Artículo Científico
Rev. Fitotec. Mex. Vol. 45 (3): 293-302, 2022
MARKER DEVELOPMENT FOR QUALITY PROTEIN MAIZE BREEDING AND AN
INTERACTION STUDY BETWEEN Opaque-2 AND Ask2 GENES
DESARROLLO DE MARCADORES PARA MEJORAMIENTO DE MAÍZ CON PROTEÍNA DE
CALIDAD Y ESTUDIO DE INTERACCIÓN ENTRE LOS GENES Opaco-2 Y Ask2
Martha Hernández-Rodríguez1*, Raman Babu2, Debra J. Skinner2, Natalia Palacios-Rojas2,
Marina C. M. Martins3, J. Jesús García-Zavala1, Ricardo Lobato-Ortíz1, Amalio SantacruzVarela1, Fernando Castillo-González1, Yunbi Xu2 and Boddupalli M. Prasanna2
Colegio de Postgraduados, Campus Montecillo, PREGEP-Genética, Montecillo, Texcoco, Estado de México, México. 2International Maize and Wheat
Improvement Center, Mexico City, México. 3Max-Planck-Institut für Molekulare Pflanzenphysiologie, Potsdam-Golm, Germany.
1
*Corresponding author (hernandez.martha@colpos.mx)
SUMMARY
Quality Protein Maize (QPM) kernels contain twice the amounts of lysine
and tryptophan compared to normal corn kernels. Although the opaque-2 (o2)
mutation is the underlying cause of this beneficial change, other genes such
as Aspartate kinase-2 (Ask2) affect the amino acid content in the endosperm
to a lesser degree. To date, reports on the interaction between both loci are
scarce and there are no high-throughput assays for the identification of the
alleles of these genes. The objectives of this research were: 1) to study the
interaction between the o2 and Ask2 genes with respect to the accumulation
of amino acids in the endosperm in an F2 population, 2) to identify conserved
SNPs into the o2 gene that can be used as markers, 3) to estimate the
frequency of an SNP of Ask2 associated with the accumulation of lysine in the
endosperm in CIMMYT germplasm, and 4) to develop high-throughput marker
assays for these SNPs. The interaction study showed a preponderant effect
of o2 on the accumulation of 11 amino acids (P ≤ 0.01). Ask2 appeared only
to act with o2 to enhance marginally lysine, histidine and methionine levels in
the double recessive homozygotes. Sequencing of amplicons at the o2 locus
led to the identification of an SNP in exon 1 that discriminated all QPM (C)
genotypes from non-QPM (T) genotypes. Validation of this SNP through KASP™
assays indicated that it was 92 % assertive in differentiating the o2 genotypes.
In contrast, the frequency of the Ask2 SNP in CIMMYT QPM germplasm was
low; however, an SSCP marker developed using this SNP detected five variants
indicating that other unknown base changes may confer positive lysineincreasing responses. These markers could aid the marker-assisted selection
of QPM cultivars.
Index words: Zea mays, Ask2, marker-assisted selection, Opaque-2,
SNP.
RESUMEN
Los granos de maíz de alta calidad proteica (QPM) contienen el doble de
lisina y triptófano en comparación con los granos de maíz normales. Aunque
la mutación opaco-2 (o2) es la causa subyacente de este cambio beneficioso,
otros genes como la aspartato quinasa-2 (Ask2) afectan en menor grado el
contenido de aminoácidos en el endospermo. Hasta el momento, los informes
sobre la interacción entre ambos loci son escasos y no existen ensayos de alto
rendimiento para la identificación de los alelos de estos genes. Los objetivos
de esta investigación fueron: 1) estudiar la interacción entre los genes o2 y
Ask2 con respecto a la acumulación de aminoácidos en el endospermo de
una población F2, 2) identificar SNPs conservados en el gen o2 que puedan
ser utilizados como marcadores, 3) estimar la frecuencia de un SNP de Ask2,
Recibido: 01 de marzo de 2021
Aceptado: 18 de mayo de 2022
asociado con la acumulación de lisina en el endospermo, en germoplasma
del CIMMYT, y 4) desarrollar ensayos de marcadores de alto rendimiento
para estos SNPs. El estudio de interacción mostró un efecto preponderante
del o2 sobre la acumulación de 11 aminoácidos (P ≤ 0.01). Al parecer, Ask2
solamente actúa con o2 para mejorar marginalmente los niveles de lisina,
histidina y metionina en los homocigotos recesivos dobles. La secuenciación
de amplicones en el locus o2 condujo a la identificación de un SNP en el exón
1 que discriminó todos los genotipos QPM (C) de los genotipos no QPM (T).
La validación de este SNP a través de los ensayos KASP™ indicó que fue 92 %
asertivo en la diferenciación de los genotipos o2. En contraste, la frecuencia
del SNP de Ask2 en el germoplasma QPM del CIMMYT fue baja; sin embargo,
un marcador SSCP desarrollado utilizando este SNP detectó cinco variantes,
lo que indica que otros cambios de base desconocidos pueden conferir
respuestas positivas de aumento de lisina. Estos marcadores podrían ayudar
a la selección asistida en variedades QPM.
