Breeding for Grain Quality Traits L

Agronomy Publications Agronomy 2005 Breeding for grain quality traits L. M. Pollak United States Department of Agriculture M. P. Scott Iowa State University, [email protected] Follow this and additional works at: http://lib.dr.iastate.edu/agron_pubs Part of the Agricultural Science Commons, Agronomy and Crop Sciences Commons, and the Plant Breeding and Genetics Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ agron_pubs/170. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Agronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Maydica 50 (2005): 247-257 BREEDING FOR GRAIN QUALITY TRAITS L.M. Pollak*, M.P. Scott USDA-ARS, Corn Insects and Crop Genetics Research Unit, Ames, Iowa 50011, USA Received February 9, 2005 ABSTRACT - Plant breeders have been extremely success- of vertically integrated grain utilization systems can ful at improving the yield of maize. Grain quality has re- capture the added value in an improved quality ceived less attention; however important advances have product. been made by breeders in this area as well. Maize with a The feasibility of breeding for grain quality in wide range of compositions and fractions within the major maize is best illustrated by the Illinois Long-Term grain components has resulted from breeders taking ad- Selection experiment for protein and oil. This ex- vantage of advances in biochemistry and genetics over the last fifty years. Breeding for grain quality provides end periment has been running for over a century (HOP- users with grain better suited to their needs. Maize with KINS, 1899, reviewed in DUDLEY and LAMBERT, 2004), improved amino acid balance allows animal feed to be and has resulted in germplasm representing the ex- produced at a lower cost. Maize with altered fatty acid tremes of protein and oil content known in maize. composition allows production of healthier vegetable oil. In addition, this material has provided valuable in- Maize with altered starch properties allows improvement sights into the physiology of metabolism in maize of many products that rely on starch based gels, films and (BELOW et al., 2004). adhesives. Although mutants that impact these traits have The focus of this review is on breeding for quali- been widely used, quantitative genetic approaches have al- ty traits that are determined by the constituents of so been successful when applied in long-term breeding three major components of grain (protein, oil, and programs. Two successful approaches involve elements of starch) including amino acid content, fatty acid con- both approaches, including the development of QPM based on the o2 mutant with selection for improved kernel tent, and starch quality. types and the development of high amylose maize based on the ae mutation with selection for increased amylose. AMINO ACID CONTENT KEY WORDS: Amino acid balance; Fatty acid content; Amylose; Amylopectin; Mutants; Starch quality. Why amino acid content is important and breeding objectives The majority of maize is used for animal feed. Ru- INTRODUCTION minant livestock depend on microbial populations in the rumen to make dietary protein available for me- Breeding for grain quality has become economi- tabolism. This affords ruminants flexibility in the cally beneficial because the development of niche amino acid composition of their diets. In contrast, markets for specialty grains and increasing numbers monogastric animals including humans require certain amino acids, termed essential amino acids, in their diets. Maize is deficient in lysine, methionine Abbreviations: CIMMYT, International Wheat and Maize Im- and tryptophan relative to the dietary needs of these provement Center; DSC, Differential Scanning Calorimetery; organisms. Deficiencies in these amino acids result in GEM, Germplasm Enhancement of Maize project; NIR, Near-in- poor utilization of maize protein, and nitrogen that is frared reflectance spectroscopy; NITS, Near-infrared transmit- not used is excreted as waste. In animal feed, these tance spectroscopy; QPM, Quality Protein Maize deficiencies are corrected by dietary supplementation * For correspondence (fax: +1 515 294 9359, e.mail: lmpol- with other protein sources or synthetic amino acids, [email protected]). but this adds to the cost of the diet. Thus, an impor- 248 L.M. POLLAK, M.P. SCOTT tant goal for improving maize for monogastric ani- amino acids into essential amino acids and would mals is to increase the level of the limiting amino probably involve increasing the ratio of non-zein to acids, while for ruminant animals, increasing the zein proteins. This is an attractive prospect because quantity of protein available is more important. it would not require increased nitrogen fixation or The major seed storage proteins of maize grain require increased inputs or decreased yield. This ap- belong to the prolamin family and are called the proach should be feasible because the level of es- zeins. These