FAX2 Mediates Fatty Acid Export from Plastids in Developing Arabidopsis Seeds

Degradation of FATTY ACID EXPORT PROTEIN1 by RHOMBOID-LIKE PROTEASE11 contributes to cold tolerance in Arabidopsis

Plants need to acclimate to different stresses to optimize growth under unfavorable conditions. In Arabidopsis (Arabidopsis thaliana), the abundance of the chloroplast envelope protein FATTY ACID EXPORT PROTEIN1 (FAX1) decreases after the onset of low temperatures. However, how FAX1 degradation occurs and whether altered FAX1 abundance contributes to cold tolerance in plants remains unclear. The rapid cold-induced increase in RHOMBOID-LIKE PROTEASE11 (RBL11) transcript levels, the physical interaction of RBL11 with FAX1, the specific FAX1 degradation after RBL11 expression, and the absence of cold-induced FAX1 degradation in rbl11 loss-of-function mutants suggest that this enzyme is responsible for FAX1 degradation. Proteomic analyses showed that rbl11 mutants have higher levels of FAX1 and other proteins involved in membrane lipid homeostasis, suggesting that RBL11 is a key element in the remodeling of membrane properties during cold conditions. Consequently, in the cold, rbl11 mutants show a shift in lipid biosynthesis toward the eukaryotic pathway, which coincides with impaired cold tolerance. To test whether cold sensitivity is due to increased FAX1 levels, we analyzed FAX1 overexpressors. The rbl11 mutants and FAX1 overexpressor lines show superimposable phenotypic defects upon exposure to cold temperatures. Our re-sults show that the cold-induced degradation of FAX1 by RBL11 is critical for Arabidop-sis to survive cold and freezing periods.

Progress in understanding and improving oil content and quality in seeds

The world's population is projected to increase by two billion by 2050, resulting in food and energy insecurity. Oilseed crops have been identified as key to address these challenges: they produce and store lipids in the seeds as triacylglycerols that can serve as a source of food/feed, renewable fuels, and other industrially-relevant chemicals. Therefore, improving seed oil content and composition has generated immense interest. Research efforts aiming to unravel the regulatory pathways involved in fatty acid synthesis and to identify targets for metabolic engineering have made tremendous progress. This review provides a summary of the current knowledge of oil metabolism and discusses how photochemical activity and unconventional pathways can contribute to high carbon conversion efficiency in seeds. It also highlights the importance of ¹³C-metabolic flux analysis as a tool to gain insights on the pathways that regulate oil biosynthesis in seeds. Finally, a list of key genes and regulators that have been recently targeted to enhance seed oil production are reviewed and additional possible targets in the metabolic pathways are proposed to achieve desirable oil content and quality.

Fatty acid exporter 1 enhances seed oil content in Brassica napus

Fatty acid exporter 1 (FAX1) is an initial transporter for fatty acid (FA), in charge of transporting FA from the inside of the plastid to the outside. Brassica napus (B. napus) has nineteen members in the FAX family, of which there are six FAX1 homologous genes. Here, we generated the BnaFAX1 CRISPR mutants (BnaA09.FAX1 and BnaC08.FAX1 were both edited) and overexpression (OE) plants of BnaA09.FAX1 in B. napus. The results showed that the FA content was increased by 0.6-0.9% in OE plant leaves, and the seed oil content was increased by 1.4-1.7% in OE lines, compared to WT. Meanwhile, the content of triacylglycerol, diacylglycerol, and phosphatidylcholine was significantly increased in OE seeds. Moreover, seedling biomass and plant height of OE plants were increased compared to WT plants. However, the traits above had no significant difference between the mutants and WT. These results suggest that BnaA09.FAX1 plays a role in improving seed oil accumulation and plant growth, while the function of BnaFAX1 may be compensated by other homologous genes of BnaFAX1 and other BnaFAX genes in the mutants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01346-0.

Fatty acid export (FAX) proteins contribute to oil production in the green microalga Chlamydomonas reinhardtii

In algae and land plants, transport of fatty acids (FAs) from their site of synthesis in the plastid stroma to the endoplasmic reticulum (ER) for assembly into acyl lipids is crucial for cellular lipid homeostasis, including the biosynthesis of triacylglycerol (TAG) for energy storage. In the unicellular green alga Chlamydomonas reinhardtii, understanding and engineering of these processes is of particular interest for microalga-based biofuel and biomaterial production. Whereas in the model plant Arabidopsis thaliana, FAX (fatty acid export) proteins have been associated with a function in plastid FA-export and hence TAG synthesis in the ER, the knowledge on the function and subcellular localization of this protein family in Chlamydomonas is still scarce. Among the four FAX proteins encoded in the Chlamydomonas genome, we found Cr-FAX1 and Cr-FAX5 to be involved in TAG production by functioning in chloroplast and ER membranes, respectively. By in situ immunolocalization, we show that Cr-FAX1 inserts into the chloroplast envelope, while Cr-FAX5 is located in ER membranes. Severe reduction of Cr-FAX1 or Cr-FAX5 proteins by an artificial microRNA approach results in a strong decrease of the TAG content in the mutant strains. Further, overexpression of chloroplast Cr-FAX1, but not of ER-intrinsic Cr-FAX5, doubled the content of TAG in Chlamydomonas cells. We therefore propose that Cr-FAX1 in chloroplast envelopes and Cr-FAX5 in ER membranes represent a basic set of FAX proteins to ensure shuttling of FAs from chloroplasts to the ER and are crucial for oil production in Chlamydomonas.

