# Mining of Potential Gene Resources for Breeding Nutritionally Improved Maize
## 6 Identification of Maize Potential Gene Resources for Mineral Content Improvement  
Minerals can be used as the components of some special substances in the human body and also as a co-enzyme to participate in metabolism as well as maintain cell membrane permeability and other various functions. It is important to understand mineral transport processes, since the minerals within food need to be taken up by plants from the soil. Plants have evolved two strategies for iron absorption. In dicots, Fe\({}^{3+}\) is reduced to Fe\({}^{2+}\) and then transported into cells. Unlike this, grass plants, such as maize, could directly chelate Fe\({}^{3+}\) by mugineic acid (MA) for transport. However, rice uses both strategies for iron uptake [97]. Overexpression of nicotianamine synthase (NAS) and nicotianamine aminotransferase (NAAT), both of which participate in MA biosynthesis, facilitates the transport of iron [98, 99]. NRAMP1 and NRAMP5 are two important transporters responsible for transporting Fe from roots to above-ground tissues where Fe could be stored in seeds. Fe chelates with citrate as it flows through the vascular, and FRD3 and FRDL1 are involved in the citrate transport. Fe is stored in vacuoles in plants, and VIT1 and NRAMP4 are responsible for the positive and negative regulation of vacuolar Fe content, respectively. Vacuolar Fe stores can be used to increase endosperm Fe content by inhibiting or promoting the expression of VIT1 and NRAMP4, respectively [100, 101]. Overexpression of endosperm Fe storage protein FER significantly increases Fe content in the endosperm [102]. Many TFs have been found to regulate Fe uptake and transport from different plant species, including IRO2 [103], OsbHLH58 [104], OsbHLH59 [104], AtbHLH29 [105], GmbHHL300 [106], IDEF1 [107] and CSN6 complex [108]. In addition, Rab6a, as a subunit of small GTPase, is involved in adaption to CO\({}_{2}\) enrichment, thereby regulating photosynthesis and Fe content [109]. Fe-binding ubiquitin ligase (HRZ) is associated with the negative regulation of the Fe transport pathway [110]. Zn transport is similar to Fe, and overexpression of NAS and NAAT also increases Zn content. However, many transporters are unique for Zn transport; these include MTP1 [111], ZIF1 [112], ZIF2 [113], HMA2 [114], HMA4 [115], HMA7 [116], ZIP1 [117], ZIP8 [118] and so on. Despite the fact that improving the transport efficiency of microelements could increase their contents in plants, plants contain a special anti-nutrient myo-inositol 1,2,3,4,5,6-hexakisphosphate (InsP6), commonly known as phytic acid (PA), which seriously affects human absorption of minerals. PA strongly chelates cations to form phytate, an insoluble salt that blocks the absorption of Fe and Zn from the human gut. The first step in the PA synthesis pathway is the conversion of glucose-6-phosphate to myo-inositol-3-phosphate by myo-inositol-1-phosphate synthase (MIPS), following which the myo-inositol-3-phosphate is further phosphorylated by 2-phosphoglycerate kinase (PGK), inositol 1,3,4-trisphosphate 5/6-kinase (ITPK) and inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK), to finally form PA [119]. PA could be downregulated by either inhibiting the production of these enzymes or promoting the synthesis of phytases such as HAD and PAP. Seventy-six homologous genes involved in mineral absorption, transport and regulation were identified from the maize genome (Table 5). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Genes** & **Protein Function** & **Maize Orthology** & **Gene ID** & **Strategy** & **References** \\ \hline \multirow{4}{*}{_FPGS_} & & & _Zm00001 eb044170_ & & \\  & _FGP2_ & _Zm00001 eb0404110_ & & \\  & _BM4_ & _Zm00001 eb299330_ & & \\  & & _Zm00001 eb421680_ & & \\ \hline \multirow{2}{*}{_GGH_} & \multirow{2}{*}{\(\gamma\)-glutamyl hydrolase_} & & _Zm00001 eb199250_ & & \\  & & _Zm00001 eb353180_ & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._