A Plastid Phosphatidylglycerol Lipase Contributes to the Export of Acyl Groups from Plastids for Seed Oil Biosynthesis

Abstract

The lipid composition of thylakoid membranes inside chloroplasts is conserved from leaves to developing embryos. A finely tuned lipid assembly machinery is required to build these membranes during development. Contrary to thylakoid lipid biosynthetic enzymes, the functions of most predicted chloroplast lipid-degrading enzymes remain to be elucidated. Here, we explore the biochemistry and physiological function of an Arabidopsis thylakoid membrane-associated lipase, PLASTID LIPASE1 (PLIP1). PLIP1 is a phospholipase A In vivo, PLIP1 hydrolyzes polyunsaturated acyl groups from a unique chloroplast-specific phosphatidylglycerol that contains 16:1 as its second acyl group. Thus far, a specific function of this 16:1 -containing phosphatidylglycerol in chloroplasts has remained elusive. The gene is highly expressed in seeds, and mutant seeds contain less oil and exhibit delayed germination compared with the wild type. Acyl groups released by PLIP1 are exported from the chloroplast, reincorporated into phosphatidylcholine, and ultimately enter seed triacylglycerol. Thus, 16:1 uniquely labels a small but biochemically active plastid phosphatidylglycerol pool in developing Arabidopsis embryos, which is subject to PLIP1 activity, thereby contributing a small fraction of the polyunsaturated fatty acids present in seed oil. We propose that acyl exchange involving thylakoid lipids functions in acyl export from plastids and seed oil biosynthesis.

In vivo lipid ‘tag and track’ approach shows acyl editing of plastid lipids and chloroplast import of phosphatidylglycerol precursors in Arabidopsis thaliana

“…Thus, based on our analysis, which is independent of the 16C/ 18C rule, a substantial fraction of PG is derived from the ER. It should be noted that the majority of plastid PG contains an unusual C16 fatty acid at its sn-2 position, 16:1 D3t , making it the target of a specific lipase, PLIP1 (Wang et al, 2017). The partial ER-origin of plastid PG in Arabidopsis is likely to be obscured using traditional approaches by specific acyl group remodeling of chloroplast PG, but is clearly apparent in the Myc-D6D transgenic lines using the 'tag and track' approach.…”

“…A second piece of evidence is the observation of SDA and 16:3 acyl groups in the same MGDG molecule as the first acyl group originates in the ER and the second in the plastid (Figure 4c). Acyl editing has been described for PC at the ER (Bates et al, 2007) or the plastid envelope (Tjellstrom et al, 2012), for plastid MGDG in Chlamydomonas (Li et al, 2012), and recently for plastid PG during seed development (Wang et al, 2017). Each of these reactions are proposed to contribute to the export of acyl groups from the plastid.…”

“…This acyl editing process actually receives the majority of the flux from the de novo synthesized acyl-CoA pool derived from chloroplast fatty acid export (Bates et al, 2007). Because there are many predicted plastid lipases of unknown function that could be involved in acyl editing, it is reasonable to consider acyl editing of plastid lipids as part of the plastid pathway, as has already been hinted at by others (Bates et al, 2007;Tjellstrom et al, 2012;Wang et al, 2017). However, the existence of extensive acyl editing within the chloroplast could affect the currently accepted quantitative indicator for identifying lipid precursor origin, the 16C versus 18C occupation in the plastid lipid sn-2 position.…”

The Lipase Link: Abscisic Acid Induces PLASTID LIPASES, Which Produce Jasmonic Acid Precursors

“…For lipid breakdown, the Arabidopsis thaliana genome encodes hundreds of predicted lipases; different lipases cleave at different positions on the lipid and release fatty acids that can be oxygenated and converted into other compounds, including JA, by various enzymes. The authors had previously identified PLASTID LIPASE1 (PLIP1), which localizes to the chloroplast and participates in seed oil production (Wang et al, 2017). Here, they extend their studies to the PLIP1 homologs PLIP2 and PLIP3.…”

Plastidic Δ6 fatty-acid desaturases with distinctive substrate specificity regulate the pool of c18-pufas in the ancestral picoalgaostreococcus tauri

“…Most of the following work focused on the transfer of lipid and PUFA from the ER to the chloroplast, establishing the idea that the reverse transport was negligible (Miquel & Browse, 1992; Li, N et al ., 2016). Nevertheless, the recent characterization of two Arabidopsis plastid lipases mutants highlighted that plastidic PUFA indeed contributed to PUFA remodelling of extraplastidic lipid (Wang et al ., 2017; Higashi et al ., 2018).…”