Table Of ContentDevelopments ni Plant Genetisc and Breedign
1A ISOZYMES IN PLANT GENETICS AND BREEDING, PART A
editde by S.D. Tankslye and TJ. Orton
1983 ÷ + 516 pp.
1B ISOZYMES IN PLANT GENETICS AND BREEDING, PART Â
editde by S.D. Tankslye and T..J Orton
1983 vii i+ 472 pp.
Developments ni Plant Genetisc and Breedin,g 1B
Isozymes
in Plant Genetics and Breeding
Part Β
Ed itde by
Steven D. Tankslye
Department of Horticultu reN,ew Mexico Stat eUniversi tyL,as Cruces ,NM 88003 , U.S.A.
and
Thomas J. Orton
AgnGenetics Researc hCorporatio n,14142 Denver West Parkway ,Golden ,CO 80401,
U.S.A.
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First editino 1983
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Main entry under title:
Isozymes in plant genetics and breeding.
(Developments in plant genetics and breeding ; l)
1. Plant-breeding. 2. Isoenzymes. 3. Plant genetics.
I. Tanksley, Steven D. II. Orton, Thomas J.
III. Series.
SE123.I65 19Φ3 631.5*3 63-l4ol3
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Printde ni The Netherlansd
S. Arulsekra W.H. Eshbaugh
Department of Pomology Department of Botany
Universiyt of Califoran i Miami Universyi t
Davis, CA 95616, U.S..A Oxford, OH 45056, U.S..A
P. Arus M.M. Goodman
Generalitt äde Catalunay Department of Statisst ic
Servei d'lnvestogaoc iAgrarai North Carolian Staet Universyi t
Carretaar de Cabri sI sn/ Raleigh, NC 27650, U.S..A
Cabrisl (Barcelon,a )SPAIN
M.B. Gorman
J.C. Bell Department of Plant Scienec
Divisino of Forest Researhc Universiyt of New Hampshire
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Canberra Cit,y AUSTRALIA
S.I. Guttman
R.S. Bringhurts Department of Zoology
Department of Pomology Miami Universyi t
Universiyt of Califoran i Oxford, OH 45056, U.S..A
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G.E. Hart
A.H.D. Brown Department of Plant Sciencse
Divisino of Plant Industyr Texas A & M Universyi t
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F. Dane Academy of Sciencse of Estonina Institeu t
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S.L. Desborough A.L. Kahler
Department of Horticultul rSacienec Department of Plant Sciencse
and Landscape Architecteu r South Dakota Staet Universyi t
Universiyt of Minnesoat Brookings, SD 57007, U.S..A
St. Paul, MN 55108, U.S..A
Y.T. Kiang
T. Endo Department of Plant Sciencse
National Institeu tof Genetisc Universiyt of New Hampshire
Misima 411, JAPAN Durham, ÍH 03824, U.S..A
viii
M J. McLeod CM . Rick
Biology Department Department of Vegetabel Crops
Belmont Abbey Colleeg Universiyt of Califoran i
Belmont, NC 28012, U.S..A Davis, CA 95616, U.S..A
J.B. Mittno S.J. Sheen
Department of Environmenta, lPopulatino Department of Agronomy
and Organismci Bioloyg Universiyt of Kentucky
Universiyt of Coloraod Lexington, KY 40546, U.S..A
Boulder, CO 80309, U.S..A
CR . Shielsd
G.F. Moran Department of Vegetabel Crops
Divisino of Forest Researhc Universiyt of Califoran i
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Canberra Cit,y AUSTRALIA
CW . Stuber
M. Morishma Department of Genetisc
National Institeu tof Genetisc North Carolian Staet Universyi t
Misima 411, JAPAN Raleigh, NC 27650, U.S..A
TJ. Orton A.M. Torrse
AgriGenetisc Researhc Corp. Department of Botany
14142 Denver West Parkway Universiyt of Kansas
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S.D. Tankslye and TJ. Orton (Editor,s ) 1
Isozymes i nPlan tGenetic sand Breeding ,Par tΒ
© 1983 Elsevir eScienec Publishes rB.V,. Amsterdam
MAIZE
MAJOR M. GOODMAN AND CHARLES W. STUBER
INTRODUCTION
In maize (Zea mays L.), perhaps more than in any other plant species, there
has been a broad range of studies involving isozymes. These have varied from
purely biochemical studies of enzymatic properties to plant breeding studies
concerned with the maintenance of quality control during seed production. In
addition, most of the structural genes encoding those maize isozymes that have
been studied have been localized to specific chromosomal sites. Certain iso
zymes, most notably alcohol dehydrogenase (ADH), have been studied intensively.
