A Summary of Two Decades of QTL and Candidate Genes That Control Seed Tocopherol Contents in Maize (Zea mays L.)

Tocopherols are secondary metabolites synthesized through the shikimate biosynthetic pathway in the plastids of most plants. It is well known that α–Tocopherol (vitamin E) has many health benefits for humans and animals; therefore, it is highly used in human and animal diets. Tocopherols vary considerably in most crop (and plant) species and within cultivars of the same species depending on environmental and growth conditions; tocopherol content is a polygenic, complex traits, and its inheritance is poorly understood. The objective of this review paper was to summarize all identified quantitative trait loci (QTL) that control seed tocopherols and related contents identified in maize (Zea mays) during the past two decades (2002–2022). Candidate genes identified within these QTL regions are also discussed. The QTL described here, and candidate genes identified within these genomic regions could be used in breeding programs to develop maize cultivars with high, beneficial levels of seed tocopherol contents.


Introduction
Tocopherols play a crucial role in plant stress resistance and are considered an essential nutrient for animals, due to their vitamin E activity.In plants, tocopherols exist in four forms: α-tocopherol (α-Toc), β-tocopherol (β-Toc), γ-tocopherol (γ-Toc), and δ-tocopherol (δ-Toc).These forms differ from each other in terms of biological activity and molecular structure.The α-Toc form, also known as vitamin E, is the most bioactive tocopherol molecule [1,2].These forms are synthesized through the shikimate biosynthetic pathway in the plastids of most plants, including Brassica napus, maize (Z.mays), sunflower (Helianthus annuus) [3,4], and other plant species such as pecans, Brazil nuts, and peanuts [5].
In most plants, stress tolerance is achieved by an increase in vitamin E (α-Toc) content, which deactivates reactive oxygen species (ROS) and prevents lipid peroxidation [6][7][8].Vitamin E (α-Toc) is also involved in membrane stability, cyclic electron flow during photosynthesis, and signal transduction [9][10][11], while γ-Toc has been shown to be involved in protecting unsaturated fatty acids and improving seed-desiccation tolerance [12].Also, both α-Toc and γ-Toc have been shown to reduce salt and sorbitol stresses [11,13].A study provided ample evidence that tocopherols help plants cope with biotic stresses such as drought, cold, heat, and salt stresses as well as biotic stresses such as fungal and bacterial infections [8].The role of tocopherols in resistance to biotic stresses is more likely achieved by the regulation of fatty acid biosynthesis [8].
The objective of this review paper was to summarize all identified quantitative trait loci (QTL) and candidate genes identified in maize (Z.mays L.).

QTLs That Control Seed Tocopherol Contents
Two mapping population crosses between cultivars that differ in their seed α-Toc and γ-Toc contents were analyzed and genotyped with simple sequence repeat (SSR) and restriction fragment length polymorphism (RFLP) markers, and a software package was used to identify QTL that control seed α-Toc and γ-Toc contents [28].Eight QTL that control seed α-Toc and γ-Toc contents have been identified on chromosomes 1, 4, 5, 6, and 7 in one or both populations (Table S1; Figure S1) [37].
In a previous study, two distinct population sets were employed [29].The first population was composed of Recombinant Inbred Lines (RILs), which were generated from a cross between 'W64a' (known for its high seed γ-Toc content) and 'A632' (recognized for its high seed α-Toc content) lines, with a total of 200 individuals (n = 200).The second population resulted from crossing the 200 F 2 plants with the 'AE335' line, characterized by its high seed oil content, to create the testcross population of 185 individuals (n = 185).Both populations were meticulously assessed in Urbana, IL, over two-year spans, which were from 1996 to 1997 for the RIL population and from 1999 to 2000 for the testcross population.The research approach involved employing WinQTL Cartographer's Composite Interval Mapping (CIM) to identify Quantitative trait loci (QTL) responsible for regulating seed α-Toc, γ-Toc, and δ-Toc contents and the α-Toc/γ-Toc ratio.In the F 2 population, they successfully pinpointed and mapped four QTL controlling seed α-Toc content, which were distributed across chromosomes 5, 6, and 8. Additionally, they identified and mapped four QTL governing seed γ-Toc content, which were located on chromosomes 1, 2, 5, and 7. Furthermore, three QTL governing seed δ-Toc content were identified and mapped on chromosomes 1, 5, and 8. Finally, they detected one QTL controlling seed T-Toc content and the (α/γ)-Toc ratio, which was mapped on chromosome 5.In the testcross population, Wong et al. identified five QTL regulating seed α-Toc content, which were distributed across chromosomes 3, 5, and 6.They also located three QTL governing seed γ-Toc content, which were mapped on chromosomes 5 and 7. Furthermore, one QTL controlling seed δ-Toc content was identified and mapped on chromosome 5.In addition, they discovered three QTL for seed T-Toc content, which were located on chromosomes 5 and 7, and two QTL for seed (α/γ)-Toc ratio on chromosome 5 (Table S1 and Figure S1) [29].
