The world’s largest oil-producing crop is the oil palm (Elaeis guineensis), which accumulates up to 90% of its oil in the mesocarp (Bhagya et al., 2020; Zhou et al., 2020). However, few reports are available on transcription factor (TF) networks regulating lipid metabolism in oil palm (Yeap et al., 2017; Li et al., 2020). Wang et al. suggested that EgMADS16 negatively regulates FAD2, SAD, and DGAT2 transcription during oil-palm mesocarp development, interacting with EgGLO1 to affect FA and triacylglycerol (TAG) biosynthesis. The authors also proposed a mechanism to explain the suppression of EgMADS16 expression by its target Egmir5179, a small RNA involved in oil accumulation (Figure 1A). Next, a study by Shi et al. examined how ABA treatment increases the unsaturated FA linoleic acid in oil-palm mesocarp. Through transcriptome analysis, they hypothesized that exogenous ABA increases linoleic acid (18:2) accumulation via activating ABA signaling genes PYR, PP2C, and SnRK, as well as TFs such as ABI5, resulting in upregulation of FAD2 expression (Figure 1B).

Glycerol kinase (GK) catalyzes the conversion of glycerol to glycerol-3-phosphate (G3P), but its physiological significance in rice defense against pathogens remains unclear. Xiao et al. confirmed that the GK gene OsNHO1 was upregulated in Xanthomonas oryzae pv. oryzae (Xoo) strain PXO99. Additionally, in the transgenic rice line overexpressing OsHNO1-OE, GK content and OsNHO1 transcription both increased, improving resistance to bacterial blight and blast diseases. In contrast, resistance was impaired in the OsNHO1-RNAi line. Similarly, wax content and wax-synthesis gene expression increased significantly in the overexpression line, while decreasing in the OsNHO1-RNAi line. Inhibiting OsNHO1 also led to decreased transcription of its interaction partners, OsSRC2 and OsPR. Xiao et al. concluded that OsNHO1 confers disease resistance through affecting wax content and regulating pathogenesis-related (PR) gene transcription (Figure 1C).

As a major seed energy source, TAG is stored in the form of fixed carbon (Baud and Lepiniec, 2009). Photosynthetic processes in chlorophyll-containing green seeds mainly involves FA synthesis by ATP and reducing agents (NADPH and NADH) produced in leaves and other green plant parts (Goffman et al., 2005; Hua et al., 2014; Zhang et al., 2016). However, the contribution of FA accumulation in non-green seeds remains uncertain. Nwafor et al. demonstrated that photosynthesis in Arabidopsis non-green seeds is responsible for 20% of FA synthesis, whereas photosynthesis in siliques and leaves/stems is responsible for 40%. The authors further suggested that the oxidized pentose phosphate pathway may be a source of the carbon, energy, and reducing agents required for FA synthesis during seed development.

Considerable effort has been devoted to developing elite rapeseed cultivars with high oil content, high yield, and disease resistance (Bao et al., 2018; Chen et al., 2018). QTLs are particularly important for deciphering agronomic traits. A recent study aligned 4,555 QTLs (identified over 25 years across 12 countries) to construct a quantitative genomic map containing 128 traits from 79 populations (Raboanatahiry et al.). These results revealed 517 regions of overlapping QTLs that harbor 2,744 candidate genes simultaneously affecting multiple traits. This data can be used to develop new rapeseed varieties.

Torreya grandis Fortune ex Lindl. is a commercial species of gymnosperm that produces oil-rich nuts with high unsaturated FA content in mature kernels (Shi et al., 2018). Zhang et al. compared de novo transcriptome and FA accumulation during kernel development of two varieties: high-oil (52.9%) T. grandis ‘Xifei’ and medium-oil (41.6%) T. grandis ‘Dielsii’. The authors inferred that differences in FA and TAG accumulation, along with related transcript expression, were the primary factors responsible for oil-content variation (oleic acid ratio, sciadonic acid ratio) between the two varieties.

Camellia is one of the four most commercially valued woody plants worldwide. Camellia oil is rich in polyphenols, saponins, and other nutritional components. Owing to its health benefits, camellia oil has strong economic competitiveness and broad market prospects (Zhu et al., 2010). Chen et al. identified an oil-tea Camellia species previously unknown in Hainan by comparing chloroplast genomic (cpDNA) sequences of 13 Chinese oil-tea Camellia samples. They concluded that cpDNA of oil-tea Camellia species exhibits a conserved tetrad structure with specific length polymorphisms. Additionally, simple sequence repeats (SSR) and other mutations led to an abundance of divergent hotspots in coding sequences (CDS) and intergenic space (IGS). The presence of these hotspots suggests that the entire cpDNA sequence can be used for species identification and phylogenetic analysis of Camellia.

Brassicaceae oilseeds produce a very long-chain monounsaturated FA called erucic acid (EA, C22:1), a compound extensively used in various chemical industries (Sakhno, 2010). However, EA is not easily digested and absorbed. Moreover, high-EA rapeseed (HEAR) oil often contains glucosinolates, which have been implicated in disease. Biotechnology research is thus necessary to produce low-EA rapeseed oil (LEAR) in addition to HEAR. In their review, Wang et al. examined the EA biosynthetic pathway and EA resources in various Brassicaceae crops. The available data led them to promote commercialization of genetically modified products that improved EA content in Brassica oilseeds. In the same vein, Liu et al. developed rapeseed lines with significantly lower EA content by using CRISPR/Cas9 to knock out one or both FAE1 copies in the amphidiploid plant. Knocking out BnaC03.FAE1 decreased EA content by more than 10%, whereas knocking out both BnaA08.FAE1 and BnaC03.FAE1 almost completely abolished EA content. Instead, oleic acid content increased considerably. These experiments ultimately lowered seed oil content, without affecting other agricultural characteristics.

Physaria fendleri (Brassicaceae) accumulates the long-chain HFA lesquerolic acid (20:1OH) (Dierig et al., 2011), suggesting that this oilseed species can be an alternative crop to castors for producing industrially valuable HFAs. In support of this application, seed-specific RNAi knockdown of TAG lipase SUGAR DEPENDENT 1 (SDP1) increased seed weight and total seed-oil content, without significantly affecting seedling establishment (Azeez et al.).

In their review of the biochemistry and molecular genetics underlying oil synthesis, Wallis et al. highlighted valuable tools for blocking relevant genes during seed development to lower polyunsaturated fatty acid (PUFA) content in oil. The authors introduced successful breeding programs that produced high-oleic, low-PUFA varieties of soybean, canola, and other oilseed crops. Finally, Park and Kim reviewed current case studies and future strategies for regulating FA and TAG metabolism using CRISPR/Cas9 (Figure 1D).

Vegetable oils contribute significantly to calorie intake in the human diet, but their FA composition is not ideal for nutrition nor the needs of the food industry. Therefore, the mechanisms of FA and TAG synthesis in oil crops should be identified. Clearly, to meet the growing demand for vegetable oil, further biotechnology research and better breeding programs are necessary to improve crop oil content and FA composition.

All authors listed have made substantial, direct, and intellectual contributions to the work, and approved it for publication.

This work was supported by a grant from the New Breeding Technologies Development Program (Project No. PJ016533), Rural Development Administration, Republic of Korea.

We thank all authors, reviewers, and editorial staff who contributed to the finalization of this special issue.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations or those of the publisher, editors, and reviewers. The publisher does not guarantee or endorse any product evaluated in this article, nor any manufacturer claims.