Products of two loci, Adhl and Adh2, have now been sequenced. Investigations
with null alleles and with chromosomal rearrangements have begun to demonstrate
the permissible range of dosage of structural loci and the relative effects of
modifying loci. In addition, linkages between specific isozyme alleles and
agronomically favorable genes have been used to improve yield by direct selec
tion for specific allozymes with a resulting correlated yield response. In
two cases (ADH, GDH), there has been a direct association shown between the
products of specific non-null alleles and the response of the maize plant to
certain environmental stresses.
This summary briefly describes the various isozyme systems studied in maize
and the genes responsible for their expression. Many of these loci have been
used in genetic survey work and for them much information about relative vari
ability is known. For some of the loci, mostly those encoding esterases,
peptidases, and peroxidases, the range of variability is not well documented.
The available variation among standard inbred lines of maize is documented,
for many isozyme loci, in Goodman and Stuber (1980) and S tuber and Goodman
(1982).
Many of the techniques appropriate for maize are outlined in Cardy et al.
(1980), while more general reviews and other recipes are found in Scandalios
(1969b), Shaw and Prasad (1970), Selander et al. (1971), Schaal and Anderson
(1974), and O'Malley et al. (1980). In most studies, samples used for elec
trophoresis are obtained by simple maceration of tissue in a suitable buffer,
with possibly a short centrifugation. In the case of starch gel electro
phoresis, the samples are then applied to filter paper wicks and inserted into
a gel. Immature endosperm samples are usually applied directly to the filter
paper wicks.
For several of the enzyme systems, nomenclature used in the literature for
specifying various isozyme loci has not been consistent. This frequently
causes confusion when relating work from different laboratories. A committee
2
on nomenclature for maize loci has recommended that three letters followed by
an arabic numeral be used to designate all newly described loci (Burnham et al.
1974). We have followed this convention for most of our discussion in this
chapter. However, when other designations have had a long history of use
(e.g., El^, E2_, etc., for the esterase loci), we usually have followed the
system most commonly found in the literature.
ACID PHOSPHATASE (ACP)
Acid phosphatase (ACP), E.C. 3.1.3.2, functions in the hydrolysis of phos-
phomonoesters important in a variety of biochemical reactions including the for
mation of sucrose in photosynthesis. Designations of the loci associated with
ACP isozymes have been varied including AD_ (El-Metainy and Omar, 1981), Acph
(Stuber et al., 1980, 1982), Phos (Brown and Allard, 1969), and ACD_ (Kahler,
in review). To be consistent with the recommended nomenclature, we have used
Acp in this discussion and recommend that this now be accepted as the standard.
El-Metainy and Omar (1981) reported three zones of ACP activity (designated
as API, AP2, and AP3). Isozymes associated with the first two zones are dimers
and are specified by Acpl and Acp2 (Apl and Ap2, respectively, by El-Metainy
and Omar). The genetic basis of the third zone has not been determined.
Kahler (personal communication) has now found a fourth group of monomeric ACP
isozymes specified by Acp4. Variation associated with the Acpl locus has been
most widely studied. Activity at this locus is expressed in the cytoplasm
throughout the plant, including roots, leaves, coleoptiles, scutella, and
pollen (Efron, 1970; Stuber and Goodman, unpublished).