In their study, a Recombinant Inbred Line (RIL) population was cultivated that was developed from a cross between 'By804' and 'B73' lines, consisting of 233 individuals, in China, spanning a two-year period from 2004 to 2005.They subjected this population to genotyping using 208 markers and employed Composite Interval Mapping (CIM) with WinQTL Cartographer to pinpoint Quantitative trait loci (QTL) responsible for regulating seed α-Toc, γ-Toc, δ-Toc, and T-Toc contents.Their analysis led to the identification and mapping of seven QTL controlling seed α-Toc content, which were distributed across chromosomes 1, 2, 5, 8, 9, and 10.Additionally, they identified and mapped seven QTL governing seed γ-Toc content, which were located on chromosomes 1, 2, 5, and 8. Furthermore, four QTL controlling seed δ-Toc content were identified and mapped on chromosomes 6, 7, and 8.In addition, they detected eight QTL regulating seed T-Toc content that were distributed across chromosomes 1, 2, 5, 8, and 9. Lastly, they found five QTL controlling seed (α/γ)-Toc ratio, which were mapped on chromosomes 3, 5, and 7, as detailed in Table S1 and Figure S1.In a related aspect of their research, Chander et al. ( 2008) also identified five candidate genes (P3VTE5, HPPD, VTE3, VTE4, and PSY1) involved in tocopherol biosynthesis within the genomic regions harboring the QTL responsible for regulating seed tocopherol contents.These findings offer valuable insights into the genetic factors influencing tocopherol content in seeds [24].
In 2009, a study was conducted using a natural population comprising 543 individuals in three different locations across China.This population was subjected to genotyping with a set of 56,110 SNPs using the MaizeSNP50 BeadChip.The researchers employed a Genome-Wide Association Study (GWAS) approach to identify SNPs associated with Quantitative trait loci (QTL) responsible for governing seed α-Toc, γ-Toc, δ-Toc, and T-Toc contents.Their investigation led to the identification and mapping of 24 SNPs associated with seed α-Toc content, which were distributed across chromosomes 1, 3, 4, and 5. Additionally, they discovered five SNPs linked to seed δ-Toc content, which were mapped on chromosomes 2, 3, 4, 5, and 9. Furthermore, three SNPs were associated with seed γ-Toc content, which were located on chromosomes 1, 3, and 5.They also identified six SNPs associated with seed total tocopherol (T-Toc) content, which were mapped on chromosomes 1, 2, 3, 4, 5, and 8, as detailed in Table S1 and Figure S1.Among the 24 SNPs related to seed α-Toc content, nine were of particular significance and were concentrated within a 2.4 Mb genomic region on chromosome 5. Notably, three of these highly significant SNPs were found to be situated within the ZmVTE4 gene, which encodes γ-tocopherol methyltransferase [38].
In 2009, a study involving two F 2 populations, one resulting from a cross between K22 and CI7 (comprising 237 individuals) and the other from a cross between K22 and Dan340 (comprising 218 individuals (n = 218)), was conducted.These populations were grown in two separate locations in China.The researchers carried out genotyping for each population using a set of 1536 SNPs and subsequently employed a Genome-Wide Association Study (GWAS) to identify SNPs associated with Quantitative trait loci (QTL) responsible for governing seed α-Toc, γ-Toc, δ-Toc, and T-Toc contents.Within the K22 by CI7 F 2 population (n = 237), they identified and mapped nine QTL controlling seed α-Toc content, which were distributed across chromosomes 2, 5, 6, and 7. Similarly, nine QTL were discovered that regulated seed γ-Toc content, and these were mapped on chromosomes 1, 5, and 6.Furthermore, eight QTL controlling seed T-Toc content were identified and mapped on chromosomes 1, 2, 5, and 7. Additionally, eight QTL responsible for seed (α/γ)-Toc ratio were identified and mapped on chromosomes 5, 6, and 8.These findings can be further explored in Table S1 and Figure S1 of Shutu et al.'s 2012 research.In the K22 by Dan340 F 2 population (n = 218), the researchers identified six QTL governing seed α-Toc content, mapped on chromosomes 1, 5, 8, and 10.They also detected nine QTL associated with seed γ-Toc content, which were mapped on chromosomes 1, 2, 5, and 8.Moreover, eight QTL controlling seed T-Toc content were identified and mapped on chromosomes 1, 2, and 5. Additionally, seven QTL influencing seed (α/γ)-Toc ratio were identified and mapped on chromosomes 1, 5, and 8.These results are detailed in Table S1, Figure S1 [39].
