Degradation of membrane phospholipids in plant cells cultured in sucrose-free medium.

It has been generally accepted that autophagy contributes to the degradation of cellular components under nutrient starvation conditions. In a previous study, however, we showed that the degradation of membrane phospholipids occurs mainly by mechanisms distinct from autophagy in suspension-cultured tobacco (Nicotiana tabacum) BY-2 cells. In response to deprivation of sucrose, the amounts of total phospholipids and a major phospholipid, phosphatidylcholine (PC), decreased. 3-Methyladenine, which inhibits autophagy, did not affect the degradation of total phospholipids or PC. On the other hand, glycerol inhibited PC degradation although it did not block autophagy. In the present study, we labeled intracellular phospholipids by loading cells with a fluorochrome-labeled fatty acid and observed cellular morphology by fluorescence microscopy. Most cellular membrane structures were stained at the start of starvation; but 12 h after starvation treatment, concomitant with PC degradation, fluorescence on membranes disappeared and instead the central vacuole became fluorescent. 3-Methyladenine did not inhibit this process, whereas glycerol did. These results suggest that the degradation of membrane phospholipids can be traced by light microscopy and support the notion that autophagy is not a main contributor to the degradation of membrane phospholipids in tobacco cells cultured in sucrose-free medium.