Choline resistance and phosphatidylcholine biosynthesis in tobacco cell suspensions or calli

Abstract

Addition of choline to the culture medium of tobacco cells or calli of a wild cell line promoted an increase in the level of phosphatidylcholine (PC) as well as a rise in the phosphatidylcholine: phosphatidylethanolamine (PC/PE) molar ratio. Both of these events are inconsistent with cell or tissue survival. The in vitro activities of the enzymes involved in the synthesis of PC by the choline diphosphate (CDP) pathway, were examined in tobacco cells sensitive to (ChS line) or resistant to choline (ChR line), grown in suspension in a liquid medium or as calli on a solid medium. During the whole growth cycle, ChR cells or calli showed no changes in the enzymatic activities related to the CDP pathway, in contrast to ChS cells or calli which exhibited large changes. Alterations in the levels of these enzymatic activities mostly occurred at the onset of the subculture of ChS calli or cells transferred on an enriched choline medium. During the lag phase of ChS calli development, an abrupt drop in the in vitro activity of phosphocholine cytidylyltransferase (CT) occurs, which may be linked to a high level of the microsomal PC. Moreover during this phase an increase in the phosphotransferase activity (CPT) occurred in ChS calli, by contrast with a transient decrease of this activity found in suspension-cultured ChS cells. In choline-treated ChS cultures, an extended elevation of cholinekinase (CK) activity promoted a large entry of exogenous choline into ChS cells. By comparison with the behaviour of choline-sensitive cells, the choline-resistance might be correlated with low and stable levels of CK and CT activities associated with a high level of CPT activity both in tobacco cells or calli.