The potential to breed a low-protein maize for protein-restricted diets

Authors: WOORE, MATTHEW - North Carolina State University; Flint-Garcia, Sherry; Holland, Jim - Jim

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2021
Publication Date: 11/23/2021
Citation: Woore, M.S., Flint Garcia, S.A., Holland, J.B. 2021. The potential to breed a low-protein maize for protein-restricted diets. Crop Science. 61(6):4202-4217. https://doi.org/10.1002/csc2.20600.
DOI: https://doi.org/10.1002/csc2.20600

Interpretive Summary: Genetic disorders involving amino acid metabolism such as Phenylketonuria affect hundreds of thousands of people worldwide. Patients with these disorders must adhere to a restricted diet low in specific amino acids beginning at birth in order to avoid toxic levels which lead to irreversible neurological damage. The modified diet for a patient with Phenylketonuria, for example, can be extremely costly as it requires phenylalanine-free protein substitutes. Unfortunately, the protein level of corn is generally too high for corn to be major part of a low phenylalanine diet. While corn is an extremely diverse crop species and it is likely that low phenylalanine varieties exist, the cost of testing phenylalanine content is cost prohibitive for the large number of samples needed in a breeding program. In order to make screening corn varieties for phenylalanine more cost effective, we analyzed the protein content and phenylalanine content in a collection of corn varieties using an array of methods of varying cost. We found that the expensive and time consuming, but extremely accurate phenylalanine content method based on wet lab chemistry was very strongly correlated with protein content determined by a non-destructive, fast, and cheap method based on near infrared spectroscopy. This relationship held true across a wide array of corn varieties grown at multiple locations and in multiple years. This means that we can rapidly and cheaply screen thousands of samples to identify low protein varieties which are likely also low phenylalanine. Identification of corn varieties that are naturally low in phenylalanine will expand the dietary options for phenylketonuric patients in a very cost effective manner and improve their quality of life.

Technical Abstract: Inborn errors of amino acid metabolism, such as phenylketonuria (PKU), are human genetic conditions that result in an inability to metabolize certain amino acids, requiring lifelong maintenance of a very restrictive low protein diet. In the case of PKU, this diet does not permit consumption of most maize (Zea mays L.) products, yet maize is a major part of the human diet and especially significant in some cultures. Kernel protein content, and to a lesser extent amino acid composition, is variable in maize. Breeding maize for low kernel protein content or low amounts of specific amino acids such as phenylalanine might expand the restrictive diets and improve quality of life for people with amino acid metabolic disorders. To test the potential of breeding low-protein content maize, diverse samples of hybrid or open-pollinated cultivars and inbreds were evaluated across two locations in two years for grain protein content and amino acid composition. Consistent with previous reports, kernel protein content varied widely across maize varieties are also highly heritable in the broad sense. We found that phenylalanine content (as a proportion of grain weight) and phenylalanine composition (as a proportion of total protein) are also highly heritable in the broad sense. As previously reported, reasonably accurate estimation of protein content is possible with near-infrared spectrophotometry, whereas measuring phenylalanine content requires a destructive, costly and time-consuming laboratory analysis. Phenylalanine content was strongly correlated with total kernel protein, breeding primarily aimed at reducing grain protein content should permit efficient selection of maize cultivars sufficiently low in phenylalanine to permit broader inclusion in the PKU diet.