Desiccation and Free Radical Mediated Changes in Plant Membranes

Abstract

Senaratna, T., McKersie, B. D. and Borochov, A. 1987. Desiccation and free radical mediated changes in plant membranes.—J. exp. Bot. 38: 2005‐2014. In vitro treatment of microsomal membranes from the axes of soybean (Glycine max (L.) Merr.) seeds with free radicals simulates the type of membrane injury observed following a lethal desiccation stress—the accumulation of free fatty acids in the membrane bilayer, the loss of lipid-P, and the formation of gel phase domains. The major phospholipids in the microsomal fraction were phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol. Although these treatments induced an extensive loss of total phospholipid from the microsomal fraction following desiccation, the ratio of the major phospholipids remained unchanged. Neither lysophosphatides nor phosphatidic acid accumulated in the fraction, but free fatty acid levels increased. Therefore, cleavage of both acyl chains from the phospholipid molecule occurred following desiccation of the axes and in vitro free radical treatment of the membrane. Both treatments also promoted formation of gel phase domains as shown by wide angle x-ray diffraction and increased microviscosity as determined by the fluorescent probe, DPH (1,6-diphenyl-1,3,5-hexatriene). This could be simulated in liposomes prepared from the total microsomal lipid fraction by the addition of saturated free fatty acids (16:0 and 18:0) at the levels observed following stress. In contrast, the addition of unsaturated fatty acids perturbed the bilayer and reduced microviscosity. The inclusion of both saturated and unsaturated free fatty acids as observed in vivo promoted a response similar to that observed with only the addition of the saturated free fatty acids. Desiccation of the axes also promoted a loss of microsomal protein, which was recovered in the 165 000 x g supernatant, and an apparent loss of thiol groups from the membrane as determined by a thiol specific fluorescence probe, dansylaziridine. This loss of thiol groups could also be simulated by exposure of the membranes to gamma irradiation, which was used as a non-enzymatic source of free radicals. Collectively, these data support the hypothesis that membrane disassembly following desiccation stress is mediated by a free radical mechanism, and that the consequent de-esterification of membrane phospholipid and accumulation of saturated free fatty acids alter the physical properties of the membrane.