# Mining of Potential Gene Resources for Breeding Nutritionally Improved Maize
## 3 Identification of Maize Potential Gene Resources for Oil Content Improvement  
Corn oil is a byproduct of corn wet-milling industries, and is a significant part of the human diet, useful in industrial applications and an alternative to fossil fuels. Corn oil is mainly composed of 59% polyunsaturated (PUFA), 24% monounsaturated (MUFA) and 13% saturated fatty acid (SFA) [23]. To enhance the economic value of corn, genome engineering is efficient and effective in improving the oil content of corn kernels [24]. Many enzymes, carrier proteins and transcription factors (TF) associated with the regulation of oil yield have been identified in other species. Genes encoding these proteins are potential resources for generating high oil-yielding maize by genome engineering. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Genes** & **Protein Function** & **Maize Orthologs** & **Gene ID** & **Strategy** & **References** \\ \hline \multirow{4}{*}{_SBE_} & & _SBE1_ & _Zm00001 eb228530_ & & \\  & & _SBE3_ & _Zm00001 eb357830_ & & \\  & & _SBE4_ & _Zm00001 eb084160_ & & \\  & & _AE1_ & _Zm00001 eb242610_ & & \\ \hline \multirow{4}{*}{_SS_} & & _SS1_ & _Zm0001 eb376100_ & & \\  & & _SS2_ & _Zm0001 eb070230_ & & \\  & & _SS3_ & _Zm0001 eb431240_ & & \\  & & _SS4_ & _Zm0001 eb353810_ & & \\  & & _SS5_ & _Zm00001 eb191890_ & & overexpression [17; 18; 19] \\  & & & _SS6_ & _Zm0001 eb222830_ & & \\  & & _SS7_ & _Zm00001 eb194550_ & & \\  & & _DUI_ & _Zm0001 eb413290_ & & \\  & & _SU2_ & _Zm0001 eb279740_ & & \\ \hline \multirow{2}{*}{_GBSS_} & & _WX1_ & _Zm00001 eb378140_ & & \\  & & _GBSS1_ & _Zm00001 eb305810_ & & overexpression & [20; 21; 22] \\ \hline _PTST1_ & _protein targeting to starch_ & _GPMI77_ & _Zm00001 eb231700_ & & overexpression & [20; 21] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of potential gene resources for improving resistant starch content in maize. The chemical composition of oil is triacylglycerolglycerol (TAG) formed from the sequential acylation of three fatty acids (FAs), with glycerol-3-phosphate (G3P) as the backbone. TAG de novo synthesis is catalyzed in the Kennedy pathway and is affected by the glycolysis and tricarboxylic acid cycle (TCA) processes. Glyceraldehyde-3-phosphate dehydrogenase (GAPC) catalyzes the reaction of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, and phosphoenolpyruvate carboxylase (PEPC) catalyzes the reaction of oxaloacetic acid to phosphoenolpyruvate. Overexpression of _GAPC_ or silencing of _PEPC_ promotes glycolysis and indirectly increases the content of dihydroxyacetone phosphate (DHAP) [25; 26]. Glycerol-3-phosphate dehydrogenase (GPDH) converts DHAP into glycerol-3-phosphate (G3P). Acetyl-CoA, a product of TCA, is promoted to malonyl-CoA by acetyl-CoA carboxylase (ACC), which enters the Kennedy pathway together with G3P. Glycerol-3-phosphate acyltransferase (GPAT) catalyzes G3P into lysophosphatidic acid (LPA), which is the first step in glycerolipid biosynthesis [27]. LPA or phatidylcholine (PC) are further catalyzed by a series of enzymes, including diacylglycerol acyltransferase (DGAT) and phospholipid diacylglycerol acyltransferase (PDAT), and finally form TAG (Figure 2). Studies have shown that GPAT [28], DGAT [29; 30; 31] and PDAT [30] directly affect the TAG synthesis. Transcription factors of AtMYB89 [32], AtMYB96 [33], LEC [34; 35; 36; 37],GL2 [38; 39], FUS3 [40] and HB2 [41] are involved in TAG biosynthesis regulation. Fatty acids are initially generated in the plastid and are transported to the endoplasmic reticulum for TAG synthesis. FAX1 and ABCA9 are identified as the carrier proteins for fatty acid transport from the plastid to the endoplasmic reticulum. In addition, _Oleosin_ (_OLE_) encodes the most abundant seed oil droplet-specific protein, overexpression of which increases oil levels in rice and soybean [30; 42]. On the other hand, silencing of _sugar dependent 1_ (_SDPI_), which inhibits the degradation of TAG, could also lead to increases in TAG content [31; 43; 44].