In 2011, a study involving an F 2 population of sweet corn was conducted, which originated from a cross between the 'A6' and 'A57' lines, comprising a total of 229 individuals in China.The researchers carried out genotyping for this population using 512 SSR markers and subsequently employed Composite Interval Mapping (CIM) with WinQTL Cartographer to pinpoint Quantitative trait loci (QTL) responsible for regulating seed α-Toc, γ-Toc, δ-Toc, and T-Toc contents.In their investigation, they successfully identified and mapped two QTL governing seed α-Toc content, which were located on chromosomes 1 and 2. Additionally, they discovered one QTL responsible for seed δ-Toc content, which was mapped on chromosome 10.Furthermore, two QTL controlling seed γ-Toc content were identified and mapped on chromosomes 1 and 5.In the case of seed T-Toc content, they identified and mapped four QTL on chromosomes 1, 5, and 6.Lastly, two QTL influencing seed (α/γ)-Toc ratio were identified and mapped on chromosomes 1 and 2. Comprehensive details of these findings are available in Table S1, Figure S1, and ref. [40].
In their study, the high tocopherol line SY999 was utilized and a series of backcrosses with four different maize lines were conducted, namely K140, K185, M01, and M14.The objective was to introduce the ZmVTE4 gene into these lines and enhance their tocopherol content through marker-assisted backcrossing.The outcome of their efforts resulted in a significant increase in seed tocopherol contents for three out of the four lines, namely K140, K185, and M01 [42].
In 2009-2010, Diepenbrock et al. ( 2017) conducted a comprehensive study involving two distinct groups: a Nested Association Mapping (NAM) population consisting of 5000 individuals and an association panel comprising 281 individuals.The NAM population was generated through crosses between the 'B73' line and 25 inbred lines, leading to the creation of 25 families, each containing 200 Recombinant Inbred Lines (RILs).Genotyping was carried out for these populations using a set of 14,772 markers, including 1106 Single Nucleotide Polymorphisms (SNPs).The researchers utilized both Joint Linkage (JL) and Genome-Wide Association Study (GWAS) analyses to identify Quantitative trait loci (QTL) responsible for controlling the seed contents of ten tocopherol-related traits.The study revealed a total of 162 QTL associated with one or more of these 10 tocopherolrelated traits, and they were mapped onto all ten maize chromosomes.Specifically, the researchers identified: • Thirteen QTL controlling seed α-Tocopherol (αT) on chromosomes 1, 2, 3, 4, 5, 6, 7, 8, and 9.
From 2009 to 2011, a study was conducted in China involving seven different populations.One of these populations comprised an association panel of inbred corn lines from across China totaling 508 individuals.They were subjected to genotyping with 56,110 SNPs, and the researchers employed GWAS analysis to pinpoint Quantitative trait loci (QTL) responsible for regulating seed contents of α-tocopherol (AT), γ-tocopherol (GT), δ-tocopherol (DT), and total tocopherol (TT) and the (α/γ)-tocopherol ratio.The remaining populations were composed of Recombinant Inbred Line (RIL) populations, each consisting of approximately 200 lines.For these biparental RIL populations, the researchers utilized CIM in WinQTL Cartographer to identify QTL governing seed contents of α-tocopherol (AT), γ-tocopherol (GT), δ-tocopherol (DT), and total tocopherol (TT) and the (α/γ)-tocopherol ratio (RT).In these RIL populations, the researchers successfully iden-tified and mapped a total of 41 QTL that control one or more of the five tocopherol-related traits, spanning all chromosomes (1-10).Among these QTL, ten were novel, and six of them were considered major QTL, explaining 10% or more of the phenotypic variation in seed kernel tocopherol content.In the natural population, 151 SNPs associated with QTL regulating seed AT were identified and mapped on chromosomes 1, 2, 3, 4, and 5. Additionally, 17 SNPs linked to QTL controlling seed GT were identified and mapped on chromosomes 1, 5, 6, and 8.Moreover, 61 SNPs were associated with QTL governing seed TT and were mapped on chromosomes 1, 2, 3, 4, 5, 6, 8, and 10.Lastly, 51 SNPs were linked to QTL controlling seed RT, and they were identified and mapped on chromosomes 2, 4, 5, 6, 9, and 10, as detailed in Table S1.Furthermore, numerous candidate genes were discovered within these QTL regions, including genes like GRMZM2G436226 on chromosome 1, which encodes a Zinc finger CCCH domain-containing protein; GRMZM2G055752 and GRMZM2G079236 on chromosome 2, which encode a DNA topoisomerase and an Acyl-CoA synthetase long-chain family member, respectively; GRMZM2G170013 on chromosome 3, which encodes a chlorophyll b reductase NYC1; and GRMZM5G892742 and GRMZM2G150714 on chromosome 4, which encode a DNAJ-related chaperone protein and a putative leucine-rich repeat receptor-like protein kinase family protein, respectively.Similar candidate genes were identified on other chromosomes (5-10), as listed in Table S1 [44].
