January 22, 2008

 

High-lysine corn may soon make supplementation redundant

 

 

With seed companies pouring research onto high-lysine corn, lysine supplementation for animal feed makers may soon be a thing of the past, Monsanto's researchers said in a report to Information Systems for Biotechnology.

 

Shihshieh Huang, Alessandra Frizzi and Thomas Malvar, researchers with Monsanto, highlighted recent research which was able to help corn, traditionally a poor protein source as a food and feed staple crop to overcome its limitations.

 

The nutritional quality of corn protein in high yielding hybrids is poor due to its deficiency in essential amino acids, such as lysine. The focus of corn breeding on yield, which has resulted in a shift in grain composition from protein to starch, has compounded this nutritional deficiency, the study said.

 

Advancements in agricultural biotechnology has started improving the nutritional quality of corn protein, starting with lysine enhancement.

 

Currently, the limiting factor in corn-based feed is lysine, with the insufficiency in corn meal supplemented by crystalline lysine produced via fermentation

 

To broaden the use of corn meal, several transgenic approaches have been investigated - at least two examples of genetically engineered high lysine corn have been successfully demonstrated in field-grown inbred crops, the researchers said.

 

Currently, natural corn opaque mutants have a better dietary protein profile due to low levels of the nutritionally poor corn protein known as ¦Á-zein.

 

¦Á-zeins comprise roughly 40 percent of the total kernel protein, but contain almost no lysine. By reducing ¦Á-zein production in genetically engineered corn, other lysine-containing kernel proteins were comparatively increased, raising the lysine content in corn protein from 2.8 percent to 5.4 percent (from 2400 ppm to 4800 ppm).

However, like the original opaque mutants, the soft and chalky kernel phenotype displayed by these transgenic lines makes it unmarketable.

 

Despite the setback, research is now focussed on the plant lysine metabolic pathway to boost lysine content.

 

 The free lysine level in plant cells is thought to be regulated by two enzymes, the dihydrodipicolinate synthase (DHDPS) and lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH).

 

DHDPS feedback and LKR/SDH feed-forward mechanism will inhibit lysine production. 

 

By using a lysine feedback-insensitive DHDPS and suppressing LKR/SDH, higher levels of free lysine can be achieved in transgenic corn.

 

Gaining the former is done through the deployment of another enzyme, CordapA.

 

To further enhance the accumulation of free lysine in corn, Monsanto recently developed transgenic corn lines that combine CordapA expression and LKR/SDH suppression, by using what is called a transgene cassette, which combines both functions together. 

 

Analysis of these seeds confirmed that the detection of CordapA was accompanied by the reduction of LKR/SDH in transgenic kernels.

 

Further detailed molecular analyses of these transgenic plants are presented in the original research article.

 

Scientists are designing such cassettes so that one alteration would be able to affect multiple pathways, thus reducing the number of transgene expression required.

 

Based on the progress made in engineering high lysine corn, it is possible that one day the lysine supplementation of corn meal will no longer be necessary, the researchers said.

 

Following that, engineered corn, with increases in other limited essential amino acids such as tryptophan, methionine, and threonine will likely follow.

 

The quantity and digestibility of corn protein in corn grain may also be improved through plant biotechnology.