Palabras clave: Zea mays, Ask2, Opaco-2, selección asistida por
marcadores, SNP.
INTRODUCTION
There are nine amino acids that human beings and
mono-gastric animals cannot synthesize and need to be
supplied through the diet (Galili et al., 2016). While most
proteins of animal origin have suitable proportions of
these amino acids, plant proteins are often suboptimal for
human needs; for instance, most cereal proteins are poor
in lysine and tryptophan, while those of legume origin are
lower in methionine and other sulfur-containing amino
acids (Mariotti and Gardner, 2019). In the developing
world, prolonged consumption and over-reliance on cereal
proteins can potentially lead to diseases such as pellagra,
a niacin deficiency disease, and kwashiorkor, a form of
protein-energy malnutrition that increases susceptibility to
tuberculosis and gastroenteritis (Nuss and Tanumihardjo,
2011).
A mature corn kernel contains approximately 10 % protein
and several mutants alter its amino acid profile (Zhang et al.,
DOI: https://doi.org/10.35196/rfm.2022.3.293
MOLECULAR MARKERS FOR QPM MAIZE
Rev. Fitotec. Mex. Vol. 45 (3) 2022
2018). One of them is opaque-2 (o2), a natural spontaneous
mutation with soft, opaque grains, discovered in the 1920s
(Singleton, 1939). Biochemical effects of o2 indicate that
the homozygous recessive o2 allele increases lysine (+69
%) and tryptophan in the endosperm compared to normal
maize (Mertz et al., 1964). This effect doubles the biological
value of maize protein, and only with half the amount of
o2 maize (relative to normal maize), the same biologically
usable protein can be obtained (Bressani, 1991). In spite of
its nutritional superiority, o2 maize did not become popular
among farmers because the o2 gene carries some negative
pleiotropic effects, including grain chalkiness, thicker
pericarp, larger germ size, reduced cob weight, lower grain
yield and reduced kernel weight and density (Vasal, 2000).
Hence, CIMMYT made intensive efforts to improve the
phenotype of o2 kernels by developing hard-endosperm
maize types named QPM. Today’s QPM is interchangeable
with normal maize in both phenotype and yield potential
while retaining its nutritional value (Vasal, 2001).
2005; Gupta et al., 2013; Pukalenthy et al., 2020); however,
they are not enough to achieve the full effectiveness of
molecular breeding for QPM genotypes. Marker umc1066
is easily visualized on agarose gels but it is commonly not
polymorphic, at least in CIMMYT breeding populations;
phi057 usually requires the use of polyacrylamide gels,
while phi112, which is based on a deletion in the promoter
region, is a dominant marker, and hence, cannot be used
in the identification of heterozygotes in F2/BC generations
(Babu and Prassana, 2014). These shortcomings, coupled
with low throughput and time consuming of gel-based
SSR analysis, result in a major hindrance to routine and
large-scale adoption of marker technology in the breeding
pipelines (Xu and Crouch, 2008). This requires the
identification of SNP markers, which have propelled plant
molecular breeding because of their ease of automation,
high throughput, low cost and high read accuracy (Rasheed
et al., 2017). Though the functional SNP conferring
favorable response in Ask2 has been reported (Wang et al.,
2007), high throughput assays are not yet available and the
interaction between o2 and Ask2 loci, involving breeding
germplasm, has not been documented. The objectives
of this study were: 1) to evaluate the interaction between
o2 and Ask2 in an F2 population derived from tropical
maize germplasm and validate synergistic effect, if any,
for enhanced amino acid accumulation in the endosperm,
2) to identify widely conserved SNPs within the o2 gene
that can effectively discriminate the alleles in a range of
elite genetic backgrounds, 3) to estimate the frequency
of favorable Ask2 allele in CIMMYT germplasm and, 4)
to develop medium to high throughput marker assays to
facilitate the implementation of a comprehensive MAS
scheme for rapid development of QPM germplasm.
The lysine value in QPM lines varies from 2.5 to 4.0 g
per 100 g of protein (Vivek et al., 2008). Though o2 is the
major determinant, a few minor modifier loci may likely
work in unison with o2 to moderate the effect and alter the
essential amino acid content in the homozygous (o2o2)
recessive background (Vasal, 2001). In a QTL mapping
study by Wang and Larkins (2001), free amino acid content
in o2 maize lines was analyzed and shown to vary up
to 12-fold in Oh545o2 and 3-10 times in Oh51Ao2 and
W64Ao2 from non-o2 wild type counterpart. One of four
loci identified was coincident with the Aspartate kinase
2 (Ask2) gene (Wang et al., 2001). This locus, located on
chromosome 2, has a polymorphism that causes a single
amino acid substitution in the C-terminal region of the ASK2
enzyme of Oh545o2 (Wang et al., 2007). Another study
using a RIL population from the cross between B73o2 and
CML161, identified QTLs for traits related to amino acid
content on chromosomes 7 and 8, explaining up to 39 %
of the observed phenotypic variation (Gutiérrez-Rojas et
al., 2010). These studies clearly established the existence
of additional modifier loci that could favorably increase
accumulation of amino acid levels in QPM germplasm. Of
the several loci reported, the role of Ask2 is prominent and
the responsible polymorphism has been identified and
characterized (Wang et al., 2007).