Sodium borate (pH 8.3-8.5) and L-histidine-citric acid (pH 5.7 or 6.5) buf
fers have been used for resolution of ACP isozymes (Efron, 1970; Cardy et al.,
1980; El-Metainy and Omar, 1981). ACP isozymes migrate anodally with these
buffer systems. Acpl has been localized to chromosome 9 (Stuber et al., 1980).
Acpl and Acp2 segregate independently (El-Metainy and Omar, 1981), but no
chromosomal location has been found for Acp2 or Acp4.
Thirteen alleles have been identified at the Acpl locus in maize and its
closest relative, teosinte. In a survey of nearly 700 racial collections of
maize, three alleles, Acpl-2, Acpl-3, and Acpl-4, predominated. These three
alleles were also the most frequently found types in a set of 406 inbred lines.
Nearly 53 percent of the lines were homozygous for Acpl-4, 32 percent for
Acpl-2^ and 12 percent for Acpl-3 (Stuber and Goodman, 1982).
ALCOHOL DEHYDROGENASE (ADH)
An extensive, thorough review of ADH in maize has recently been published by
Freeling and Birchler (1981). In addition, an annotated bibliography of the
Adh genes of maize has just appeared (Karoly et al., 1982). Two structural
3
loci, Adhl and Adh2, code for ADH dimers (Schwartz, 1966, 1969a; Freeling and
Schwartz, 1973; Freeling, 1974). Adhl is located on the long arm of chromo
some 1, at approximately position 127, 1.5 map units from lw (Schwartz, 1971);
Adh2 is about 20 map units distal to su_ on chromosome 4 (Dlouhy, 1980). Under
most conditions in most tissues, the predominant ADHs are those expressed by
Adhl. Anaerobic conditions can induce ADH activity in seedling roots and that
activity is a result of synthesis of both ADHl and ADH2 homodimers and intra
genic and intergenic heterodimers (Freeling and Schwartz, 1973; Ferl et al.,
1980). Dimerization between subunits of both loci appears to be random
(Freeling, 1974). SDS-dissociated ADH polypeptides have a molecular weight of
approximately 38-42,000 daltons (Ferl et al., 1979; Sachs et al., 1980).
Several natural and induced ADH variants are available which differ in
electrophoretic mobility and/or specific activity. Many maize populations
segregate for Adhl-F and Adhl-S, with the F_ allele being the most common.
Using In situ staining of pollen grains for ADH activity in conjunction with
various Adhl dysfunctional mutants, reversion frequencies and intra- and inter-
allelic recombination frequencies have been obtained by Freeling (1978).
Striking differences in intragenic recombination between alleles and between
different allelic combinations appear to exist. Similarly, Freeling and
Woodman (1979; see also Woodman and Freeling, 1981) have studied a wide range
of naturally occurring Adhl variants with electrophoretic mobilities identical
to the standard Adhl-S or Adhl-F variants. Many of these carried linked, cis-
acting modifiers affecting the quantitative variation of ADHl activity. Both
natural (Stuber and Goodman, unpublished) and induced (see Freeling and
Birchler, 1981) nulls also exist for Adhl.
Adh2 has been studied much less than Adhl. Both natural and induced vari
ants (including nulls) occur; many of these are listed by Freeling and Birchler
(1981). Although polypeptides encoded by Adhl predominate in most tissues, the
Adh2 product predominates in at least two--the immature ear peduncle and the
mature stem node cells. A recent Russian report (Monastyreva, 1978) suggests
that Adh2 activity is also very high in the cortical parenchyma of 12-day post-
pollination corncobs. While ADH is a dispensable enzyme for maize under normal
growing conditions, after prolonged soaking, Adhl null kernels do not germinate
(Schwartz, 1969b). Homozygous plants null for both Adhl and Adh2 germinate
poorly and drown more rapidly than plants having active variants at either
locus. Plants lacking only Adh2 activity appear completely normal (Freeling
and Birchler, 1981). Marshall et al. (1973) reported that under flooding con
ditions, there are differential growth effects between the common Adhl alleles,
F and S_, with S_ being the more favorable allele.