In 2011-2012, Alves et al. ( 2020) conducted a study in Portugal that grew an association panel comprising 132 inbred corn lines.They performed genotyping on this panel using 48,778 SNPs and applied GWAS analysis to identify Quantitative trait loci (QTL) governing the seed contents of α-tocopherol (AT), γ-tocopherol (GT), and δ-tocopherol (DT).Additionally, they investigated seed carotenoid, phenolic compound, and hydroxycinnamic acid contents.The study successfully identified and mapped a total of 14 QTL controlling seed AT (ATLOG) content, located on chromosomes 4, 5, and 10.They also found 19 QTL regulating seed DT (DTLOG) content, which were distributed across chromosomes 1, 2, 3, 4, 5, 6, and 7. Furthermore, five QTL governing seed GT content were identified and mapped to chromosomes 1, 4, and 8, as detailed in Table S1 and [46].

Seed Tocopherol Candidate Genes
As mentioned earlier, Li et al. (2012) identified 24 SNPs associated with seed α-, δ-, γ-, and total tocopherol contents, and 3 of these SNPs fell within the ZmVTE4 gene encoding γ-tocopherol methyltransferase (Table S1) [38].Lipka et al. (2013) identified 88 SNPs associated with tocopherol-related traits among which a strong cluster of SNPs on chromosome 5 that contains the candidate gene GRMZM2G035213 (ZmVTE4) encoding γ-tocopherol methyltransferase (γ-TMT).Other candidate genes have been identified in the same region such as GRMZM2G161641 encoding an amino acid permease, GRMZM5G823157 encoding a WYRKY transcription factor, and GRMZM2G325019 encoding a pentatricopeptide repeat-containing (PPR) protein.The candidate gene GRMZM5G833760 that encodes a phytosulfokine receptor has been identified within the QTL region that controls seed γT3 content on chromosome 9 (Table S1) [41].Diepenbrock et al. (2017) identified 162 QTL that control seed tocopherol-related traits and many candidate genes have been identified within these QTL regions, including genes that encode γ-tocopherol methyltransferase (VTE4), Arogenate/prephenate dehydrogenase family protein, ABC kinase (tocopherol cyclase kinase), 1-deoxy-D-xylulose 5-phosphate synthase 1 (dxs1), 1-deoxy-D-xylulose 5-phosphate synthase 2 (dxs2), chorismate mutase, homogentisic acid geranylgeranyl transferase 1 (HGGT1), shikimate biosynthesis protein, 4-hydroxyphenylpyruvate dioxygenase, isopentenyl pyrophosphate isomerase, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase, MPBQ/MSBQ methyl transferase (VTE3), and many other proteins (Table S1) [43].Fenton et al. (2018) identified 81 QTL that control seed contents of six tocopherol-related traits in two RIL populations and several candidate genes within these QTL regions, including ZmVTE1 that encodes a tocopherol cyclase chromosome 5; ZmVTE2 that encodes a homogentisate phytyltransferase on chromosome 9; ZmVTE4 that encodes a γ-T methyltransferase on chromosome 5; ZmHPPD1 that encodes a hydroxyphenylpyruvate deoxygenase on chromosome 5; and ZmHGGT1 that encodes a homogentisic acid geranyl transferase on chromosome 9 (Table S1) [37].Wang et al. (2018) identified 41 QTL and 151 SNPs associated with seed tocopherol-related traits and many candidate genes within these QTL regions, including GRMZM2G436226 that encodes a Zinc finger CCCH domain-containing protein on chromosome 1, GRMZM2G055752 that encodes a DNA topoisomerase and GRMZM2G079236 that encodes a Acyl-CoA synthetase long-chain family member on chromosome 2, GRMZM2G170013 that encodes a chlorophyll b reductase NYC1 on chromosome 3, GRMZM5G892742 that encodes a DNAJ-related chaperone protein and GRMZM2G150714 that encodes a putative leucine-rich repeat receptor-like protein kinase family protein on chromosome 4, GRMZM2G035213 that encodes a γ-tocopherol