MATERIALS AND METHODS
Interaction analysis
Nine genotypic classes were determined after
genotyping 400 seeds of an F2 population resulting from
a cross between CML172 (QPM) and CML323 (non-QPM).
Genotyping was carried out with molecular markers for
o2 and Ask2 using DNA from endosperm. DNA extraction
followed a protocol based on CTAB-lauroylsarcosine
described by Gao et al. (2008). To genotype o2, SSR marker
phi057 was amplified by PCR reactions of 15 µL containing
1X Taq buffer, 2.5 mM MgCl2, 150 μM each dNTP, 0.3 μM of
each primer (forward 5´-CTCATCAGTGCCGTCGTCCAT-3´
and reverse 5´-CAGTCGCAAGAAACCGTTGCC-3´), 1 U
homemade Taq polymerase and 20 ng of DNA. The PCR
program consisted of an initial denaturation of 1 min at 94
°C followed by 30 cycles of 30 s at 94 °C, 1 min at 58 °C, 1
min at 72 °C and a final extension of 5 min at 72 °C using a
MJ Tetrad thermalcycler (MJ Research, St. Bruno, Quebec,
The cloning and characterization of the O2 locus, located
on chromosome 7 (Hartings et al., 1989; Schmidt et al.,
1990), followed by detection of three SSR markers viz.
phi057, phi112 and umc1066 (Chin et al., 1996) led to
effective differentiation of the O2 and o2 alleles in elite
QPM and non-QPM germplasm (Babu et al., 2005). These
o2-specific SSR markers are the foundation for markerassisted selection (MAS) in QPM breeding (Babu et al.,
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HERNÁNDEZ-RODRÍGUEZ et al.
Rev. Fitotec. Mex. Vol. 45 (3) 2022
Canada). Three microliters of each PCR product was
migrated in 6 % acrylamide:bis-acrylamide (29:1) gels at
250 V for 2 h in 1X TBE (90 mM Tris-borate, 2 mM EDTA pH
8.0) in a MGV-116-33 system (CBS Scientific®, San Diego,
California, USA). The reference of molecular weight was
ϕX174/HaeIII. After electrophoresis, the gels were silverstained according to CIMMYT (2005).
thermostatic column holder (560-CIL, ERC, Riemerling,
Germany) at a flow rate of 0.8 mL min-1. Eluent A was 8 mM
Na2HPO4 (pH 6.8) containing 0.4 % (v/v) tetrahydrofuran.
Eluent B was a mixture of 250 mL 40 mM Na2HPO4 (pH 6.8),
150 mL methanol and 105 mL acetonitrile. Two different
gradients determined major and minor amino acids. For
major amino acid columns were equilibrated for 10 min
with 10 % A and separation conditions were as follows:
0-5 min, isocratic elution with 0 % B; 5-15 min, linear
gradient from 0 % to 15 % B; 15-15.5 min, linear gradient
from 15 to 100 % B; 15.5-21 min 100 % B. For minor amino
acids, columns equilibrated for 10 min with 100 % A and
separation conditions were as follows: 0-5 min, isocratic
elution with 0 % B; 5-16 min, linear gradient from 0 to 15 %
B; 16-26 min, linear gradient from 15 to 50 % B; 26-42 min,
linear gradient from 50 to 60 % B; 42-46 min from 60 to 100
% B; 46-52 min 100 % B. Amino acids were identified and
quantified by comparison with original standards (SigmaAldrich) using Chromeleon software (version 6.8, Dionex).
Individual analyses of variance per amino acid and principal
component analysis were applied using PROC GLM and
PRINCOMP of SAS, respectively (SAS Institute, 2002).
To genotype Ask2, primers flanking the functional SNP
of Ask2 (Wang et al., 2007) were designed to amplify a
product of 151 pb. PCR mix included 1X Taq buffer, 2.0
mM MgCl2, 166 µM each dNTP, 0.1 µM of each primer
(forward 5´-CTTGGTGGTCCATGACAGTG-3´ and reverse
5´-CTAATGGCTGTGGATTGTGC-3’), 1 U homemade Taq
polymerase and 50 ng of DNA in a reaction volume of
15 µL. The PCR program consisted of one cycle of 94
°C for 2 min, 30 cycles at 94 °C for 1 min, 58°C for 40 s,
72°C for 1 min, and a final extension at 72 °C for 5 min
performed in a MJ Tetrad thermalcycler (MJ Research, St.
