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a Dep. of Agronomy and Plant Genetics, and Microbial and Plant Genomics Institute, 1991 Upper Buford Cir., Univ. of Minnesota, St. Paul, MN 55108
b Monsanto Corn Research, 2440 Hwy. 19 Blvd., Stanton, MN 55018
c Pioneer Hi-Bred International, Inc., 7300 NW 62nd Ave., P.O. 1004, Johnston, IA 50131
* Corresponding author (phill005{at}umn.edu).
ABSTRACT
Maize (Zea mays L.) is a primary energy-supplying grain for animal feed in the United States. However, it is deficient in the essential amino acid methionine. Of the maize used for animal feed, 20% is fed to poultry, where the methionine requirement is particularly important. Significant increases in maize methionine levels in the lines reported here can be effectively used in poultry nutrition as well as in human diets where corn and bean (Phaseolus L.) play a major role. Eleven inbred lines of high-methionine maize (58611 Inbred A632 [Reg. No. GP-557, PI 648423], 58609 A632 (Meth) BC5S4 [Reg. No. GP-558, PI 648424], 58610 A632 (Meth) BC5S4 [Reg. No. GP-559, PI 648425], 58612 Inbred B73 [Reg. No. GP-560, PI 648426], 58613 B73 (Meth) BC5S4 [Reg. No. GP-561, PI 648427], 58614 B73 (Meth) BC5S4 [Reg. No. GP-562, PI 648428], 58615 B73 (Meth) BC5S4 [Reg. No. GP-563, PI 648429], 58801 Inbred Mo17 [Reg. No. GP-564, PI 648430], 58802 Mo17 (Meth) BCS3 [Reg. No. GP-565, PI 648431], 58803 Mo17 (Meth) BCS3 [Reg. No. GP-566, PI 648432], and 58804 Mo17 (Meth) BCS3 [Reg. No. GP-567, PI 648433]) were developed by the University of Minnesota and released on 26 Oct. 2007 by the Minnesota Agricultural Experimental Station. These materials were produced by crossing BSSS53 with inbred lines A632, B73, and Mo17 and backcrossing to the respective inbred with intermittent selfing. Maize inbred lines A632, B73, and Mo17 are released with methionine elevated as much as 12.5, 25.0, and 50.0%, respectively, above the recurrent parent. The germplasm source of the high methionine trait is a random line isolate from the Iowa Stiff Stalk Synthetic, identified as BSSS53 and later released by the Iowa State Experiment Station as B101 after obtaining the high methionine information from the University of Minnesota.
Abbreviations: BC, backcross • HPLC, high-performance liquid chromatography • NIRS, near infrared reflectance spectroscopy • QTL, quantitative trait locus • S, self
Eleven inbred maize (Zea mays L.) lines that have been developed by the Department of Agronomy and Plant Genetics, University of Minnesota (St. Paul) and released by the Minnesota Agricultural Experiment Station have elevated levels of the essential amino acid methionine ((58611 Inbred A632 [Reg. No. GP-557, PI 648423], 58609 A632 (Meth) BC5S4 [Reg. No. GP-558, PI 648424], 58610 A632 (Meth) BC5S4 [Reg. No. GP-559, PI 648425], 58612 Inbred B73 [Reg. No. GP-560, PI 648426], 58613 B73 (Meth) BC5S4 [Reg. No. GP-561, PI 648427], 58614 B73 (Meth) BC5S4 [Reg. No. GP-562, PI 648428], 58615 B73 (Meth) BC5S4 [Reg. No. GP-563, PI 648429], 58801 Inbred Mo17 [Reg. No. GP-564, PI 648430], 58802 Mo17 (Meth) BCS3 [Reg. No. GP-565, PI 648431], 58803 Mo17 (Meth) BCS3 [Reg. No. GP-566, PI 648432], and 58804 Mo17 (Meth) BCS3 [Reg. No. GP-567, PI 648433]; see Table 1 for experimental numbers).
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In countries where a corn–bean (Phaseolus L.) diet is prevalent, an overabundance of beans in the diet can lead to a methionine deficiency in humans. High-methionine maize may allow a balanced diet even with an excessive amount of methionine-deficient beans.
