Molecular mechanisms for regulating seed amino acid levels in maize (<i>Zea mays</i>)
The majority of worldwide maize production is used for food or feed; as such it is imperative that its amino acid balance mirror the needs of humans and animals. However, lysine and methionine, two essential amino acids necessary for proper growth and development, are lacking in the maize endosperm. In this thesis, molecular mechanisms to increase both lysine and methionine deposition in the kernel are explored.
ZmbZIP1, a maize transcription factor, was identified through differential expression in a microarray experiment as a potential candidate to increase the lysine content of the endosperm. ZmbZIP1 was physically mapped to maize chromosome 9, contig 380, and a rice orthologue, RISBZ5 was found through synteny. ZmbZIP1 mRNA was present in all tested maize tissues to varying extents. An attempt to make a ZmbZIP1 knock-down mutant did not result in changes in kernel alpha-zein deposition, the main determinant of endosperm lysine content. These results suggest a functionally redundant protein to ZmbZIP1 exists, the knock-down allele retained sufficient activity to be functional, or phenotypes produced by disruption of ZmbZIP1 are not detectable by current methods.
A molecular mechanism for increasing methionine deposition into maize endosperm was investigated with regards to the 10kDa delta-zein ( Dzs10), a methionine-rich storage protein in endosperm, and cysteine synthase (Cys2), a key regulator in methionine biosynthesis. Dzs10 mRNA was differentially expressed in maize endosperm and a correlation of 0.4535 was found between Dzs10 mRNA and kernel methionine levels, suggesting that a change in Dzs10 expression is partially responsible for kernel methionine deposition. In Cys2 , the level of kernel methionine deposition corresponds to a G/A SNP present in the gene, suggesting different Cys2 alleles may also alter the kernel methionine levels.