Bruno, Quebec, Canada). PCR products were resolved as
single-stranded conformation polymorphisms (SSCP) in
16 % polyacrylamide gels applying a modified Laemmli´s
protocol that consisted of preparing each layer of the gel
(stacking and resolving) without SDS (Laemmli, 1970). The
PCR products were diluted with 5 µL of loading buffer (10
mM NaOH, 95 % formamide, 0.05 % bromophenol blue,
0.05 % xylene cyanol), denatured during 10 min at 94 °C
and quick quenching on ice. Five μL were run for 15-16 h at
150 V and 8 °C in a MGV-116-33 system (CBS Scientific®,
San Diego, California, USA) using 1X TBE as running
buffer. The gels were stained with 1X SYBR® Gold (Life
Technologies, Carlsbad City, California, USA) for 20 min
and documented by scanning in a Typhoon 8600 imager
(Amersham Biosciences Ltd., Chicago, Illinois, USA).
Polymorphism identification
Fifteen elite inbred lines were chosen to search for
polymorphisms within the o2 gene. Seven inbred lines
were QPM, of diverse agro-ecological zones, with excellent
kernel modifying capacity and of extensive use in the QPM
conversion programs. The other eight lines were nonQPM, with wide adaptability and superior performance
in Asia, Latin America and East African highlands (Table
1). Leaves of four seedlings per line were used separately
to extract genomic DNA following a protocol based
on CTAB for small-scale isolation of high quality DNA
(CIMMYT, 2005). Two sets of primers were designed to
amplify two genomic fragments of o2 (477 and 530 pb)
and to identify regions with high degree of conservation.
The reference sequence X15544, which belongs to the
zein regulatory gene o2 of Zea mays (http://www.ncbi.
nlm.nih.gov/), plus six cDNA sequences of o2 gene
(AJ41297, AJ491298, AJ491299, AJ491300, M29411
and X16618) were used as templates for the design of
primers
O2F1:
5´-CTAGTGTTTGCTTCTCCCTTCC-3´,
O2R1:
5´-TCCTCAGTATGGCATTGTACTCC-3´
and
O2F3:
5´-TGTTGTGACCTCAGATCAACG-3´,
O2R3:
5´-TTCCAGTTCTTTCAGGTGAGC-3. The PCR reactions
were optimized to produce single bands without smear by
applying a lower number of cycles and a large volume of
reaction (30 µL). In total, two PCR fragments per seedling
were amplified and purified with Qiagen spin columns
(Qiagen®, Hilden, Germany), and cloned into pGEM®-T
Easy Vector (Promega, USA) according to manufacturer
recommendations. Recombinant plasmids were selected
After genotyping, each marker was scored by hand
and seeds with the same genotype were bulked to form
a genotypic class. Only 20 seeds per class were used
to quantify 11 amino acids by high-performance liquid
chromatography (HPLC). Ethanolic extracts by HPLC
(consisting of a pump P680, Dionex, autosampler 465
and fluorimetric detector SFM 25, Kontron, Eching,
Germany) after derivation of the primary amino group with
o-phthaldialdehyde (OPA) were carried out as described by
Jarrett et al. (1986). Aliquots (35 µL) of ethanolic extracts
were combined with 35 µL of OPA reagent [25 mg OPA
dissolved in 500 µL methanol, the addition of 4.5 mL 0.8
M borate buffer (pH 10.4) and 50 µL of mercaptopropionic
acid], mixed and kept for 2.5 min at 4 °C to allow complete
derivation. The extract was injected into a pre-column
(Hypersil 10 µm, Phenomenex, Torrance City, California,
USA) before separation on a C18 column (Hypersil ODS,
150 × 4.6 mm, 3 µm particle size, Thermo Fisher Scientific,
Bonn, Germany) maintained at constant 18 °C using a
295
MOLECULAR MARKERS FOR QPM MAIZE
Rev. Fitotec. Mex. Vol. 45 (3) 2022
significantly higher amounts of lysine, tryptophan and
aspartic acid relative to the double dominant homozygous
class (G5). In this regard, G1 had 2.3-fold more lysine,
1.3-fold more tryptophan and 5.3-fold more aspartic acid
than G5. Lysine levels of G1 were also 60 % higher relative
to G2, the recessive dominant class. A fast analysis using
biplot analysis indicated that the first two components
explained 96.24 % of the variance (Figure 1). PC1 explained
89.27 % of variation while PC2 explained 6.93 %. All vectors
had a positive direction toward G1, G2 and G3 where o2
was recessive. No vector pointed toward genotypes with
the dominant allele of o2 (G4 to G9). On the contrary, Ask2
locus on its own was not found to have any significant
effect on any of the amino acids analyzed, but appeared
to interact synergistically with o2 and it significantly
enhanced lysine, histidine and methionine levels in the
double recessive homozygous class (G1).
Table 1. Maize inbred lines used to identify polymorphisms
in the o2 locus.
