Transcriptional and biochemical responses of monoacylglycerol acyltransferase-mediated oil synthesis and associated senescence-like responses in Nicotiana benthamiana

Genome-wide identification and expression of monoacylglycerol lipase (MAGL) gene family in peanut (Arachis hypogaea L.) and functional analysis of AhMGATs in neutral lipid metabolism

Monoacylglycerol lipase (MAGL) involved in regulating plant growth and development and stress responses, hydrolyzes monoacylglycerol (MAG) into free fatty acid and glycerol, which is the last step of triacylglycerol (TAG) breakdown. Here, a genome-wide characterization of MAGL gene family from cultivated peanut (Arachis hypogaea L.) was performed. In total, 24 MAGL genes were identified and unevenly distributed on 14 chromosomes, encoding 229-414 amino acids with molecular weights ranging from 25.91 to 47.01 kDa. Spatiotemporal and stress-induced expression was analyzed by qRT-PCR. Multiple sequence alignment revealed that AhMAGL1a/b and AhMAGL3a/b were the only four bifunctional enzymes with conserved regions of hydrolase and acyltransferase, which could also be named as AhMGATs. GUS histochemical assay showed that AhMAGL1a and -1b were strongly expressed in all tissues of the plants; whereas both AhMAGL3a and -3b were weakly expressed in plants. Subcellular localization analysis indicated that AhMGATs were localized in the endoplasmic reticulum and/or Golgi complex. Seed-specific overexpression of AhMGATs in Arabidopsis decreased the oil content of the seeds and altered the fatty acid compositions, indicating that AhMGATs were involved in TAG breakdown but not TAG biosynthesis in plant seeds. This study lays the foundation for better understanding AhMAGL genes biological function in planta.

Cross-Talk between Transcriptome Analysis and Physiological Characterization Identifies the Genes in Response to the Low Phosphorus Stress in Malus mandshurica

Phosphorus (Pi) is a macronutrient essential for plant growth, development, and reproduction. However, there is not an efficient available amount of Pi that can be absorbed by plants in the soil. Previously, an elite line, MSDZ 109, selected from Malus mandshurica, was justified for its excellent tolerance to low phosphorus (low−Pi) stress. To date, however, the genes involved in low−Pi stress tolerance have not yet been unraveled in this species. Currently, the physiological responses of this line for different days to low−Pi stress were characterized, and their roots as well as leaves were used to carry out transcriptome analysis, so as to illuminate the potential molecular pathways and identify the genes involved in low−Pi stress−response. After exposure to low−Pi treatment (32 µmol/L KH₂PO₄) for 20 day after treatment (DAF) the biomass of shoots was significantly reduced in comparison with that of the stress−free (control), and root architecture diversely changed. For example, the root growth parameters e.g., length, surface area, and total volume somewhat increase in comparison with those of the control. The activity of acid phosphatase (ACP) increased with the low−Pi treatment, whereas the photosynthetic rate and biomass were declining. The activity of antioxidant enzymes, e.g., superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), were substantially elevated in response to low−Pi treatment. Many enzyme−related candidate genes e.g., MmCAT1, MmSOD1 and MmPOD21 were up−regulated to low−Pi treatment. Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that the processes of photosynthesis, plant hormone signal transduction, and MAPK signaling pathway were affected in the low−Pi response. In combination with the physiological characterization, several low−Pi−responsive genes, e.g., PHT, PHO, were identified, and the genes implicated in Pi uptake and transport, such as MmPHT1;5, MmPHO1, MmPAP1, etc., were also obtained since their expression status varied among the exposure times, which probably notifies the candidates involved in low−Pi−responsive tolerance in this line. Interestingly, low−Pi treatment activated the expression of transcription factors including the WRKY family, MYB family, etc. The available evidences will facilitate a better understanding of the roles of this line underlying the high tolerance to low−Pi stress. Additionally, the accessible data are helpful for the use of the apple rootstock M. mandshurica under low−Pi stress.

Metabolic engineering of medium-chain fatty acid biosynthesis in Nicotiana benthamiana plant leaf lipids

Various research groups are investigating the production of oil in non-seed biomass such as leaves. Recently, high levels of oil accumulation have been achieved in plant biomass using a combination of biotechnological approaches which also resulted in significant changes to the fatty acid composition of the leaf oil. In this study, we were interested to determine whether medium-chain fatty acids (MCFA) could be accumulated in leaf oil. MCFA are an ideal feedstock for biodiesel and a range of oleochemical products including lubricants, coatings, and detergents. In this study, we explore the synthesis, accumulation, and glycerolipid head-group distribution of MCFA in leaves of Nicotiana benthamiana after transient transgenic expression of C12:0-, C14:0-, and C16:0-ACP thioesterase genes. We demonstrate that the production of these MCFA in leaf is increased by the co-expression of the WRINKLED1 (WRI1) transcription factor, with the lysophosphatidic acid acyltransferase (LPAAT) from Cocos nucifera being required for the assembly of tri-MCFA TAG species. We also demonstrate that the newly-produced MCFA are incorporated into the triacylglycerol of leaves in which WRI1 + diacylglycerol acyltransferase1 (DGAT1) genes are co-expressed for increased oil accumulation.

Expression of Mouse MGAT in Arabidopsis Results in Increased Lipid Accumulation in Seeds

Worldwide demand for vegetable oil is projected to double within the next 30 years due to increasing food, fuel, and industrial requirements. There is therefore great interest in metabolic engineering strategies that boost oil accumulation in plant tissues, however, efforts to date have only achieved levels of storage lipid accumulation in plant tissues far below the benchmark to meet demand. Monoacylglycerol acyltransferase (MGAT) is predominantly associated with lipid absorption and resynthesis in the animal intestine where it catalyzes monoacylglycerol (MAG) to form diacylglycerol (DAG), and then triacylglycerol (TAG). In contrast plant lipid biosynthesis routes do not include MGAT. Rather, DAG and TAG are either synthesized from glycerol-3-phosphate by a series of three subsequent acylation reactions, or originated from phospholipids via an acyl editing pathway. Mouse MGATs 1 and 2 have been shown to increase oil content transiently in Nicotiana benthamiana leaf tissue by 2.6 fold. Here we explore the feasibility of this approach to increase TAG in Arabidopsis thaliana seed. The stable MGAT2 expression resulted in a significant increase in seed oil content by 1.32 fold. We also report evidence of the MGAT2 activity based on in vitro assays. Up to 3.9 fold increase of radiolabeled DAG were produced in seed lysate which suggest that the transgenic MGAT activity can result in DAG re-synthesis by salvaging the MAG product of lipid breakdown. The expression of MGAT2 therefore creates an independent and complementary TAG biosynthesis route to the endogenous Kennedy pathway and other glycerolipid synthesis routes.