The Molecular Mechanism of the Low- Temperature Tolerance of Plants Studied by Gene Technology of Membrane Lipids

Abstract

It has been stated that the fatty-acid unsaturation of membrane lipids is related to the tolerance (or sensitivity) of plants to high- and low-temperature stresses. This claim has not been supported directly in previous studies because the level of unsaturation has always been altered indirectly by changing the growth temperature. The change in growth temperature may be expected to alter not only the lipids, but also a number of other cellular components and enzymes. Therefore, to study the relationship between temperature tolerance and the lipid, it is necessary to establish an experimental system in which only the fatty-acid unsaturation of membrane lipids is altered, while the growth temperature remains constant. Such a system is now available through gene technology that modifies membrane lipids using genes for the desaturation of fatty acids. Recently, we have isolated genes termed desA and desB) for fatty-acid desaturation at the Δ12 and Δ15 positions from the cyanobacterium Synechocystis PCC6803. By mutagenesis and transformation with desaturase genes, we obtained strains of this cyanobacterium that had discretely different levels of fatty-acid unsaturation of their membrane lipids. The characteristic features of temperature tolerance of these strains suggest that the fatty-acid unsaturation of membrane lipids is related to the tolerance against low- temperature stress, but not to that against high-temperature stress. A second line of research has been the gene-technological modification of fatty-acid unsaturation of phosphatidylglycerol, one of the membrane lipids of higher- plant chloroplasts. In these studies, we cloned genes for acyl-ACP:glycerol- 3-phosphate acyltransferase from Arabidopsis thaliana and Cucurbita moschata (squash) and introduced them into Nicotiana tabacum (tobacco). Leaves of the resultant tobacco revealed distinctly different levels of unsaturation of the phosphatidylglycerol. By comparing the tolerance of the transformants against low-temperature stress, we have obtained evidence that low-temperature tolerance is enhanced by unsaturation of the membrane lipid.