Oliver Nelson and Quality Protein Maize

Corresponding author: James F. Crow, University of Wisconsin, 445 Henry Mall, Madison, WI 53706-1574.

MAIZE is the leading livestock food in North America. In much of the world, particularly Latin America and parts of Africa and Asia, it is the basic cereal in human diets. Yet, its deficiency as the sole source of protein has been recognized for years. Native Americans seemingly knew this, for they often incorporated beans in their diets. The word "succotash" is of Native American origin. Early in the century, OSBORNE and MENDEL 1914 found that the most limiting factor is lysine. They showed that rats fed on zein, the major protein of maize endosperm, died of protein starvation. Knowing that zein was devoid of lysine and tryptophan, they added these amino acids to the diet, and the rats remained healthy and grew normally. This was the first proof that specific amino acids are essential for a healthy diet.

Edwin T. Mertz (1909–1999) at Purdue University thought that it might be possible to improve the amino acid quality of maize by suppressing the amount of zein. This would permit the nitrogen normally used in zein synthesis to be diverted to other proteins containing lysine and tryptophan. In the fall of 1962, Oliver E. Nelson attended a seminar by Mertz in which this idea was presented. Nelson suggested that the opaque and floury mutants would be good candidates for correcting the zein deficiency. His reasoning was this: Earlier he had noticed that lines selected for high protein content had hard, glassy endosperm with increased amounts of poor quality zein proteins, while those with low protein content were soft and floury. The opaque and floury mutants seemed to mimic this latter phenotype and might therefore be expected to produce more of the proteins with the needed amino acids.

The idea worked. Both opaque2 and floury2 had substantially higher lysine and tryptophan content than wild-type strains (MERTZ et al. 1964 ; NELSON et al. 1965 ). This immediately led to nutrition tests. Weanling rats fed on a diet of 90% opaque2 (o2) maize gained weight more than three times faster than those fed on standard hybrid maize. Furthermore, the o2 maize could substitute for added soybean oil meal, a standard source of lysine and aromatic amino acids (MERTZ et al. 1965 ). Since it usually has more lysine than floury2, opaque2 has received most of the attention and has become the standard. Trials using pigs and humans were run soon after and were equally convincing. Studies on Guatemalan children showed that o2 maize was almost as effective as milk protein. Moreover, children with the protein deficiency disease, kwashiorkor, responded to diets with o2 protein (NELSON 2001 ).

One might expect that high-lysine maize containing the o2 gene would quickly become standard throughout the world's corn-producing areas. But this was not to be. The early introgression of o2 into inbred lines of high-yielding hybrids led to a substantial reduction of yield, 10% or more. Furthermore, the kernels dried slowly and consequently were more subject to fungal infections. And the soft endosperm of the mutant strains broke easily, making machine harvesting difficult and requiring different milling. Finally, the lysine content was unusually susceptible to environmental fluctuations. To formulate an efficient ration, farmers need to have maize with a predictable lysine content. For these reasons, research with o2 foundered for many years.

Some research continued, however. One approach was to abandon o2 and simply select for strains with the desired endosperm composition. This was never successful. There was also a search for modifiers that could offset the undesirable aspects of o2 while retaining the high lysine. This was more successful. Yet the only major hybrid maize producer to utilize o2 was Crow's Hybrid Corn Company in Illinois (no relation to J. F. Crow). When other companies gave up, Crow's persisted. For several years this company has produced high-performance o2 strains, comparable to the best hybrids. The endosperm is still soft, but has been modified to eliminate most breakage and disease problems. It is even possible that the softness improves digestibility (NELSON 2001 ).

The most extensive program for developing high-lysine corn was carried out by researchers at the International Maize and Wheat Improvement Center in Mexico (Centro Internacional de Mejoramiento de Maiz y Trigo, CIMMYT). They introduced modifiers that change the endosperm texture to a more desirable phenotype, and of course there was continued selection for yield and other aspects of performance. The hard endosperm o2 stocks they developed were designated quality protein maize (QPM) to distinguish them from soft o2 strains. Not all of the hard endosperm o2 lines retained high levels of the critical amino acids. For this reason, and because the most desirable selections resembled standard dent phenotypes, it was necessary to monitor amino acid levels chemically. Again, the Mertz and Nelson laboratories stepped in to assist in developing rapid, inexpensive colorimetric assays specific for lysine and tryptophan.

As with many major practical findings, the development of QPM has not been without controversy. Just as the CIMMYT program was becoming successful, it suffered a setback. An influential nutritionist on its board believed that the amino acid deficiency of standard maize was not important and that the main problem was insufficient calories and total protein. For this reason the QPM breeding program was terminated in the early 1990s. But various studies throughout the world repeatedly demonstrated the value of high-lysine corn, and the breeding program was reinstated in 1996. At present, largely because of CIMMYT's efforts, QPMs are being used extensively and increasingly in Latin America, Asia, and Africa.

The first QPM varieties were synthetics; later, hybrids were developed. For an account of this extensive work, see VASAL 2000 . In recognition of this achievement, S. K. Vasal, who headed the CIMMYT breeding program, and Evangelina Villegas, a Mertz student who led the biochemical work, were awarded the World Food Prize (Borlaug Award) in 2000.