Grain color and adaptation zone
QPM line Non-QPM line
White, tropical lowland
CML159
CML491
CML502
CML492
CML161
CML165
CML176
Yellow, tropical lowland
White, sub-tropical
White, highland
White, Ethiopian highland
CML343
CML495
CL RCW22
CML348
CML244
CML349
F7287
A7018
after transforming DH5α competent cells of E. coli by
heat-shock according to CIMMYT (2005). Plasmid DNA
extraction was with Wizard® Plus SV Minipreps DNA
Purification System (Promega, USA) and verification of
inserts was by digestion with EcoR1. Two plasmids of
each genotype were sequenced at CINVESTAV facilities
(Guanajuato, México), aligned using Clustal X version 1.8
(Thompson et al., 1997) and refined by hand.
Polymorphism identification in the o2 locus
Sequencing the amplicons of o2 allowed to locate
an important discriminatory SNP in the region of exon 1
(Figure 2). The sequencing included part of the 5’ flanking
region, four exons (1, 2, 3 and 4) and two introns (2 and
3). In total, a length of 1 kb of the genomic sequence was
analyzed, 65 polymorphic sites were identified, and one
polymorphism each 30 bp was averaged. In general, all
QPM lines had the same sequence, indicating a possible
common donor of o2 in CIMMYT lines.
Markers assays validation
A panel of 88 inbred lines from the Maize Program or
CIMMYT Genbank was used to validate polymorphisms
using genomic DNA as template. DNA extraction protocol
was the same as that used for polymorphism identification.
Two SNPs were assayed at LGC Biosearch Technologies
(https://www.biosearchtech.com/) using the KASP™
chemistry. One of them was the o2-specific SNP, which
differentiated QPM and normal lines and the other was
the Ask2-specific SNP amplified to determine genotypic
classes. This last one was also screened as SSCP marker
to explore the natural variation of Ask2 and to estimate the
frequency of the favorable allele in CIMMYT germplasm.
Of the several polymorphisms that differentiated the QPM
lines from non-QPM, the following four merit attention: 1)
two SNP in intron 2 (positions 10,800,027 and 10,800,032
bp concerning to o2 reference sequence of Zea mays, ID
542375 of B73 RefSeq_v3) which defined three alleles with
the haplotypes GT/AG/GT (donor/recipient/recipient), 2)
two consecutive SNP in intron 2 (positions 10,800,038 and
10,800,039) which defined the alleles AT/TC/AC (donor/
recipient/recipient), 3) two InDels in intron 2 (positions
10,799,978 to 10,799,982 and 10,799,988 to 10,799,990),
which corresponded to 5-base and 3-base deletions in all
QPM donor lines as well as for two non-QPM lines, and
4) an SNP of transition type in exon 1 (position 10,799,047
according to sequence ID 542375 of B73 RefSeq_v3, and
position 1790 according to accession X15544 of the
GenBank). The SNP corresponds to C in QPM lines and to
T in non-QPM lines. This SNP was named 2-exon1-SNP, it
causes an amino acid substitution (valine to alanine) and
was selected for the subsequent high throughput assays.
Sequences with this SNP were submitted to the GenBank
of NCBI under accessions KY807178 to KY807200.
RESULTS
Interaction analysis between o2 and Ask2
The interaction between o2 and Ask2 was studied
in a tropical germplasm background by developing a
segregating population from a cross between CML172,
which is a QPM line possessing favorable alleles for both
o2 and Ask2 loci (based on ‘2’ type SSCP conformer),
and CML323, a non-QPM line. The interaction analysis
was based on the concentration of 11 amino acids of
nine genotypic classes (Table 2). Among these classes,
the double recessive homozygous class (G1) contained
296
Genotypic
class
o2
Ask2
CML172
1
CML323
Amino acids
297
Asp
Glu
Asn
Thr
His
Met
Trp
Phe
Ile
Leu
Lys
1
4969.74
2759.53
4631.05
286.31
61.10
27.98
53.14
210.12
159.79
268.78
719.06
2
2
268.04
329.00
656.81
64.00
41.36
39.11
45.95
36.17
41.80
55.04
81.75
G1
1
1
3383.40
1203.85
1469.11
297.81
230.78
100.67
105.13
155.70
199.92
224.46
431.92
G2
1
2
3095.29
1294.23
2312.75
349.47
160.47
47.47
112.76
156.63
192.91
249.35
270.02
G3
1
H
1979.50
1108.96
1189.26
306.18
120.90
60.67
66.57
112.90
169.72
250.83
364.71
G4
2
1
409.54
393.07
351.05
179.56
68.63
31.10
37.67
58.01
81.12
171.76
94.02
G5
2
2
541.97
557.88
589.25
222.23
98.03
36.49
46.35
67.43
94.24
177.15
132.78
G6
2
H
460.78
366.26
374.36
180.02
104.14
38.35
49.94
48.73
62.50
129.35
124.24
G7
H
1
298.44
247.67
309.08
149.93
73.33
32.13
37.31
46.06
46.36
103.12
102.82
G8
H
2
471.34
342.96
385.82
170.18
95.32
39.26
38.24
50.65
72.13
128.60
126.01
G9
H
H
315.77
300.76
300.61
159.87
88.22
31.29
39.22
43.81
57.83
114.87
96.99
G1,G2,G3
1
-
317.82 ª
170.72
69.61
94.82 ª
141.74 ª
187.52 ª
241.55 ª
355.55 ª
G4,G5,G6
2
-
470.76b
439.07 b
438.22 b
193.94 b
90.27
35.31
44.66 b
58.05 b
79.29 b
159.42 b
117.01 b
G7,G8,G9
H
-
361.85b
297.13 b
331.83 b
159.99 c
85.62
34.23
38.25 b
46.84 b
58.77 b
115.53 b
108.61 b
HERNÁNDEZ-RODRÍGUEZ et al.
