Impact of Selection for Grain Methionine Content.

Methionine is a nutritionally limiting amino acid in some corn (Zea Mays L.) based diets.  In order to develop germplasm with altered methionine content, we initiated a recurrent selection program for grain methionine content.  After three cycles of divergent selection in two starting populations we determined that methionine content changed about 0.4 % (g methionine/g grain) per cycle.  Selection for methionine content had relatively little impact on grain composition, including the content of other amino acids, protein oil and starch.  Changes to plant morphology were observed when high and low methionine populations were compared, but the magnitude of these changes was dependant on the starting population. Because the diverged populations vary widely in methionine content, they provide a unique opportunity to understand the genetic and biochemical mechanisms that control grain methionine content.  After 5 cycles of selection, we analyzed storage protein levels as well as allele frequencies and transcript levels of candidate genes in the high and low methionine populations.  The Cys2 gene which is involved in sulfate assimilation did not show significant differences between the high and low methionine populations in transcript levels, however the two populations had very different allele frequencies at this locus.  In contrast, the Dzs10 gene encoding a methionine-rich seed storage protein, did not exhibit different allele frequencies between the two populations, but did show significantly different transcript levels.  Taken together, these data provide insight into the genetic and biochemical mechanisms that control grain methionine content.