methyltransferase (VTE4) and GRMZM2G024739 that encodes a Cryptochrome-1 (CRY1) on chromosome 5, GRMZM2G127299 that encodes a transcription factor GTE8 and GR-MZM2G146190 that encodes a peptidyl-prolyl cis-trans isomerase (CYP59) on chromosome 6, GRMZM2G003022 that encodes a COPII vesicle coat on chromosome 8, GRMZM5G818232 that encodes a Clavata3/esr-related16 on chromosome 9, GRMZM2G004534 that encodes a pyruvate kinase 2 on chromosome 10, and many other genes (Table S1) [44].Zhang et al. (2019) created transgenic seeds of maize and Arabidopsis thaliana and overexpressed the gene ZmTMT (VTE4) that encodes γ-tocopherol methyltransferase in those seeds and observed a 4-5-fold increase in α-tocopherol content in Arabidopsis and 6.5-fold increase in maize transgenic plants while the (α/γ)-tocopherol ratio increased by 15-fold and 17-fold, respectively, which proves that increasing tocopherol content in corn is feasible [48].Zhan et al. (2019) grew three RIL populations (n = 170-188 lines) from the crosses of Zong3 by Yu8701 (ZXY), K22XCI7, and B73 by BY804 (BXB) in two environments in China and genotyped them with 50 k SNPs.They used CIM of WinQTLCart2.5 for QTL analysis and identified one major QTL that controls seed γ-tocopherol and total tocopherol (qVE5) on chromosome 5 [48].The major QTL qVE5 was also identified in a previous study [44] and confirmed here in these three RIL populations [49].This qVE5 genomic region between NF129 and NF11 markers was narrowed to 170 kb within which the candidate gene GRMZM2G073351 that encodes a chloroplastic protochlorophyllide oxidoreductase (ZmPORB2) has been identified, and its overexpression increases tocopherol contents in both seeds and leaves, which was determined to occur mainly by maternal effect [49].Most genes involved in domestication-related traits in maize are summarized by Liu et al. (2020) [50].

Expression Analysis of the Candidate Gene Involved in the Tocopherol and Tocotrienol Biosynthetic Pathway
The tocopherol and tocotrienol biosynthetic pathway, including the genes and compounds involved, was previously reported in the model plant A. thaliana [36,54].The reconstruction of this pathway in Z. mays was conducted using the reverse BLAST of the previously reported genes in A. thaliana [54].
In total, 14 genes underlying the tocopherol and tocotrienol pathway in Z. mays were identified (Figure 2, Table 1).HGGT is present only in monocots [55], and not in Arabidopsis.The gene ID of this gene in Z. mays was obtained by searching the available data at Phytozome database (https://phytozome-next.jgi.doe.gov/,accessed on 7 April 2024).The name of the gene was used as a query in a search of the Z. mays reference genome (Z.mays RefGen_V4), and the obtained genes are represented in Figure 2 and Table 1.The publicly available RNA-seq database from MaizeGDB [56] was used to perform expression analysis of the genes involved in the corn tocopherol and tocotrienol biosynthetic pathway.The expression pattern of these genes in corn seeds and embryos was produced in version 4 of Gene Model ID.The obtained data was converted into a heatmap using the Heatmapper website [57].Most of the analyzed genes were expressed in the analyzed tissues except for the ZmHPPD1 gene, Zm00001d019365, that was not expressed in any of the analyzed tissues (Figure 2).In contrast, the ZmVTE3 gene, Zm00001d031071, and the ZmGGDR gene, Zm00001d018034, were highly expressed in the embryo.Five genes had moderate expression profiles, including the ZmHPPD1 gene, Zm00001d015356; the ZmGGDR gene, Zm00001d040356; the ZmVTE5 gene, Zm00001d001896; the ZmVTE1 gene, Zm00001d015985; and the ZmVTE2 gene, Zm00001d046909.The remaining genes presented lower expression patterns (Figure 2).