Table 2. Amino acids concentration (nmol g-1 FW) of kernels of the genotypic classes of o2 and Ask2 in CML172 × CML323.
o2 classes
2819.40ª 1202.35 ª 1657.04 ª
G1,G4,G7
-
1
1363.79
614.86
709.74
209.10 ª
124.25
54.64
60.04
86.59
109.13
166.45
209.59
G2,G5,G8
-
2
1369.53
731.69
1095.94
247.30b
117.94
41.08
65.78
91.57
119.76
185.04
176.27
G3,G6,G9
-
H
918.69
591.99
621.41
215.36b
104.42
43.44
51.91
68.48
96.68
165.02
195.31
Genotype nomenclature: 1: double homozygous recessive, 2: double homozygous dominant, H: heterozygous. Asp: aspartic acid, Glu: glutamic acid, Asn: asparagine, Thr: threonine, His:
histidine, Met: methionine, Trp: tryptophan, Phe: phenylalanine, Ile: Isoleucine, Leu: leucine, Lys: lysine. Means with different letters within columns are statistically different (Tukey, P ≤ 0.05).
Rev. Fitotec. Mex. Vol. 45 (3) 2022
Ask-2 classes
MOLECULAR MARKERS FOR QPM MAIZE
Rev. Fitotec. Mex. Vol. 45 (3) 2022
1.00
Methionine
0.75
Histidine
G2
PC2 (6.93 %)
0.50
Lysine
0.25
G6
G8
G9 G7
0.00
G5
-0.25
Aspartic
Phenylalanine
Tryptophan
Isoleucine
G3
Glutamic
G4
Threonine
Leucine
Asparagine
-0.50
G1
-0.75
-1.00
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
PC1 (89.27 %)
Figure 1. Biplot of principal components of 11 free amino acids content in nine genotypic classes (G) from an F2 population
between CML172 (QPM) and CML323 (non-QPM). G1: o2/o2|ask2/ask2, G2: o2/o2|Ask2/Ask2, G3: o2/o2/Ask2/ask2, G4:
O2/O2/ask2/ask2, G5: O2/O2/Ask2/Ask2, G6: O2/O2/Ask2/ask2, G7: O2/o2/ask2/ask2, G8: O2/o2/Ask2/Ask2, G9: O2/
o2/Ask2/ask2.
CML161-KY807181
CML165-KY807180
CML159-KY807184
CML491-KY807185
CML502-KY807187
CML492-KY807188
CML176-KY807189
CML348-KY807191
CML343-KY807192
CML495-KY807195
CLRCW22-KY807197
CML244-KY807196
CML349-KY807179
F7287-KY807199
A7018-KY807200
O2-exon1-SNP
o2
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGCGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTGGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTAGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
GTTACTGGAAGAGGAGGCTCTGACGACAAGCACACCGCCGCCGGTGGTGGTGGTGCCGAA
****** ************************************* ***************
[A>V]
336
336
336
336
336
336
336
336
348
348
336
336
345
336
341
Figure 2. The o2-exon1-SNP and its context sequence. The SNP is highlighted in yellow. Allele C discriminates to QPM
lines (in italics). Allele T discriminates non-QPM lines (no italics). Name of each maize line plus its GenBank ID is to the
left of the nucleotide sequence.
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Rev. Fitotec. Mex. Vol. 45 (3) 2022
(CML328 and CML 370). The other cluster with unfavorable
allele T (in blue) comprised 81 lines of which 55 were QPM
and 26 non-QPM, indicating a low frequency of this allele
in CIMMYT germplasm.
Validation of high throughput marker assays
Two high throughput KASP™ assays non gel-based
were developed to facilitate large-scale MAS in pedigree/
back-cross breeding programs involving QPM × non-QPM
lines. One of these assays, designed to screen the o2exon1-SNP, was firstly tested in 16 lines. The analysis of
allele calls clearly grouped all genotypes into two clusters
–one corresponding to allele C of QPM and another
corresponding to allele T of non-QPM lines. Thus, this
analysis was extended to a larger set of 88 lines in which
60 lines were QPM and the remaining non-QPM. Allele calls
for o2-exon1-SNP differentiated two clusters again (Figure
3A), the cluster in red color corresponding to lines with the
allele C and the cluster in blue color to lines with the allele
T. The cluster with allele C included 53 QPM lines while the
cluster with allele T comprised 35 lines, of which 28 were
normal and seven were QPM. CML145, CML167, CML168,
CML177, CML180, CML184 and CML188 were the QPM
lines that possessed the ‘normal’ allele (T), contrary to
expectation. Therefore, prediction value of o2-exon1-SNP
was 92 % for the QPM trait.