Tracing Key Molecular Regulators of Lipid Biosynthesis in Tuber Development of Cyperus esculentus Using Transcriptomics and Lipidomics Profiling

Cyperus esculentus is widely representing one of the important oil crops around the world, which provides valuable resources of edible tubers called tiger nut. The chemical composition and high ability to produce fats emphasize the role of tiger nut in promoting oil crop productivity. However, the underlying molecular mechanism of the production and accumulation of lipids in tiger nut development still remains unclear. Here, we conducted comprehensive transcriptomics and lipidomics analyses at different developmental stages of tuber in Cyperus esculentus. Lipidomic analyses confirmed that the accumulation of lipids including glycolipids, phospholipids, and glycerides were significantly enriched during tuber development from early to mature stage. The proportion of phosphatidylcholines (PC) declined during all stages and phosphatidyl ethanolamine (PE) was significantly declined in early and middle stages. These findings implied that PC is actively involved in triacylglycerol (TAG) biosynthesis during the tubers development, whereas PE may participate in TAG metabolism during early and middle stages. Comparative transcriptomics analyses indicated several genomic and metabolic pathways associated with lipid metabolism during tuber development in tiger nut. The Pearson correlation analysis showed that TAG synthesis in different developmental stages was attributed to 37 candidate transcripts including CePAH1. The up-regulation of diacylglycerol (DAG) and oil content in yeast, resulted from the inducible expression of exogenous CePAH1 confirmed the central role of this candidate gene in lipid metabolism. Our results demonstrated the foundation of an integrative metabolic model for understanding the molecular mechanism of tuber development in tiger nut, in which lipid biosynthesis plays a central role.

Lipidomic and transcriptomic profiling of developing nodules reveals the essential roles of active glycolysis and fatty acid and membrane lipid biosynthesis in soybean nodulation

Symbiotic rhizobia-legume interactions are energy-demanding processes, and the carbon supply from host cells that is critically required for nodulation and nitrogen fixation is not fully understood. Investigation of the lipidomic and carbohydrate profiles with the transcriptome of developing nodules revealed highly activated glycolysis, fatty acid (FA), 2-monoacylglycerol (2-MAG), and membrane lipid biosynthesis and transport during nodule development. RNA-sequence profiling of metabolic genes in roots and developing nodules highlighted the enhanced expression of genes involved in the biosynthesis and transport of FAs, membrane lipids, and 2-MAG in rhizobia-soybean symbioses via the RAML-WRI-FatM-GPAT-STRL pathway, which is similar to that in legume-arbuscular mycorrhizal fungi symbiosis. The essential roles of the metabolic pathway during soybean nodulation were further supported by analysis of transgenic hairy roots overexpressing soybean GmWRI1b-OE and GmLEC2a-OE. GmLEC2a-OE hairy roots produced fewer nodules, in contrast to GmWRI1b-OE hairy roots. GmLEC2a-OE hairy roots displayed different or even opposite expression patterns of the genes involved in glycolysis and the synthesis of FAs, 2-MAG, TAG, and membrane lipids compared to GmWRI1b-OE hairy roots. Glycolysis, FA and membrane lipid biosynthesis were repressed in GmLEC2a-OE but increased in GmWRI1b-OE hairy roots, which may account for the reduced nodulation in GmLEC2a-OE hairy roots but increased nodulation in GmWRI1b-OE hairy roots. These data show that active FA, 2-MAG and membrane lipid biosynthesis are essential for nodulation and rhizobia-soybean symbioses. These data shed light on essential and complex lipid metabolism for soybean nodulation and nodule development, laying the foundation for the future detailed investigation of soybean nodulation.

Towards model-driven characterization and manipulation of plant lipid metabolism

Plant lipids have versatile applications and provide essential fatty acids in human diet. Therefore, there has been a growing interest to better characterize the genetic basis, regulatory networks, and metabolic pathways that shape lipid quantity and composition. Addressing these issues is challenging due to context-specificity of lipid metabolism integrating environmental, developmental, and tissue-specific cues. Here we systematically review the known metabolic pathways and regulatory interactions that modulate the levels of storage lipids in oilseeds. We argue that the current understanding of lipid metabolism provides the basis for its study in the context of genome-wide plant metabolic networks with the help of approaches from constraint-based modeling and metabolic flux analysis. The focus is on providing a comprehensive summary of the state-of-the-art of modeling plant lipid metabolic pathways, which we then contrast with the existing modeling efforts in yeast and microalgae. We then point out the gaps in knowledge of lipid metabolism, and enumerate the recent advances of using genome-wide association and quantitative trait loci mapping studies to unravel the genetic regulations of lipid metabolism. Finally, we offer a perspective on how advances in the constraint-based modeling framework can propel further characterization of plant lipid metabolism and its rational manipulation.

Plant Unsaturated Fatty Acids: Biosynthesis and Regulation

In most plants, major unsaturated fatty acids (UFAs) are three C18 species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds play multiple crucial roles in planta and are also important economic traits of oil crops. The enzymatic steps of C18 UFA biosynthesis have been well established. However, the associated FA/lipid trafficking between the plastid and the endoplasmic reticulum remains largely unclear, as does the regulation of the expression and activities of the involved enzymes. In this review, we will revisit the biosynthesis of C18 UFAs with an emphasis on the trafficking, and present an overview of the key enzymes and their regulation. Of particular interest is the emerging regulatory network composed of transcriptional factors and upstream signaling pathways. The review thereby provides the promise of using physical, biochemical and/or genetic means to manipulate FA composition and increase oil yield in crop improvement.