The oil content is a complex quantitative trait influenced by multiple genes and subject to complex genetic interactions with the environment. The balance between lipid and protein synthesis in plants depends on the activity of key enzymes in their metabolism [
88]. Pyruvate carboxylase (PEPC) is a key enzyme controlling the ratio of protein/oil content and is involved in the regulation of seed storage protein and fatty acid metabolism in plant seeds [
56,
86]. The activity of
PEPC may be coordinated through post-translational regulation, with the overexpression of
PEPC genes accelerated after nitrogen application [
6,
92]. Acetyl-CoA carboxylase (ACCase) is the rate-limiting enzyme in fatty acid biosynthesis [
93]. The expression of the
ACCase gene in rapeseed treated with different nitrogen changes significantly after 31 days of pollination [
6]. After nitrogen application, the expression of the
ACCase gene decreases in the high oil-content material, while it increases in the high protein-content material [
6]. Phospholipid phosphatase (PPase) is the rate-limiting enzyme in the biosynthesis of triacylglycerol [
56]. The oil content of seeds is related to the activity of the
PPase gene.
PPase activity in oilseeds is highest at 35 and 55 days after flowering under high nitrogen treatment, followed by medium and low nitrogen treatment [
57]. Diacylglycerol acyltransferase (DGAT2) catalyzes the synthesis of triacylglycerol (TAG) from diacylglycerol (DAG) in plants and is the last step and the rate-limiting step of TAG synthesis [
6]. Overexpression of the
DGAT2 gene in tobacco can increase the oil content and the content of oleic acid and linoleic acid in leaves [
6,
94]. Similarly, the expression of the
DGAT2 gene in the accession with high oil content was decreased by 12-fold after 31 days of pollination, compared to the case without nitrogen application [
6]. Pyruvate kinase (PK) catalyzes phosphoenolpyruvate (PEP) to pyruvate (PYR) and is the last irreversible enzyme in the glycolysis pathway [
95]. Glycerin-3-phosphate dehydrogenase (G3PDH) is involved in energy metabolic pathways such as glycolysis, gluconeogenesis, and the Calvin cycle, and plays an important role in promoting lipid synthesis [
96]. The expression of the
G3PDH and
PK genes is found to be enhanced with increasing nitrogen application levels [
97]. While many genes regulate lipid synthesis after nitrogen application, the regulatory network behind it is complex and huge, as oil content in rapeseed is a complex trait. Therefore, the molecular mechanism of nitrogen levels affecting the oil content of rapeseed still needs to be further investigated.
Figure 1.
Distribution of carbon and nitrogen source-sink nutrients in rapeseed. The sympathetic transporter (NRT) transports nitrate (NO
3-) to mesophyll cells, and the nitrate reductase (NR) converts nitrate (NO
3-) to nitrite (NO
2-). Nitrite (NO
2-) binding H
+ is transferred to the stroma of chloroplasts and converted to ammonium (NH
4+) by nitrite reductase (NiR). Ammonium (NH
4+) is converted to glutamate by glutamine synthetase (GS) and glutamate synthetase (GOGAT). Glutamic acid is converted to other amino acids in the cytoplasm [
29]. The root absorbs N and transports it to the leaves for photoassimilation, and the assimilated product is transported through the phloem to the developing leaves and growing roots. Nutrients from old leaves are remobilized into developing seeds for lipid and protein synthesis. The green line shows the path of nitrogen transfer and the red line shows the path of redistribution of photosynthetic products. PEP: phosphoenolpyruvate; PYR: pyruvate.
Figure 1.
Distribution of carbon and nitrogen source-sink nutrients in rapeseed. The sympathetic transporter (NRT) transports nitrate (NO
3-) to mesophyll cells, and the nitrate reductase (NR) converts nitrate (NO
3-) to nitrite (NO
2-). Nitrite (NO
2-) binding H
+ is transferred to the stroma of chloroplasts and converted to ammonium (NH
4+) by nitrite reductase (NiR). Ammonium (NH
4+) is converted to glutamate by glutamine synthetase (GS) and glutamate synthetase (GOGAT). Glutamic acid is converted to other amino acids in the cytoplasm [
29]. The root absorbs N and transports it to the leaves for photoassimilation, and the assimilated product is transported through the phloem to the developing leaves and growing roots. Nutrients from old leaves are remobilized into developing seeds for lipid and protein synthesis. The green line shows the path of nitrogen transfer and the red line shows the path of redistribution of photosynthetic products. PEP: phosphoenolpyruvate; PYR: pyruvate.