Glycerolipid synthesis in Avena leaves during greening of etiolated seedlings I. Lipid changes in leaves.

Abstract

Etiolated seedlings of Avena sativa L. were illuminated under fluorescent light of 1,500 lux at 25°C and the lipid changes in their first leaves and isolated plastids were followed during 24 hr of greening. Lipid changes were divided into a degradation phase [0 hr of illumination (hr-L) to 3 hr-L] and a synthetic phase (3 hr-L to 24 hr-L). In the degradation phase, which paralleled prolamellar body transformation in the plastids, both plastidic lipids, monogalactosyldiglyceride (MGDG), digalactosyldiglyceride (DGDG), sulfoquinovosyldiglyceride (SQDG) and phosphatidylglycerol (PG), and extraplastidic lipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI), were partially degraded. Plastidic lipids (except DGDG) began to increase at 1.5 hr-L without the de novo synthesis of fatty acids, at the same time that there was a temporary accumulation of diglyceride, whose fatty acids were similar to the fatty acids of phospholipids such as PC, PE and PI, but different from the fatty acids of DGDG. This suggests that there is conversion of phospholipids to plastidic lipids during the later degradation phase. During the later degradation phase and the early synthetic phase (until 6 hr-L), plastid division occurred, resulting in a 30% increase in the plastid number per cell. Plastidic lipids were synthesized rapidly during the synthetic phase, in accordance with the beginning of light-enhanced fatty acid synthesis and thylakoid proliferation. In addition, the fatty acid composition of the plastidic lipids changed markedly throughout the synthetic phase : in MGDG, DGDG and SQDG, α-linolenate increased as linoleate decreased at both the C-1 and C-2 positions of sn-glycerol; in PG, α-linolenate increased in compensation for the decrease in linoleate at C-1 and hexadecenoate (3t) appeared and increased in compensation for the decrease in palmitate at C-2. This is evidence of the parallel desaturation of linoleate to linolenate at both C-1 and C-2 of MGDG, DGDG and SQDG, and also of the position-specific desaturation of linoleate to linolenate at C-1 of PG and of palmitate to hexadecenoate at C-2 of PG. The formation of hexadecenoate was entirely light-dependent. Extraplastidic lipids increased slightly in the early synthetic phase (3 hr-L to 6 hr-L), but declined later.