Genetic control of fatty acid desaturation during cold acclimation and de-acclimation of birch

Among the metabolic modifications induced by a low temperature exposure, an increase in fatty acid (FA) unsaturation of membrane lipids is a common feature that allows membranes to retain their functional fluidity. To understand the mechanism of this increased FA unsaturation, the correlation between frost resistance, modification of membrane lipids, and the expression of the genes involved in the desaturation of linoleic (18:2) to linolenic (18:3) acids, the omega-3 fatty acid desaturase (omega-3 FAD) is studied in birch. Three birch omega-3 FAD clones, homologous to the Arabidopsis FAD7, FAD8 and FAD3 genes were isolated. Analysis of birch seedlings exposed to low temperature in greenhouse conditions showed that 18:3 increased in leaves by 5% after 1d at low temperature but no significant change in 18:3 was measured in stems when total FA were analysed. FAD8 was only slightly induced in leaves and stems, and FAD7 slowly down regulated. FAD3 was constitutively expressed in both leaves and stems. Bark samples (phellogen) from adult birch were collected at three different high latitudes in 2001. Water content, chlorophyll content and stem photosynthesis were good markers of the cold acclimation process. Lipid analysis showed that levels of FA unsaturation was maximum in mid-winter in the phospholipid fraction, but at the end of the growing season in the galactolipid fraction. The amount of 18:3 was highest at time of budburst in both lipid fractions. Surprisingly, the expression of FAD8 was correlated with high levels of freezing resistance during the early step of de-acclimation. FAD7 was expressed mainly during the growing season and FAD3 was relatively constitutive all over the year. The expression pattern of the birch omega-3 FAD genes was in agreement with data described in other plant species, except for the expression of FAD8 during the very early step of the de-acclimation in bark.