Lipidomic and transcriptomic analyses of Chlamydomonas reinhardtii under heat stress unveil a direct route for the conversion of membrane lipids into storage lipids.

Studying how photosynthetic cells modify membrane lipids in response to heat stress is important to understand how plants and microalgae adapt to daily fluctuations in temperature and to investigate new lipid pathways. Here, we investigate changes occurring in lipid molecular species and lipid metabolism genes during early response to heat stress in the model photosynthetic microorganism Chlamydomonas reinhardtii. Lipid molecular species analyses revealed that, after 60 min at 42 °C, a strong decrease in specific polyunsaturated membrane lipids was observed together with an increase in polyunsaturated triacylglycerols (TAGs) and diacylglycerols (DAGs). The fact that decrease in the major chloroplastic monogalactosyldiacylglycerol sn1-18:3/sn2-16:4 was mirrored by an accumulation of DAG sn1-18:3/sn2-16:4 and TAG sn1-18:3/sn2-16:4/sn3-18:3 indicated that newly accumulated TAGs were formed via direct conversion of monogalactosyldiacylglycerols to DAGs then TAGs. Lipidomic analyses showed that the third fatty acid of a TAG likely originated from a phosphatidylethanolamine or a diacylglyceryl-O-4'-(N,N,N,-trimethyl)-homoserine betaine lipid species. Candidate genes for this TAG synthesis pathway were provided through comparative transcriptomic analysis and included a phospholipase A2 homolog and the DAG acyltransferase DGTT1. This study gives insights into the molecular events underlying changes in membrane lipids during heat stress and reveals an alternative route for TAG synthesis.

CYP77A19 and CYP77A20 characterized from Solanum tuberosum oxidize fatty acids in vitro and partially restore the wild phenotype in an Arabidopsis thaliana cutin mutant.

Cutin and suberin represent lipophilic polymers forming plant/environment interfaces in leaves and roots. Despite recent progress in Arabidopsis, there is still a lack on information concerning cutin and suberin synthesis, especially in crops. Based on sequence homology, we isolated two cDNA clones of new cytochrome P450s, CYP77A19 and CYP77A20 from potato tubers (Solanum tuberosum). Both enzymes hydroxylated lauric acid (C12:0) on position ω-1 to ω-5. They oxidized fatty acids with chain length ranging from C12 to C18 and catalysed hydroxylation of 16-hydroxypalmitic acid leading to dihydroxypalmitic (DHP) acids, the major C16 cutin and suberin monomers. CYP77A19 also produced epoxides from linoleic acid (C18:2). Exploration of expression pattern in potato by RT-qPCR revealed the presence of transcripts in all tissues tested with the highest expression in the seed compared with leaves. Water stress enhanced their expression level in roots but not in leaves. Application of methyl jasmonate specifically induced CYP77A19 expression. Expression of either gene in the Arabidopsis null mutant cyp77a6-1 defective in flower cutin restored petal cuticular impermeability. Nanoridges were also observed in CYP77A20-expressing lines. However, only very low levels of the major flower cutin monomer 10,16-dihydroxypalmitate and no C18 epoxy monomers were found in the cutin of the complemented lines.

Mechanism and dynamics of fatty acid photodecarboxylase.

Fatty acid photodecarboxylase (FAP) is a photoenzyme with potential green chemistry applications. By combining static, time-resolved, and cryotrapping spectroscopy and crystallography as well as computation, we characterized Chlorella variabilis FAP reaction intermediates on time scales from subpicoseconds to milliseconds. High-resolution crystal structures from synchrotron and free electron laser x-ray sources highlighted an unusual bent shape of the oxidized flavin chromophore. We demonstrate that decarboxylation occurs directly upon reduction of the excited flavin by the fatty acid substrate. Along with flavin reoxidation by the alkyl radical intermediate, a major fraction of the cleaved carbon dioxide unexpectedly transformed in 100 nanoseconds, most likely into bicarbonate. This reaction is orders of magnitude faster than in solution. Two strictly conserved residues, R451 and C432, are essential for substrate stabilization and functional charge transfer.

Oil-bodies as substrates for lipolytic enzymes.