Molecular characterization revealed manifold effects of the opaque2 mutation. The locus codes for a transcription factor that regulates synthesis of the largest family of zein proteins (SCHMIDT et al. 1987 ). Early analysis demonstrated a compensating increase in the desirable albumin and globulin protein fractions. Curiously, a minor fraction of zeins is also increased (GENTINETTA et al. 1975 ). The balance of free amino acids is shifted in favor of the aspartate family, including lysine (WANG and LARKINS 2001 ). Other recent reports from the Larkins laboratory provide a detailed account of modified free amino acid and protein composition, the accumulation of storage proteins in o2 endosperm, and the tracking down through genome analysis of the genes responsible for the alterations. After the 1960s, Nelson himself had little direct involvement in the biochemical characterization of o2 and fl2. The studies themselves were done elsewhere, although Nelson's influence was ever present. The cloning was done in the lab of F. A. and B. Burr, former postdocs, using transposon tagging, per Nina Federoff and Nelson. The fractionation of zeins was performed in the lab of F. Salamini, also a former postdoc. And B. A. Larkins was partial heir to the Nelson legacy at Purdue, before moving to Arizona. Although Oliver Nelson was an arm's length away, it was a short arm's length.

It has taken more than 35 years of selective breeding to improve the agronomic properties of high-lysine corn so that it can compete with standard hybrids. Hybrid corn has been genetics' biggest success story (CROW 1998 ), but the nutritional deficit still exists. There is every reason to believe that QPM strains will be increasingly grown, both in the corn belt and, especially, in the tropics, where the nutritional deficiency is more crucial. Their major impact is yet to come, and they could go a long way toward alleviating a major food problem.

Maize is not the only crop in which high-lysine mutants have been useful. They have been found and have been incorporated in both barley and sorghum, which, unlike wheat and rice, have large quantities of lysine-deficient storage proteins (LARKINS and MERTZ 1994 ).

Oliver Nelson was born on August 16, 1920, in Seattle. Throughout most of his life he was robust and athletic. He was an avid golfer and gardener and swam every day, in good weather in a pond at his rural home, otherwise in the University natatorium. But in his last few years his physical–but not his mental–activity was brought to near zero. He was afflicted with a number of physical impairments and hardships, the worst being a deterioration of the spinal column that caused a great deal of pain. Despite this and great difficulty walking and talking, he persisted in his research until close to the end. He died on November 6, 2001. He is survived by his wife, Gerda.

High-lysine corn, although his best known work, is only one of many of Oliver Nelson's accomplishments. In the early 1950s he devised a way to use the waxy mutant in pollen grains to screen an enormous number of gametes for rare recombinants. This was the first example of an extensive fine-structure analysis in a higher plant (NELSON 1987 ). One of us (J.F.C.) vividly remembers his excitement in hearing Nelson report this result at a Genetics Society meeting, little realizing that he and Oliver Nelson would later be colleagues. It is no coincidence that this work was done at the same time that Nelson's close friend Seymour Benzer was doing similar studies with bacteriophage T4. Nelson showed that the waxy locus encodes for a starch-bound ADP-glucose glucosyl transferase, one of the earliest associations of a plant phenotype with an underlying enzymatic defect. This was the first of several gene-enzyme associations made in the Nelson laboratory using endosperm starch mutations.

In the late 1960s, he began to focus on developing systems using transposable elements to study gene action. Collaborating with Nina Federoff, he cloned the bronze gene, the first successful use of transposon tagging in a higher plant. He showed that phenotypically similar reversions from an insertion in a coding region resulted from a heterogeneous set of internal deletions and nearly complete excisions. Partial deletion restored the wild-type phenotype by creating an acceptor splice site, leading to removal of the element from the transcript.

Oliver Nelson started his genetic career at the Connecticut Agricultural Experiment Station, working as a graduate student with D. F. Jones (NELSON 1993 ). After receiving his Ph.D. from Yale in 1947, he joined the faculty of Purdue University. His move to Wisconsin came in 1969. There, until his retirement, he carried out an active research program and mentored a number of students and postdocs, several of whom are now leaders in plant genetics. He also served as Chairman of the Genetics Department at Wisconsin. He was elected to the National Academy of Sciences in 1972 and soon after received the Stephen Hales Prize for noteworthy contributions to plant physiology. In 1997 he was awarded the Thomas Hunt Morgan Medal for a lifetime contribution to genetics (GANETZKY 1998 ). In honor of his 70th birthday, a group of his students dedicated a commemorative issue of Maydica to him (PETERSON 1990 ).

View larger version (112K): In this window In a new window Download PPT slide   Oliver E. Nelson in 1970, shown with a photograph of rats used in nutrition experiments. The rats on the right, fed opaque2 maize, are noticeably larger than those fed only standard maize.

	ACKNOWLEDGMENTS

A fuller biography is being prepared for the Biographical Memoirs of the National Academy of Sciences.

	LITERATURE CITED

TOP LITERATURE CITED

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GENTINETTA, E., T. MAGGIORE, F. SALAMINI, C. LORENZONI, and F. PIOLI et al., 1975  Protein studies in 46 opaque2 strains with modified endosperm texture. Maydica 20:145-163.

LARKINS, B. A., and E. T. MERTZ (Editors), 1994 Quality Protein Maize: 1964–1994 (Proc. Int. Symp. Quality Protein Maize). Sete Lagoas, MG, Brazil.

MERTZ, E. T., L. S. BATES, and O. E. NELSON, 1964  Mutant gene that changes the protein composition and increases the lysine content of maize endosperm. Science 145:279-280[Abstract/Free Full Text].

MERTZ, E. T., O. VERON, L. S. BATES, and O. E. NELSON, 1965  Growth of rats fed opaque-2 maize. Science 148:1741-1742[Abstract/Free Full Text].

NELSON, O. E., 1987  The waxy locus in maize twenty-five years later. Genetics 116:339-342[Free Full Text].

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NELSON, O. E., E. T. MERTZ, and L. S. BATES, 1965  Second mutant gene affecting the amino acid pattern of maize endosperm proteins. Science 150:1469-1470[Abstract/Free Full Text].

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