Methods
BSSS53, a random line isolate from Iowa Stiff Stalk Synthetic (Hallauer and Wright, 1995), was identified by a laboratory whole-kernel screening assay searching for lines resistant to feedback inhibition by lysine-plus-threonine–supplemented tissue culture medium (Green and Phillips, 1974; Phillips et al., 1981). A lysine-plus-threonine resistant line, BSSS53, was discovered. However, inhibition was detected only when embryos instead of entire kernels were germinated on a lysine-plus-threonine–supplemented medium, indicating that methionine was being supplied to the embryo by the endosperm, thus preventing the feedback inhibition in whole kernels. BSSS53 was found to exhibit a 30 to 70% methionine increase over other Corn Belt inbred lines (Olsen and Phillips, 2001). Analysis of endosperm storage proteins indicated that a zein-2 protein with 21% methionine residues was elevated in BSSS53 (Phillips and McClure, 1985). Genetic analysis of the control of this zein-2 protein indicated a gene on the long arm of chromosome 9 (Benner et al., 1989) and that another gene affecting the accumulation of the protein was located on chromosome 4 (Benner, 1988; Benner et al., 1989; Chaudhuri and Messing, 1995). The chromosome 9 structural gene was cloned by isolating purified 10 kDa zein from BSSS53 and an oligonucleotide probe formed to obtain cDNA and genomic clones (Kirihara et al., 1988a,b). Attempts to increase methionine levels by backcrossing (five or six backcrosses [BC] with subsequent selfing [S]) the chromosome 9 BSSS53 zein-2 gene into four inbred lines (A619, A632, B73, and Mo17) did not result in significantly elevated methionine levels in any of the backcross-derived inbreds or hybrids (Krone, 1994). Transgenic plants possessing this gene also did not exhibit elevated methionine levels (Kleese et al., 1991). A quantitative trait locus (QTL) analysis using an F2:3 population from crossing Mo17 and BSSS53 located QTLs on chromosomes 1, 5, 6, and 10; in addition, a region on chromosome 7 was retained among high-methionine BC3S2:3 lines (Olsen, 1999). The finding of several QTLs on various chromosomes not including chromosome 9 prompted us to simply select for high methionine in a backcross program with A632, B73, and Mo17, measuring the trait by near infrared reflectance spectroscopy (NIRS); the correlation of genotype means of NIRS-predicted methionine and genotype means of high-performance liquid chromatography (HPLC)–measured methionine was 0.91 (Olsen, 1999). High-methionine BC2S2:3 versions of A632, B73, and Mo17 were developed by Krone (1994). These materials were subsequently used to develop high-methionine BC3S2:3 and BC4S1:2 lines (Olsen et al., 2003). High-methionine lines referenced in this release were developed from these BC3S2:3 and BC4S1:2 lines at the Minnesota Agricultural Experiment Station in St. Paul.
Subsequently, from 2001 to 2005, these lines were further backcrossed and self-pollinated to evaluate methionine levels. In each year, the lines were planted in single-row plots with two replicates at two locations. The rows were 9.1 m (30 ft) in length; the 32 seeds per row were spaced 23 cm (9 in) apart. Within each plot, 20 plants were self-pollinated. Equal volumes of seed were bulked to form a sample for NIRS analysis, performed in the Forage Laboratory, Department of Agronomy and Plant Genetics, at the University of Minnesota. The materials were evaluated in 2003 and 2005 (see Table 1).
The methionine level in these lines was analyzed using HPLC and NIRS at the University of Minnesota. The NIRS determination used Foss North America (Model 6500) instrumentation. To efficiently screen for methionine levels, a NIRS equation was developed for predicting methionine levels of ground kernels. On an individual sample basis, the correlation between NIRS-predicted methionine levels and HPLC-measured methionine was 0.79. The correlation between genotype means of NIRS-predicted methionine and genotype means of HPLC-measured methionine was 0.91. Samples of A632, B73, and Mo17 backcrossed derived lines as well as the recurrent parent lines and the donor line (BSSS53) grown each year from 1995 to 1998 and from 2000 to 2003 were used for calibration development. Representative samples from all the genetic backgrounds and from each growing season were included in the calibration development. A total of 230 samples were used for calibration development. Reference amino acid levels were measured by the HPLC for these 230 samples.
Two self-pollinated generations interceded each backcross to have sufficient uniform material to analyze. Replicate plantings were placed in adjacent fields, and two samples were analyzed from each replicate. Two years of data are presented in Table 1. Most of the converted lines are BC4S3 and BC5S4. Statistical analyses of the NIRS data used the Tukey and Dunnett mean comparison tests using the general linear model procedure of SAS (Cary, NC).
A linear relationship was observed between hybrid and the midparent methionine levels (Olsen et al., 2003). The coefficient of variation was r2 = 0.42.
Characteristics
Methionine levels are significantly elevated in all lines at the 0.05 significance level, compared with the corresponding inbreds. The percentage increase in methionine was relatively consistent in 2003 and 2005 for B73 and Mo17 high-methionine lines, whereas A632 showed more variation. The range of the percentage increase in methionine levels was 11.5 to 12.5, 21.4 to 25.0, and 12.5 to 50.0 for A632, B73, and Mo17 released lines, respectively (Table 1). Olsen et al. (2003) indicated that both increased protein levels and increased methionine as a percentage of total protein contribute to the high-methionine phenotype of these lines.
Higher methionine levels for A632 lines were obtained in 2003 compared with 2005, perhaps the result of sampling. All 2005 lines were derived from the 2003 lines as given in the Table 1.
All lines appeared to have the same phenotypic characteristics as their recurrent parent. No agronomic performance data have been collected for these materials.
Availability
Seed is available in lots of 25 kernels from the corresponding author. We request that the source of the lines be acknowledged and this registration cited when these genetic materials contribute to a research publication or cultivar release.
Acknowledgments
These high-methionine maize lines were developed through the support of the Minnesota Corn Grower Research and Promotion Council and the Minnesota Agricultural Experiment Station.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication November 30, 2007.
References
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