# Mining of Potential Gene Resources for Breeding Nutritionally Improved Maize
## 5 Identification of Maize Potential Gene Resources for Vitamin Content Improvement
Transcription factors cytosine-mismatch-binding protein 1 (CMB1) and stay-green protein (SGR1) were found to regulate carotenoid accumulation during fruit ripening in tomatoes [78; 79]. As an antioxidant, \(\beta\)-carotene is easily degraded by light, heat and oxygen. The inhibition of carotenoid cleavage dioxygenase (CCD) and lipoxygenase (LOX) can delay the degradation of \(\beta\)-carotene [80; 81; 82]. For mammals, deficiency in vitamin E is associated with some cancers, as well as neurodegenerative and cardiovascular diseases. Vitamin E is made up of four tocopherols and four tocopherols and four tocopherol is the most active form. The phytol moiety of tocopherols could be derived from chlorophyll. Chlorophyll debutyrate(CLD) and chlorophyll synthase (CHLG) are involved in the synthesis and decomposition of the process, respectively. \(\beta\)-hydroxyphenylpyruvate dioxygenase (HPD), homogenistate phyttransferase (HPT), homogenistic acid geramyfleary  
but their mechanisms are not well understood. In the case of folate, GTP is its synthetic substrate, which is first catalyzed by GTP cyclohydrolase I (GTPCHI). After entering the mitochondrion, it merges with para-aminobenzzoate (p-ABA) from plastid and is catalyzed by aminodeoxycharismate synthase (ADCS) and other enzymes. Overexpression of two vital enzymes, dihydrofolate synthetase (DHFS) or folypolyglutamate synthase (FPGS), improves the efficiency of folate synthesis [87]. These enzymes add glutamate to folate and increase its stability, while \(\gamma\)-glutamyl hydrolase (GGH) hydrolyzes it. On the contrary, overexpression of GGH decreases the level of folate [88]. The key enzymes involved in vitamin A, B, C and E synthesis are shown in Figure 4. Fifty-six homologous genes involved in vitamin synthesis were identified from the maize genome (Table 4). \begin{table}
\begin{tabular}{c c c c c c} \hline
**Genes** & **Protein Function** & **Maize Orthologs** & **Gene ID** & **Strategy** & **References** \\ \hline \multirow{2}{*}{_DXS_} & 1-deoxyxylulose 5-phosphate synthase & _DXS1_ & _Zm00001 eb287860_ & overexpression & [89] \\ \hline \multirow{2}{*}{_DXR_} & 1-deoxy-D-xylulose & _DXR1_ & _Zm00001 eb126690_ & overexpression & [72] \\  & 5-phosphate reductioisomerase & _DXR2_ & _Zm00001 eb334370_ & overexpression & [90] \\ \hline _PDS_ & phytoene desaturase & _VP5_ & _Zm00001 eb006300_ & overexpression & [90] \\ \hline _OR_ & coactivator & & _Zm00001 eb249060_ & overexpression & [73; 74] \\ \hline \multirow{2}{*}{_DET1_} & \multirow{2}{*}{transcription factor} & \multicolumn{2}{c}{_Zm0001 eb317230_} & \multirow{2}{*}{knockout} & [75; 76] \\  & & & _Zm00001 eb341540_ & & \\ \hline _BZR1_ & transcription factor & _BES1_ & _Zm00001 eb325550_ & overexpression & [77] \\ \hline \end{tabular}
\end{table} 
Table 4: List of potential gene resources for enhancing vitamin contents in maize. Figure 4: Schematic representation of vitamin biosynthetic pathways in plants. (G3P, glycerol-3-phosphate; DXS, 1-deoxy-D-xylulose 5-phosphate synthase; DXP, 1-deoxy-D-xylulose-5-phosphate; DXR, 1-Deoxy-D-xylulose 5-phosphate reductioisomerase; IPP, isopentenyl diphosphate isomerase; PDS, phytoene desaturase; HPPD, \(\varepsilon\)-hydroxyphenylpyruvate dioxygenase; HPT, homogenistate phytyltransferase; MPBQ, methylphytylbenzzoquinol; HGGT, homogenistic acid geranylfearse; TC,tocopherol cyclase; MPBQMT, 2-methyl-6-pyridylbenzamidequinol methyltransferase; DMPBQ, dimethylphytylbenzoquinone; PEP, phosphoenolpyruvate; E4P, erythrose 4-phosphate; ADCS, aminodeoxycharismate synthase; p-ABA, para-aminobenzzoate; DDH, dihydroproteate; GTPCHI, GTP cyclohydrolase I; DHFS, dihydrofolate synthetase; DHF, dihydrofolate; THF, tetrahydrofolate; FGPS, folypolyglutamate synthase; G6P, glucose-6-phosphate; GME, GDP-mannose 3,5-epimerase; GGP, GDP-L-galactose phosphorylase; GPP, L-galactose-1-phosphate; GDH, L-galactose dehydrogenase; GalLDH, L-galactono-1,4-lactone dehydrogenase).