# Mining of Potential Gene Resources for Breeding Nutritionally Improved Maize
## 4 Identification of Maize Potential Gene Resources for Essential Amino Acid Content Improvement  
Essential amino acids are vital for protein synthesis, tissue repair and nutrient absorption. For instance, both lysine and tryptophan are important components of neurotransmitters. However, humans and animals cannot synthesize essential amino acids and can only get them from diets rich in proteins. Grains are low in lysine, while beans are poor in methionine. In maize kernels, protein content ranges from 7% to 14%, depending on genotype and environmental effects [1]. Here, we focus on improving the content of three important essential amino acids: methionine, lysine and tryptophan, as well as total protein (Figure 3).  
Both lysine and methionine are synthesized from aspartate via different pathways. Aspartate kinase (AK) catalyzes the first step, which is a rate-limiting step that requires ATP, and this step is also regulated by subsequent steps in a feedback manner. Single amino acid substitution mutants of _ak_ are insensitive to the feedback inhibition of lysine synthesis and elevate lysine content [52]. Asparagraphyl-tRNA synthetase (SYNC) mediates the process of linking amino acids to tRNAs, and over-accumulation of the SYNC has been shown to increase lysine levels [53]. Increasing the abundance of proteins rich in lysine, such as VSP and BiP, elevates lysine content [54; 55; 56]. Cystathionine \(\gamma\)-synthesize (CGS), the first specific enzyme for methionine synthesis using cysteine as a precursor, is critical for the control of methionine content [57; 58]. The backflow from S-methylmethionine to methionine is mainly catalyzed by homocysteine S-methyltransferase  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Genes** & **Protein Function** & **Maize Orthologs** & **Gene ID** & **Strategy** & **References** \\ \hline _SDP1_ & sugar dependent & \begin{tabular}{c} _TGL1_ \\ _AY110479_ \\ \end{tabular} & \begin{tabular}{c} _Zm00001 eb370460_ \\ _Zm00001 eb062080_ \\ \end{tabular} & knockout & [31; 43; 44] \\ \hline _FAD2_ & delta-12 fatty acid & \begin{tabular}{c} _Zm00001 eb188990_ \\ _Zm00001 eb252730_ \\ \end{tabular} & \begin{tabular}{c} _Zm00001 eb300860_ \\ _Zm00001 eb409700_ \\ \end{tabular} & knockout & [50; 51] \\ \hline _FAD3_ & delta-12 fatty acid & \begin{tabular}{c} _FAD7_ \\ _desaturase_ \\ \end{tabular} & \begin{tabular}{c} _Zm00001 eb013340_ \\ _FAD8_ \\ \end{tabular} & knockout & [50] \\ \hline _FAE1_ & fatty acid elongase & \
\begin{tabular}{c} _KCS1_ \\ _KCS16_ \\ \end{tabular} &
\begin{tabular}{c} _Zm00001 eb344070_ \\ _Zm00001 eb296230_ \\ \end{tabular} & knockout & [51] \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 3: Schematic representation of amino acid biosynthetic pathways in plants. Curved arrows with a (\(-\)) sign represent major feedback inhibition loops by the end product amino acids and arrows with a (\(+\)) sign represent activation. (AK, aspartate kinase; TS, threonine synthase; CGS, cystathionine \(\gamma\)-synthesize; HMT, homocysteine S-methyltransferase).  
is a key enzyme for threonine synthesis but affects methionine formation as it competes with TS for their common substrate O-phosphophomoserine. More TS makes the resource flow to threonine, which is not conducive to the formation of methionine [59]. Studies have shown that mutations in _ts_ boost the methionine level [60; 61]. In the pathway of tryptophan synthesis, anthranilate synthase (ASA) is a key enzyme of the process and affects the pathway [62]. Aspartate aminotransferase (AAT) is also one of the important targets for improving protein content as it participates in the regulation of carbon and nitrogen metabolism through the transfer of the amino group [63]. Asparagine synthetase (ASN) has a similar function to AAT. Carrier protein SUT1 [64], AAP6 [65] and TF TaNAC100 [66] were also found to affect the protein content of grains. Twenty-eight homologous genes involved in essential amino acid biosynthesis were identified from the maize genome (Table 3).