Natural variation of Ask2
Given that the favorable allele A at the Ask2 locus was
only detected in five out of 60 QPM lines, to determine
other possible functional variants of Ask2, a gel-based
SSCP assay was developed and tested in 82 maize lines.
Five distinct SSCP band patterns were unambiguously
identified (Figure 4). SSCP variants were as follows: variant
1 in 60 lines (46 QPM and 14 normal), variant 2 in 11 lines
(10 QPM and 1 normal), variant 3 in 5 lines (all QPM),
variant 4 in 5 lines (all normal) and variant 5 in 1 line (QPM),
suggesting that other polymorphisms could be involved in
the high lysine level in the endosperm.
DISCUSSION
The interaction between o2 and Ask2 in tropical
germplasm background was studied in nine different
genotypic classes resulting from an F2 segregating
population. The single locus tests established the overriding
effect of o2 on most of the amino acids, irrespective of the
genotypic constitution at Ask2 locus (Figure 1). On the
contrary, Ask2 locus on its own was not found to have
any significant effect on any of the amino acids analyzed
but did interact synergistically with o2 and significantly
enhanced lysine, histidine and methionine levels in the
The remaining assay was designed to test the functional
polymorphism A (‘lysine enhancing’) of the C-terminal
region of the ASK2 protein of the Oh545o2 maize line (Wang
et al., 2007), which identified several lines possessing
the favorable allele (Figure 3B). In this case, allele calls
distinguished one cluster (in red) of seven inbred lines with
the favorable allele A, of which five were QPM (CML148,
CML161, CML162, CML179 and CML188) and two normal
A
o2
B
Ask2
A:A
Y- signal fluorescence HEX
C:C
T:T
T:T
X- signal fluorescence FAM
Figure 3. Genotyping cluster plot of KASP™ assays for o2 (A, left) and Ask2 (B, right) SNP. Each data point represents an
individual inbreed line. Similar fluorescence values mean the same genotype. Red dots: homozygous for allele labeled
with HEX, blue dots: homozygous for allele labeled with FAM, green dots: heterozygous, black dots: no template control.
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Rev. Fitotec. Mex. Vol. 45 (3) 2022
Figure 4. The five SSCP variants of Ask2 detected in QPM germplasm. Mobility differences correspond to base changes in
the amplified sequence (151 pb, from 3’ end) that cause single-stranded DNA to migrate differently. Bands were resolved
on 16 % polyacrylamide gel.
double recessive homozygous class (G1).
Brennecke et al. (1996) inferred that Ask1 is being directly
or indirectly regulated by O2 because AK in these double
mutants is less sensitive to lysine feedback inhibition than
that of Ask1 mutation alone. Though there is no direct
evidence for Ask2 being regulated by O2, such a possibility
may partially explain the insignificant effect of Ask2 locus
independent of o2.
The elevated levels of lysine in o2 endosperm are due to
enhanced synthesis of lysine-containing proteins (Zhang
et al., 2018) as well as high free lysine content (Wang and
Larkins, 2001). Lysine is synthesized through a branch of
aspartate pathway and is regulated by feedback inhibition
loops, in which the accumulating lysine suppress the
activity of two key enzymes, aspartate kinase (AK) and
dihydrodipicolinate synthase (DHDPS) (Galili, 1995; Galili
et al., 2016). The native mono-functional AK enzyme is a
tetramer composed of two α- and two β-subunits, encoded
by Ask1 and Ask2 genes, respectively (Dotson et al., 1989;
1990). Mutant alleles in either Ask1 or Ask2 loci result in
AK enzyme that is less sensitive to feedback inhibition and
lead to overproduction of lysine, threonine, methionine and
isoleucine (Dotson et al., 1990). In maize, Ask2 explains a
significant portion of variance for the free lysine levels in
the o2 endosperm (Wang and Larkins, 2001), and thus, it
represents an important target for enhancing free lysine
levels in seeds. In this study, the mutation in the Ask2 locus
that resulted in a favorable SSCP allele in CML172 is either
insensitive or less sensitive to lysine inhibition and, thereby,
contributed to increased accumulation of lysine and other
aspartate pathway amino acids, similar to the mutant Ask2
allele of Oh545o2; however, it was apparent that the Ask2
locus on its own did not have any significant impact on
the lysine or other amino acid levels in this investigation.
Based on studies involving double mutants of Ask1 and o2,
Though the three o2-specific SSR markers offer an
excellent opportunity for MAS, their use has a limited
potential in plant breeding. In this investigation, an
important discriminatory SNP in the exon 1 region of o2
was identified and an easy to use, high throughput assay,
non-gel-based to facilitate large-scale MAS was developed.
