Genetic and biochemical differences in populations bred for extremes in maize grain methionine concentration

Mark A Newell,M Paul Scott,Muhammad Ali,Marie Adams,Adrienne N Moran Lauter,Karla E Vogel,Nevzat Aydin,Anastasia L Bodnar

doi:10.1186/1471-2229-14-49

Abstract

BackgroundMethionine is an important nutrient in animal feed and several approaches have been developed to increase methionine concentration in maize (Zea mays L.) grain. One approach is through traditional breeding using recurrent selection. Using divergent selection, genetically related populations with extreme differences in grain methionine content were produced. In order to better understand the molecular mechanisms controlling grain methionine content, we examined seed proteins, transcript levels of candidate genes, and genotypes of these populations.ResultsTwo populations were selected for high or low methionine concentration for eight generations and 40 and 56% differences between the high and low populations in grain methionine concentration were observed. Mean values between the high and low methionine populations differed by greater than 1.5 standard deviations in some cycles of selection. Other amino acids and total protein concentration exhibited much smaller changes. In an effort to understand the molecular mechanisms that contribute to these differences, we compared transcript levels of candidate genes encoding high methionine seed storage proteins involved in sulfur assimilation or methionine biosynthesis. In combination, we also explored the genetic mechanisms at the SNP level through implementation of an association analysis. Significant differences in methionine-rich seed storage protein genes were observed in comparisons of high and low methionine populations, while transcripts of seed storage proteins lacking high levels of methionine were unchanged. Seed storage protein levels were consistent with transcript levels. Two genes involved in sulfur assimilation, Cys2 and CgS1 showed substantial differences in allele frequencies when two selected populations were compared to the starting populations. Major genes identified across cycles of selection by a high-stringency association analysis included dzs18, wx, dzs10, and zp27.ConclusionsWe hypothesize that transcriptional changes alter sink strength by altering the levels of methionine-rich seed storage proteins. To meet the altered need for sulfur, a cysteine-rich seed storage protein is altered while sulfur assimilation and methionine biosynthesis throughput is changed by selection for certain alleles of Cys2 and CgS1.

Highlights

Methionine is an important nutrient in animal feed and several approaches have been developed to increase methionine concentration in maize (Zea mays L.) grain
Divergent mass selection was implemented for high and low grain methionine concentration in two independent starting populations. This led to four selection tracks, with populations in each track designated according to the following convention: BS11 or BS31, depending on the starting population; High methionine (HM) or Low methionine (LM), depending on whether selection was for high or low methionine concentration; C# indicating the cycle of selection
It should be noted that the data from each cycle were produced in different environments, so comparisons should only be made between the HM and LM populations of a given cycle

Summary

Introduction

Methionine is an important nutrient in animal feed and several approaches have been developed to increase methionine concentration in maize (Zea mays L.) grain. Genetic approaches to increase the level of methionine in maize (Zea mays L.) grain would reduce the level of supplementation required, decreasing the cost of animal diets. These approaches would be facilitated by an understanding of molecular mechanisms controlling methionine deposition in the grain. Methionine and cysteine are the major sulfur-containing amino acids; levels of grain methionine may be related to sulfur uptake and/or assimilation. Sulfate from the environment is activated to 5′-adenlylysulfate by the enzyme adenosine - 5′-triphosphate sufurylase This activated sulfur is reduced to sulfide that is fixed in the form of cysteine by o-acetyl-L-serine (thiol)lyase, known as cysteine synthase, which is encoded by the Cys gene in maize. A study focusing on genes in amino acid biosynthesis [4] concluded that genes with reduced diversity tend to be clustered in a few pathways and most amino acid biosynthesis genes have sufficient diversity for continued improvement through breeding

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