Roles of plastoglobules and lipid droplets in leaf neutral lipid accumulation during senescence and nitrogen deprivation.

Upon abiotic stress or senescence, the size and/or abundance of plastid-localized plastoglobules and cytosolic lipid droplets, both compartments devoted to neutral lipid storage, increase in leaves. Meanwhile, plant lipid metabolism is also perturbed, notably with the degradation of thylakoidal monogalactosyldiacylglycerol (MGDG) and the accumulation of neutral lipids. Although these mechanisms are probably linked, they have never been jointly studied, and the respective roles of plastoglobules and lipid droplets in the plant response to stress are totally unknown. To address this question, we determined and compared the glycerolipid composition of both lipid droplets and plastoglobules, followed their formation in response to nitrogen starvation, and studied the kinetics of lipid metabolism in Arabidopsis leaves. Our results demonstrated that plastoglobules preferentially store phytyl-esters, while triacylglycerols (TAGs) and steryl-esters accumulated within lipid droplets. Thanks to a pulse-chase labeling approach and lipid analyses of the fatty acid desaturase 2 (fad2) mutant, we showed that MGDG-derived C18:3 fatty acids were exported to lipid droplets, while MGDG-derived C16:3 fatty acids were stored within plastoglobules. The export of lipids from plastids to lipid droplets was probably facilitated by the physical contact occurring between both organelles, as demonstrated by our electron tomography study. The accumulation of lipid droplets and neutral lipids was transient, suggesting that stress-induced TAGs were remobilized during the plant recovery phase by a mechanism that remains to be explored.