The lack of a perfect correlation between the underlying
allelic state at this SNP locus and the QPM status indicates
the non-causal nature of this variation, which however has
a high predictive value for the QPM trait. Assays SSCP and
KASP were designed for Ask2 which are able to screen a
large collection of QPM and normal lines in the CIMMYT
germplasm and to identify several lines possessing
the favorable allele. Interestingly, the frequency of the
unfavorable allele at the Ask2 locus was significantly higher
in the QPM germplasm compared to normal endosperm,
which could be due to the high selection pressure exerted
by breeders during the process of QPM germplasm
development through rigorous biochemical tests for high
lysine and tryptophan. The higher free amino acid content
in the o2 endosperm could either be due to increased lysine
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Rev. Fitotec. Mex. Vol. 45 (3) 2022
synthesis or loss of lysine ketoglutarate reductase (LKR)
activity, which degrades lysine as endosperm matures
(Arruda et al., 2000). Apart from Ask2, several genes
have been postulated to play key roles in the enhanced
lysine synthesis such as aspartate kinase-homoserine
dehydrogenase-2 and dihydro dipicolinate synthase (Galili
et al., 2016; Wang and Larkins, 2001). Some of such loci
may likely be at play in those QPM lines that are rich in
lysine but do not have the favorable Ask2 allele. Identifying
and characterizing such loci in the high lysine QPM lines
may provide further incremental gains on the enhanced
amino acid levels conferred by o2 and Ask2.
have never been subjected to biochemical tests, 2) as an
informed choice of parental lines based on the favorable
allele composition and, consequently, as a predictive
tool to cut down the number of crosses in QPM breeding
program, 3) to fix the favorable alleles at o2 and Ask2 loci in
homozygous condition at an early stage of recombination
such as F2/BC1/BC1F2 in pedigree/backcross breeding
schemes, and 4) as fixed effect covariates in a genomic
selection index to efficiently predict the likely high-lysine/
tryptophan entries among the biochemically untested
germplasm such as conventionally bred material or DH
lines that are generated routinely in large numbers in the
breeding program.
The gel-based SNP detection assay developed in this
investigation for the Ask2 locus combines reasonable
throughput of modern gel electrophoresis platforms
with robust discriminative power of the SSCP technique
(Orita et al., 1989). The SSCP-SNP assay is efficient
and sensitive enough for most of the PCR products in
the size range of 100-750 bp, cost-competitive and
requires no sophisticated instrumentation (Sunnucks et
al., 2000). Notably, some of the lines (e.g. CML172) that
were identified as possessing favorable allele at the Ask2
locus (‘2’ type), as per SSCP conformers, had indeed the
unfavorable allele (T) according to KASP genotype calls.
This discrepancy between SSCP and KASP genotypes in
the Ask2 locus was possibly due to other unknown base
changes in the Ask2 amplified region. Based on the results
of these genetic analyses in the segregating F2 progenies, it
was demonstrated that unknown base changes in the exon
region of Ask2 may also confer positive responses (lysine
enhancing) similar to the previously established functional
SNP. It is indeed, possible to obtain higher genetic gains
for enhanced lysine and tryptophan levels if the QPM lines
identified in this investigation, that harbor favorable alleles
(either A as per KASP assay or ‘2’ type SSCP conformer)
at the Ask2 locus, are preferred as QPM donors in the new
line conversions or pedigree starts and tracked through
MAS in the subsequent multiple filial generations.
CONCLUSIONS
Results pointed to an independent effect of o2 and
Ask2 rather than an interaction effect between both loci
concerning to amino acids content in the maize endosperm.
The o2 variant alone provided a highly significant effect for
high accumulation of amino acids in the double recessive
genotypes and this accumulation could be positively
influenced and modified for lysine, histidine and methionine
by Ask2. Evidence is provided from high-throughput assays
that could enhance the efficacy of marker selection for QPM
germplasm development. One of them, the SNP of exon 1
of the o2 locus had a highly predictive diagnostic value for
the QPM trait, while the other one, the SSCP marker led
to the identification of genetic variants in QPM germplasm
that opens the opportunity to find other polymorphisms
associated with the high accumulation of lysine.
ACKNOWLEDGMENTS
The study was supported by the CGIAR Research
Program Maize. First author acknowledges her PhD
scholarship to Consejo Nacional de Ciencia y Tecnología
(CONACYT) of Mexico. We also thank John Lunn from
Max Planck Institute for Molecular Plant Physiology for his
support for the free amino acid analysis and Dr. Gregorio
Alvarado-Beltrán† for biplot analysis.
With the development of an array of low-cost and highthroughput genotyping platforms (Semagn et al., 2014),
MAS has emerged as an attractive proposition in most
commercial and large-scale public-sector maize breeding
programs and quickly becoming a compelling option
especially ever since doubled haploids are integrated as
a mainstream breeding tool in the maize improvement
pipeline (Chaikam et al., 2019; Maqbool et al., 2020). Four
possible ways are envisioned in which the high throughput
SNP markers developed in this investigation for o2 and
Ask2 could be deployed in the QPM breeding programs:
1) as an inexpensive allele mining tool to identify putative
donor lines in large and diverse germplasm collections,
including non-adapted genotypes and wild relatives that
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