**5. Conclusions**

Stearoyl-ACP-desaturase, omega-6, and omega-3 desaturases are diiron cofactor, histidine box motif enzymes that introduce, respectively, the first, second or third double bond into the specific C18 fatty acid substrate to yield oleate (18:1), linoleate (18:2), or linolenate (18:3). The expression and activity of these enzymes significantly determines the fatty acid composition and overall quality of soybean oil, and also contributes to the physiological adaptation to environmental temperature and the induction of defense responses to pathogens. Investigations of the regulation of desaturase expression and activity by temperature and pathogens in soybean are relatively recent, but initial findings suggest similarities with *Arabidopsis* and other plants. Down regulation of the *SACPD* gene expression results in plants with reduced 18:1, elevated 18:0, the formation of spontaneous lesions, increased salicylic acid accumulation, and constitutively expressed pathogenesisrelated genes (Kachroo & Kachroo 2009). These plants exhibit enhanced resistance to bacterial and oomycete pathogens. In both soybean seed and leaf tissues, the levels of 18:2 and 18:3 gradually increase as temperature decreases, but the transcript levels of the omega-6 desaturases do not increase at low temperature, suggesting that post-translational regulatory mechanisms likely play an important role in modulating the omega-6 (FAD2-1) enzyme activities. Transcript expression of the omega-3 desaturases *FAD8* and *FAD3* do change in response to changes in ambient temperature. *FAD8* is cold-inducible and the increased 18:3 level in chloroplast membranes due to upregulated *FAD8* expression is associated with low temperature tolerance. Upregulation of *FAD7* and increased 18:3 levels in chloroplasts modulate plant defense responses to pathogens through increased production of oxylipin antimicrobial and signaling molecules. SACPD, ω-6, and ω-3 fatty acid desaturase genes are present as multiple copies in the soybean genome as expected given the evidence (Schmutz et al. 2010, Ha et al., 2010) from cytogenetics, genetic mapping, and genomic sequencing that soybean is a paleopolyploid species that underwent at least two major genome duplications. The soybean genome possesses tissue-specific alleles for all three of C18 desaturase enzymes involved in the biosynthesis of triacylglycerols. The occurrence of seed-specific alleles of these genes provides for the accommodation of the great increase in lipid biosynthesis that occurs as the developing soybean seeds produce storage lipid reserves (Tang et al., 2005). Genomic (Schmutz et al., 2010) and gene expression analysis (Upchurch & Ramirez, 2010) using the Williams 82 soybean genome database is expected to expand knowledge of soybean gene regulatory sequences and their interaction with transcription complexes. Development of soybean SNP markers (Ha et al., 2010), mapping and dissection of Quantitative Trait Loci (Bachlava et al., 2008, Bachlava et al., 2009A, Bachlava et al., 2009B) and gene silencing analyses (Singh et al., 2011) may lead to the discovery of new genes for fatty acid biosynthesis and stress adaptation, and the potential epigenetic interactions between them. Since the capacity to induce host pathogen defenses is associated with specific desaturase-mediated changes in the levels of unsaturated C18 fatty acids in plant lipid, global climate change (Garrett et al. 2006) may potentially negatively impact plant defenses.
