FIGURE 9.

Putative pathways leading to TAG accumulation in Arabidopsis cell cultures during nitrogen deprivation or following MTX treatment. (A) In the presence of MTX, cell growth and membrane synthesis stopped, and the net increase of FA resulted from TAG synthesis. Because TAG composition was rich in unsaturated FA and similar to the one of PC, we hypothesized that the newly synthesized FA were first incorporated into PC for desaturation. Since NPC were strongly enhanced whereas PDCT, PDAT, and LPCAT were downregulated, we propose that NPC is the main way from PC to TAG. We cannot exclude that a small pool of edited acyl-CoA contributed also to TAG synthesis. Since the level of Pi-choline was reduced in the presence of MTX, but not the level of PC, we hypothesized a strong recycling of Pi-choline. (B) During nitrogen deprivation, membrane lipids declined whereas TAG increased, but the total amount of FA remained almost unchanged. Because the FA composition in TAG resembled the one of PC, together with the decrease of GPAT expression, we hypothesized that these FA originated more from membrane lipids than de novo synthesis. The significant increase of NPC and DGAT expressions also strongly suggests a PC to DAG to TAG pathway. However, NPC was less expressed than in the MTX condition whereas more TAG accumulated, suggesting that other activities such as PDCT and/or PDAT might be involved. Alternatively, a small pool of edited acyl-CoA resulting from the breakdown of membrane lipids could also contribute to the production of TAG. The main routes are shown by wider arrows.