. A small amount of desaturase transcript was also detected in uninfected plants, which suggests that the gene does not belong to the strict nodule-specific sequences. The desaturase provides unsaturated fatty acids for additional cell membrane synthesis. During nodule and symbiosome development a peribacteroid membrane is formed and the requirement for membrane surface increases, thus the level of desaturase expression is also higher. Transgenic plants of Nicotiana tabacum with overexpression of the full-length lupine stearoyl-ACP desaturase sequence were obtained. They revealed higher content of unsaturated fatty acids (especially oleic acid) in comparison with control plants.