Membrane Lipid Integrity Relies on a Threshold of ATP Production Rate in Potato Cell Cultures Submitted to Anoxia

In this paper we report on our study of the changes in biomass, lipid composition, and fermentation end products, as well as in the ATP level and synthesis rate in cultivated potato (Solanum tuberosum) cells submitted to anoxia stress. During the first phase of about 12 h, cells coped with the reduced energy supply brought about by fermentation and their membrane lipids remained intact. The second phase (12–24 h), during which the energy supply dropped down to 1% to 2% of its maximal theoretical normoxic value, was characterized by an extensive hydrolysis of membrane lipids to free fatty acids. This autolytic process was ascribed to the activation of a lipolytic acyl hydrolase. Cells were also treated under normoxia with inhibitors known to interfere with energy metabolism. Carbonyl-cyanide-4-trifluoromethoxyphenylhydrazone did not induce lipid hydrolysis, which was also the case when sodium azide or salicylhydroxamic acid were fed separately. However, the simultaneous use of sodium azide plus salicylhydroxamic acid or 2-deoxy-D-glucose plus iodoacetate with normoxic cells promoted a lipid hydrolysis pattern similar to that seen in anoxic cells. Therefore, a threshold exists in the rate of ATP synthesis (approximately 10 μmol g−1 fresh weight h−1), below which the integrity of the membranes in anoxic potato cells cannot be preserved.

Protein Hydrolysis and Nitrogen Remobilisation in Plant Life and Senescence

In plant cells, as in all other cells, proteins are submitted to permanent turnover, and the intracellular content of a given protein depends on its rate of both synthesis and degradation. The life time of most proteins is shorter than that of the cell. Thus, in young leaves of Lemna minor, the average half-life of protein was estimated to be 7 days, and it was shorter under stress conditions (Davies 1982). Such observations mean that nitrogen and amino acid fluxes are both cylic and permanent. Although protein turnover may appear wasteful, in terms of energy, numerous studies have shown that proteolysis provides multiple functions in cell physiology, and is an essential regulatory mechanism of cell metabolism and development.