proteins make up 40-60% of the total sential amino acids has been demonstrated to be a endosperm protein and are so abundant that their heritable trait (DOTY et al., 1946). A strategy to im- properties have a large impact on the properties of prove protein quality by reducing the level of zeins maize grain protein as a whole. Zeins lack the es- while increasing the concentration of tryptophan sential amino acids lysine and tryptophan, and this and lysine was proposed by FREY et al. (1949). The deficiency is reflected in the amino acid balance of feasibility of this approach was demonstrated with the grain. Thus, plant breeding strategies for im- one cycle of selection for tryptophan content which proving the amino acid balance often involve some resulted in an increase in the tryptophan content of type of modification of the zein content. the population (FREY et al., 1949). Similarly, two cycles of selection for lysine content were conducted Quantitative genetic approaches to improving in three maize populations resulting in improve- amino acid content ments to lysine content accompanied by much Quantitative genetic approaches treat amino acid smaller changes in protein content (ZUBER and HELM, levels as a multigenic trait with continuous varia- 1972). A third cycle of selection was conducted and tion, and incremental improvements are made by all cycles were evaluated in the same environment. recurrent selection. This is analogous to the way Two of the three populations studied had increased that total protein was manipulated in the Illinois lysine levels and total protein content following each Long-term Selection experiment (DUDLEY and LAM- cycle of selection (ZUBER, 1975). Lysine content was BERT, 2004). One approach to improving levels of reported to have a relatively high heritability, and essential amino acids is to select for total protein high lysine levels could be maintained in subse- content. Not surprisingly, a positive correlation ex- quent generations of breeding (CHOE et al., 1976). ists between essential amino acids and total protein While recurrent selection to increase amino acid content (MILLER et al., 1950); however, the correla- levels has been shown to be successful, there are at tion is greater between tryptophan and non-zein least two impediments to recurrent selection for in- protein than between tryptophan and total protein creasing amino acid levels: the lack of inexpensive, (FREY et al., 1949). Consistent with these observa- high-throughput assays and the time investment re- tions, several studies have found that the levels of quired to generate substantial improvements with one or more essential amino acids are negatively multiple cycles of selection. correlated with total protein content when ex- pressed as a percentage of the total protein (FLYNN Use of mutants in breeding et al., 1954; TELLO et al., 1965). An additional prob- for amino acid content lem with this approach is illustrated in the Illinois The poor amino acid balance of maize protein is Long Term Selection populations in which a high a direct consequence of the amino acid composition protein population yielded less grain per land area of the major seed storage proteins, the zeins. Several than did a low protein population (BELOW et al., mutants exist in which zein content is altered and 2004). This exemplifies a well known correlation therefore the amino acid balance of the kernel is al- between protein content and grain yield. While ni- so altered. Kernels carrying the mutation, opaque2, trogen fertilization is an effective way to increase have elevated levels of the essential amino acids ly- grain yield of the high protein varieties (BELOW et sine and tryptophan due to a reduced content of al., 2004), increasing total protein remains a costly zeins (MERTZ et al., 1964). From the time of this dis- approach to increasing essential amino acid levels. covery to the present, opaque2 and other mutants Selection for the levels of specific amino acids that improve the amino acid balance such as floury2 and thereby an increase essential amino acid content have been used extensively in breeding programs. without increasing total protein content proportion- However, kernels carrying these mutations tend to ally should be possible. Physiologically, this could have a number of pleiotropic effects that reduce occur by reallocation of nitrogen from non-essential their agronomic adaptability. They tend to be soft GRAIN QUALITY TRAITS 249 (LOESCH et al., 1977), which makes them susceptible content of the grain. In addition, oils with certain to mechanical damage (LAMBERT et al., 1969). In addi- fatty acid compositions are well suited to certain end tion, mutant plants have poor germination (LOESCH et uses. Although corn oil is relatively stable to oxida- al., 1978) and reduced grain yield (LAMBERT et al., tive changes during storage because of its native fat- 1969; SREERAMULU and BAUMAN, 1970).

Load more