Plastidial ω-3 desaturase FAD7 is a major contributor to trienoic fatty acid biosynthesis in the leaves of Arabidopsis plants. However, the precise contribution of the other plastidial ω-3 desaturase, FAD8, is poorly understood. Fatty acid and lipid analysis of several ω-3 desaturase mutants, including two insertion lines of AtFAD7 and AtFAD8, showed that FAD8 partially compensated the disruption of the AtFAD7 gene at 22 °C, indicating that FAD8 was active at this growth temperature, contrasting to previous observations that circumscribed the FAD8 activity at low temperatures. Our data revealed that FAD8 had a higher selectivity for 18:2 acyl-lipid substrates and a higher preference for lipids other than galactolipids, particularly phosphatidylglycerol, at any of the temperatures studied. Differences in the mechanism controlling AtFAD7 and AtFAD8 gene expression at different temperatures were also detected. Confocal microscopy and biochemical analysis of FAD8-YFP over-expressing lines confirmed the chloroplast envelope localization of FAD8. Co-localization experiments suggested that FAD8 and FAD7 might be located in close vicinity in the envelope membrane. FAD8-YFP over-expressing lines showed a specific increase in 18:3 fatty acids at 22 °C. Together, these results indicate that the function of both plastidial ω-3 desaturases is coordinated in a non-redundant manner.

This study analysed the contribution of each omega-3 desaturase to the cold response in soybean. Exposure to cold temperatures (5 °C) did not result in great modifications of the linolenic acid content in leaf membrane lipids. However, an increase in the GmFAD3A transcripts was observed both in plant leaves and soybean cells whereas no changes in GmFAD3B or GmFAD3C expression levels were detected. This increase was reversible and accompanied by the accumulation of an mRNA encoding a truncated form of GmFAD3A (GmFAD3A-T), which originated from alternative splicing of GmFAD3A in response to cold. When the expression of plastidial omega-3 desaturases was analysed, a transient accumulation of GmFAD7-2 mRNA was detected upon cold exposure in mature soybean trifoliate leaves while GmFAD7-1 transcripts remained unchanged. No modification of the GmFAD8-1 and GmFAD8-2 transcripts was observed. The functionality of GmFAD3A, GmFAD3B, GmFAD3C and GmFAD3A-T was examined by heterologous expression in yeast. No activity was detected with GmFAD3A-T, consistent with the absence of one of the His boxes necessary for desaturase activity. The linolenic acid content of Sacharomyces cerevisiae cells overexpressing GmFAD3A or GmFAD3B was higher when the cultures were incubated at cooler temperatures, suggesting that reticular desaturases of the GmFAD3 family, and more specifically GmFAD3A, may play a role in the cold response, even in leaves. The data point to a regulatory mechanism of omega-3 fatty acid desaturases in soybean affecting specific isoforms in both the plastid and the endoplasmic reticulum to maintain appropriate levels of linolenic acid under low temperature conditions.