Plant seeds store triacylglycerols (TAGs) in intracellular organelles called oil-bodies or oleosomes, which consist of oil droplets covered by a coat of phospholipids and proteins. During seed germination, the TAGs of oil-bodies hydrolysed by lipases sustain the growth of the seedlings. The mechanism whereby lipases gain access to their substrate in these organelles is largely unknown. One of the questions that arises is whether the protein/phospholipid coat of oil-bodies prevents the access of lipase to the oil core. We have investigated the susceptibility of almond oil-bodies to in vitro lipolysis by various purified lipases with a broad range of biochemical properties. We have found that all the enzymes assayed were capable of releasing on their own free fatty acids from the TAG of oil-bodies. Depending on the lipase, the specific activity measured on oil-bodies using the pH-stat technique was found to range from 18 to 38% of the specific activity measured on almond oil emulsified by gum arabic. Some of these lipases are known to have a dual lipase/phospholipase activity. However, no correlation was found to exist between the ability of a lipase to readily and efficiently hydrolyse the TAG content of oil-bodies and the presence of a phospholipase activity. Kinetic studies indicate that oil-bodies behave as a substrate as other proteolipid organelles such as milk fat globules. Finally we have shown that a purified water-soluble plant lipase on its own can easily hydrolyse oil-bodies in vitro. Our results suggest that the lipolysis of oil-bodies in seedlings might occur without any pre-hydrolysis of the protein coat.

Assaying Arabidopsis lipase activity.

A low lipase activity from a crude extract of Arabidopsis seedlings was assayed using three sensitive methods (radiolabelled triacylglycerols, commercial resorufin ester and triacylglycerols containing the naturally fluorescent parinaric acid as substrates). The specific activity of the extract was found to be similar using the three methods. However, the plant lipase activity measured using the radioactivity and the fluorescence assays could be abolished by heating the extract, contrary to the apparent activity measured using the commercial colorimetric assay. Unlike the radioactivity assay, the fluorescence assay can be monitored continuously. The parinaric acid-based method is therefore the only one to provide a sensitive, specific and continuous assay.

Use of naturally fluorescent triacylglycerols from Parinari glaberrimum to detect low lipase activities from Arabidopsis thaliana seedlings.

The aim of this study was to design a convenient, specific, sensitive, and continuous lipase activity assay using natural long-chain triacylglycerols (TAGs). Oil was extracted from Parinari glaberrimum seed kernels and the purified TAGs were used as a substrate for detecting low levels of lipase activities. The purified TAGs are naturally fluorescent because more than half of the fatty acids from Parinari oil are known to contain 9,11,13, 15-octadecatetraenoic acid (parinaric acid) in its esterified form. The presence of detergents (sodium taurodeoxycholate, CHAPS, Sulfobetaine SB12, Tween 20, Brij 35, Dobanol, n-dodecylglucoside) above their critical micellar concentration dramatically increases the fluorescence of the parinaric acid released by various lipases. This increase in the fluorescence intensity is linear with time and proportional to the amount of lipase added. This new method, performed under non-oxidative conditions, was applied successfully to detecting low lipase levels in crude protein extracts from plant seeds and could be scaled down to microtiterplate measurements. Quantities as low as 0.1 ng of pure pancreatic lipase could be detected under standard conditions (pH 8). Lipase activity can also be assayed in acidic media (pH 5) using human gastric lipase. This simple and continuous assay is compatible with a high sample throughput and might be applied to detecting true lipase activities in various biological samples.

Use of the tape stripping technique for directly quantifying esterase activities in human stratum corneum.

The enzymes secreted in the intercellular spaces of stratum corneum (SC), the outermost layer of the epidermis, are thought to be involved in normal desquamation and skin barrier function. Their activity can barely be measured due to the difficulty in isolating enough biological material. Human SC layers were obtained from the forearm of healthy volunteers by the tape stripping technique. Assays for esterase activities were carried out in specially designed plates which contained the SC blotted on tape strips, using various fluorescent methylumbelliferone acyl esters as substrates. Triacylglycerol hydrolase activities were also studied by this method. By using radiolabeled triolein and fluorescent 4-methylumbelliferyl 7-oleate as substrates, true lipase activities could be detected and quantitated in SC at pH 5.5 and 7.5. These activities were shown to be strongly inhibited by tetrahydrolipstatin while this was not the case with 4-methylumbelliferyl 7-heptanoate. The method described here combines the painless tape stripping technique with a sensitive plate assay analysis. Since the whole process needs little manipulation, this method can permit rapid quantitation of multiple enzyme activities from a single strip. Therefore, it will permit the study of the involvement of enzyme activities in epidermis aging and skin pathologies.