Glycerolipid-fatty-acid desaturase deficiencies in chloroplasts from fruits of Capsicum annuum L.

Abstract

Chloroplasts from fruits and leaves of Capsicum annuum cv. 'Bell Tower' were purified on sucrose gradients, and the lipids were separated by column and thin-layer chromatography. The glycerolipids mono- and digalactosyldiacylglycerol (MGDG, DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG) were quantified, and the fatty-acid composition at the 1 and 2 positions of the glycerol moiety (sn-1 and sn-2) was determined after hydrolysis with position-specific lipases. In fruit chloroplasts, Δ (3)-trans hexadecenoate (trans-3-16∶1) was absent and replaced by palmitate (16∶0) at sn-2 of PG, and Δ (7,10,13)-hexadecatrienoate (16∶3) at sn-2 of MGDG was greatly reduced and largely replaced by linoleate (18∶2). The ratio of 18∶2 to linolenate (18∶3) was consistently greater in glycerolipids from fruit compared with leaf chloroplasts. The lower percentage of C-16 fatty acids at sn-2 indicated that "prokaryotic" molecular species were reduced by ≤15% in DGDG, ∼40% in SQDG, and ∼90% in MGDG, in fruit compared with leaf chloroplasts. The MGDG∶DGDG ratios in fruit and leaf chloroplasts were 1.2∶1 and 2.2∶1, respectively. Taken together, the data indicate that chloroplasts in Capsicum fruit are deficient in three desaturases: those that convert 1) 16∶0 to Δ (3)-trans-16∶1 at sn-2 of PG, 2) 16∶0 to Δ (7)-cis-16∶1 at sn-2 of MGDG, and 3) 18∶2 to 18∶3 at both sn-1 and sn-2 of various chloroplast glycerolipids.