Abstract
BACKGROUND: A balanced composition of amino acids in seed flour is critical because of the demand on essential amino acids for nutrition. However, seed proteins in cereals like maize, the crop with the highest yield, are low in lysine, tryptophan, and methionine. Although supplementation with legumes like soybean can compensate lysine deficiency, both crops are also relatively low in methionine. Therefore, understanding the mechanism of methionine accumulation in the seed could be a basis for breeding cultivars with superior nutritional quality. RESULTS: In maize (Zea mays), the 22- and 19-kDa α-zeins are the most prominent storage proteins, nearly devoid of lysine and methionine. Although silencing synthesis of these proteins through RNA interference (RNAi) raises lysine levels in the seed, it fails to do so for methionine. Computational analysis of annotated gene models suggests that about 57% of all proteins exhibit a lysine content of more than 4%, whereas the percentage of proteins with methionine above 4% is only around 8%. To compensate for this low representation, maize seeds produce specialized storage proteins, the 15-kDa β-, 18-kDa and 10-kDa δ-zeins, rich in methionine. However, they are expressed at variant levels in different inbred lines. A654, an inbred with null δ-zein alleles, methionine levels are significantly lower than when the two intact δ-zein alleles are introgressed. Further silencing of β-zein results in dramatic reduction in methionine levels, indicating that β- and δ-zeins are the main sink of methionine in maize seed. Overexpression of the 10-kDa δ-zein can increase the methionine level, but protein analysis by SDS-PAGE shows that the increased methionine levels occur at least in part at the expense of cysteines present in β- and γ-zeins. The reverse is true when β- and γ-zein expression is silenced through RNAi, then 10-kDa δ-zein accumulates to higher levels. CONCLUSIONS: Because methionine receives the sulfur moiety from cysteine, it appears that when seed protein synthesis of cysteine-rich proteins is blocked, the synthesis of methionine-rich seed proteins is induced, probably at the translational level. The same is true, when methionine-rich proteins are overexpressed, synthesis of cysteine-rich proteins is reduced, probably also at the translational level. Although we only hypothesize a translational control of protein synthesis at this time, there are well known paradigms of how amino acid concentration can play a role in differential gene expression. The latter we think is largely controlled by the flux of reduced sulfur during plant growth.
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Claims 10
Claim · model-written summary
Silencing major alpha-zeins raised lysine but did not raise methionine.
- Treatment
- silencing synthesis of these proteins through RNA interference (RNAi)e0
- Gene
- 22- and 19-kDa α-zeinse1
- Outcome
- lysine levels in the seede0
- Direction
- raisese0
Zea maysNot reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“silencing synthesis of these proteins through RNA interference (RNAi) raises lysine levels in the seed, it fails to do so for methionine”
e1“the 22- and 19-kDa α-zeins are the most prominent storage proteins”
t0“maize seed”
Claim · model-written summary
Silencing major alpha-zeins failed to raise seed methionine.
- Treatment
- silencing synthesis of these proteins through RNA interference (RNAi)e1
- Outcome
- methioninee0
- Direction
- fails to do soe0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“it fails to do so for methionine”
e1“silencing synthesis of these proteins through RNA interference (RNAi) raises lysine levels in the seed”
t0“maize seed”
Claim · model-written summary
About 57% of annotated proteins exceeded 4% lysine content.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“about 57% of all proteins exhibit a lysine content of more than 4%”
t0“maize seed”
Claim · model-written summary
Around 8% of proteins exceeded 4% methionine content.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the percentage of proteins with methionine above 4% is only around 8%”
t0“maize seed”
Claim · model-written summary
Methionine-rich storage proteins were expressed at variant levels among inbred lines.
- Material
- different inbred linese0
- Gene
- 15-kDa β-, 18-kDa and 10-kDa δ-zeinse1
- Outcome
- expressed at variant levelse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“they are expressed at variant levels in different inbred lines”
e1“15-kDa β-, 18-kDa and 10-kDa δ-zeins, rich in methionine”
t0“maize seed”
Claim · model-written summary
Introgression of two intact delta-zein alleles raised methionine relative to null allele A654.
- Material
- A654e0
- Treatment
- two intact δ-zein alleles are introgressede0
- Gene
- δ-zein allelese0
- Outcome
- methionine levelse0
- Direction
- significantly lowere0
- Comparator
- null δ-zein allelese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“A654, an inbred with null δ-zein alleles, methionine levels are significantly lower than when the two intact δ-zein alleles are introgressed.”
t0“maize seed”
Claim · model-written summary
Further beta-zein silencing sharply reduced methionine.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Further silencing of β-zein results in dramatic reduction in methionine levels”
t0“maize seed”
Claim · model-written summary
Overexpressing 10-kDa delta-zein increased methionine.
- Treatment
- Overexpression of the 10-kDa δ-zeine0
- Gene
- 10-kDa δ-zeine0
- Outcome
- methionine levele0
- Direction
- increasee0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Overexpression of the 10-kDa δ-zein can increase the methionine level”
t0“maize seed”
Claim · model-written summary
Increased methionine partly came at the expense of cysteines in beta- and gamma-zeins.
- Gene
- β- and γ-zeinse0
- Outcome
- increased methionine levelse0
- Direction
- increasede0
- Qualifier
- at least in parte0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the increased methionine levels occur at least in part at the expense of cysteines present in β- and γ-zeins”
t0“maize seed”
Claim · model-written summary
Silencing beta- and gamma-zeins increased 10-kDa delta-zein accumulation.
- Treatment
- β- and γ-zein expression is silenced through RNAie0
- Gene
- 10-kDa δ-zeine0
- Outcome
- 10-kDa δ-zeine0
- Direction
- higher levelse0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“when β- and γ-zein expression is silenced through RNAi, then 10-kDa δ-zein accumulates to higher levels”
t0“maize seed”
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See all 30 papers →Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.