(courtesy of Adeyemo)
Recently, genetic engineering (rDNA) technique was used for the development of rice which contains the precursor to vitamin A from daffodil plant called Golden rice (www.bioscience-explained.org). Research for biofortified maize is focusing on finding genetic markers to facilitate selection for enhancing provitamin A levels. The use of DNA marker tools could be harnessed for production of numerous markers at important target genes involved in ß – carotene biosynthetic pathway that may be used in MAS. Techniques like molecular marker may be used for enhancing nutritional value of many important crops like maize.
The genetic mechanism involved in the accumulation of carotenoids in maize have been identified. Through association analysis, linkage mapping, expression analysis, and mutagenesis, lycopene epsilon cyclase (LCYE) locus has been shown to alter flux down ?-carotene versus ß–carotene branches of the carotenoid pathway and that it plays a key role in controlling this ratio (Harjes et al., 2008).
Moreover, with availability of the published rice genomic DNA sequence (http://portaltmri-org/rice/). Gallagher et al. (2004) were able to identify the two different PSY genes, designated PSY 1 and PSY 2 on chromosomes 6 and 12, respectively, and through comparison to available cDNAs, annotated their exon/intron structures. They also found out that maize also contained duplicate PSY gene. The carotene desaturase (zds) gene was identified as candidate genes for levels of carotenoids (Li et al., 2007), and when the maize ZDS gene was isolated, it was mapped to VP 9 on chromosome 7. This finding enhances the likelihood of the validity of psy and zds as QTLs in maize.
Identifying these SNPs markers associated with some of the genes which play major roles in the ß – carotene biosynthetic pathway will be a valuable tool for marker – assisted selection and for assessing how variation is distributed across the genome. Haplotyping an individual will help identify alleles at a number of linked SNP loci associated with each chromosome of a homologous pair. The identification of genes and alleles that modify the ß–carotene is the most direct approach using genomics for plant breeding. Allele-specific markers when discovered will discriminate between two alleles. Once the interesting alleles have been identified for the candidate genes, then complete sequencing of the genes for representative alleles will be done. This will identify candidate polymorphisms that may cause phenotypic variation. These identified alleles and markers could be used in actual plant breeding.
Simple sequence repeats (SSR) are also other PCR-based molecular markers that have been widely used in detection of QTL to evaluate genes of interest. They are relatively simple, can be automated and SSR markers are highly informative (Matsuoka et al., 2002). Additionally this project will be using, SSR markers for genotyping and QTL/association analysis for those specific genes.
In addition, the yellow maize inbred lines may be analyzed based on amplified fragment length polymorphism (AFLP) markers. AFLP markers are suitable for analysis or detection of genetic diversity in maize germplasm because it is highly polymorphic (Li et al., 2004 and Oliveria et al., 2004). Also, it is useful for association between traits and germplasm for guiding breeding program. This study will also attempt to investigate the genetic diversity in yellow maize inbred lines by AFLP markers.
For evaluation of the entire genome for identification of the genome for identification of genes and alleles, that control genetic traits, linkage disequilibrium mapping or association mapping may be applied (Breseghello and Sorrells, 2006). For the purpose of this project, association analysis is being proposed. This will go further to characterize the yellow maize RILs and it will provide the estimate of linkage disequilibrium (LD) in maize genes associated with provitamin A biosynthetic pathway which is a necessary strategy to implement MAS for breeding programs.
There are few cases where MAS has been used. MAS has been successfully developed and implemented for resistance against bean golden mosaic virus and also in rice, lines homozygous for resistance genes against bacterial blight pathogen were selected using RFLP and RAPD markers linking to these genes (Martinez, 2006). The efficiency of MAS in improving of traits such as b carotene in maize plant breeding programs is also desired. These results will go a long way to produce a maize crop containing sufficient b carotene for people in the sub-Sahara.
The identification of genes and alleles that modify the ß–carotene is the most direct approach using genomics for plant breeding. Yellow endosperm maize will be analyzed for SNPs, SSRs and AFLP polymorphisms within the b carotene candidate genes by designing allele specific primers, they could be used in an efficient MAS selection process to facilitate increasing levels in maize grain.
The objectives of the study:
Relevant links
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