Three genes presented a moderate expression pattern in the seeds, including the ZmVTE2 gene, Zm00001d046909; the ZmVTE3 gene, Zm00001d031071; and the ZmVTE5 gene, Zm00001d001896.The remaining genes presented lower expression profiles (Figure 2).
The QTL and candidate genes identified by these studies may be used in MAS to develop maize cultivars with high seed tocopherol contents.The publicly available RNA-seq database from MaizeGDB [56] was used to perform expression analysis of the genes involved in the corn tocopherol and tocotrienol biosynthetic pathway.The expression pattern of these genes in corn seeds and embryos was produced in version 4 of Gene Model ID.The obtained data was converted into a heatmap using the Heatmapper website [57].Most of the analyzed genes were expressed in the analyzed tissues except for the ZmHPPD1 gene, Zm00001d019365, that was not expressed in any of the analyzed tissues (Figure 2).In contrast, the ZmVTE3 gene, Zm00001d031071, and the ZmGGDR gene, Zm00001d018034, were highly expressed in the embryo.Five genes had moderate expression profiles, including the ZmHPPD1 gene, Zm00001d015356; the ZmGGDR gene, Zm00001d040356; the ZmVTE5 gene, Zm00001d001896; the ZmVTE1 gene, Zm00001d015985; and the ZmVTE2 gene, Zm00001d046909.The remaining genes presented lower expression patterns (Figure 2).Three genes presented a moderate expression pattern in the seeds, including the ZmVTE2 gene, Zm00001d046909; the ZmVTE3 gene, Zm00001d031071; and the ZmVTE5 gene, Zm00001d001896.The remaining genes presented lower expression profiles (Figure 2).
The QTL and candidate genes identified by these studies may be used in MAS to develop maize cultivars with high seed tocopherol contents.

Conclusions
In conclusion, the exploration of Quantitative trait loci (QTL) and candidate genes associated with seed tocopherol contents in maize (Z.mays L.) has significantly advanced our understanding of the genetic bases underpinning the biosynthesis and accumulation of tocopherols.The identification and mapping of numerous QTL across diverse maize populations and environments underscore the complex polygenic nature of tocopherol content, which is influenced by both genetic and environmental factors.Studies utilizing mapping populations, Recombinant Inbred Lines (RILs), and genome-wide association studies (GWAS) have identified critical genomic regions and candidate genes that contribute to variations in α-Toc, β-Toc, γ-Toc, δ-Toc, and total tocopherol contents.Notably, genes such as ZmVTE4, which encodes γ-tocopherol methyltransferase, have been highlighted for their central role in converting γ-tocopherol to α-tocopherol, which is the most bioactive form of vitamin E.
The discovery of these QTL and candidate genes not only enriches our genetic knowledge of tocopherol biosynthesis in maize but also opens avenues for marker-assisted selection (MAS) and genetic improvement strategies aimed at enhancing tocopherol content in maize seeds.Such genetic improvements hold promise for developing maize cultivars with increased nutritional quality, offering potential health benefits to consumers and adding value to maize as a crop.Moreover, the identification of specific genes and their expression patterns provides a molecular foundation for further research into the regulatory mechanisms controlling tocopherol biosynthesis, offering insights that could be leveraged across different plant species.
Future research efforts should focus on fine-mapping identified QTL, validating the function of candidate genes, and elucidating the complex regulatory networks that govern tocopherol accumulation.Additionally, integrating genomic, transcriptomic, and metabolomic data could yield comprehensive insights into the tocopherol biosynthetic pathway, facilitating the development of biofortified maize varieties that are tailored to meet nutritional requirements and adapt to changing environmental conditions.Ultimately, the integration of genetic, molecular, and breeding approaches will be crucial in harnessing the full potential of maize tocopherols for improving crop nutritional quality and addressing global health challenges.

Figure 2 .
Figure 2. Seed and embryo expression heatmap of the tocopherol and tocotrienol biosynthesis candidate genes in Z. mays.

Figure 2 .
Figure 2. Seed and embryo expression heatmap of the tocopherol and tocotrienol biosynthesis candidate genes in Z. mays.

Table 1 .
Genes involved in the corn tocopherol and tocotrienol biosynthetic pathway.