The production of dysfunctional mutants by means of pollen selection using
allyl alcohol vapor has played an important role in assembling a set of Adhl
4
variants (Schwartz and Osterman, 1976; Freeling and Cheng, 1978). There is
some evidence that the ADH variants induced by different methods (EMS, heavy
radiation, mutator genes) can be categorized differently, with those induced by
mutator loci most closely resembling those occurring naturally (Freeling and
Birchler, 1981).
An alternative nomenclature for ADH was suggested by Scandalios (1967,
1969a) [who uses Adh2 for the more active and more intensively studied ADH
locus, rather than the usual Adhl locus notation in general use (Coe and
Neuffer, 1977)] in support of a two-locus, linked gene hypothesis to explain
the variation observed at that time for ADH. The evidence for that model was
based largely upon the apparent discovery of three recombinant genotypes among
some 20,000 F^ individuals (Scandalios, 1969a). Support for the hypothesis and
its associated nomenclature largely dissipated when Lewontin (1970) pointed out
that two of the three "recombinant" zygotes would have required simultaneous
crossovers in both the male and female gametes (Gottlieb, 1981), a highly
improbable result.
The Adhl and Adh2 genes share similar nucleotide sequences, and at least
some of the antibodies produced against purified ADHl homosimers cross react
with ADH2 homosimers (Freeling and Birchler, 1981). The protein coding regions
of Adhl and Adh2 have approximately 807o nucleotide sequence homology and about
90% amino acid homology (Ferl, personal communication; Gerlach et al., 1982;
Sachs et al., 1982), suggesting that one arose by gene duplication from the
other.
Except for Mel and Got3, Adhl was the least variable isozyme locus studied
in a survey of nearly 700 racial collections of maize from Latin America. Most
collections were fixed for the common fast migrating form (Stuber and Goodman,
unpublished). In a study of 406 inbred lines of maize, 86 percent were homo
zygous for the common fast variant, and 13 percent for the common slow variant
(Stuber and Goodman, 1982).
AMYLASE (AMY)
Two forms of amylase have been reported in the developing endosperm of young
maize kernels (Chao and Scandalios, 1969, 1971, 1972; Finnegan, 1969;
Scandalios et al., 1978). Amy2 (the Sd locus of Finnegan) governs the inheri
tance of the major form of ß-amylase (E.C. 3.2.1.2), a developmentally tran
sient enzyme with high activity in the maturing kernel. AMY2 activity de
creases with sporophytic development, and it is not usually detected in mature
plants (Scandalios et al., 1978).
Both Amyl, which governs the major forms of ^-amylase (E.C 3.2.1.1), and
Amy2 have co-dominant alleles (Two reported at each locus) which encode mono
mers and which do not appear to interact. Both loci are customarily studied by
5
means of Polyacrylamide gel electrophoresis. The two loci have differential
stabilities with respect to pH and temperature, with Amy! losing more activity
at low pH and Amy2 being more susceptible to heat inactivation (Scandalios et
al., 1978). The same authors have estimated the molecular weight of AMYl to be
about 44,000.
CATA LA SΕ (CAT)
Three loci encode the various catalase (E.C. 1.11.1.6) isozymes which have
been extensively studied in maize. Catl, on 5S, and Cat2, on IS, encode tran
sient isozymes expressed differently during kernel and seedling development.
Both loci encode tetramers and, during the brief time span shortly after seed
imbibition when both loci are expressed, hybrid catalase molecules are formed.
Catl is expressed in the developing kernels, primarily in the endosperm and
scutellum, while Cat2 is expressed in the germinating seedling, primarily in
the scutellum (Roupakias et al., 1980). Cat3, which has yet to be unequivo
cally localized to a chromosome, is expressed in the hypocotyl and shoot of the
germinating seedling (Sorenson and Scandalios, 1975; Goodman and Stuber, 1982a,
1983). It encodes a tetrameric enzyme associated with the mitochondria, while
Catl and Cat2 are cytosolic, although catalases encoded by Catl and Cat2 are
transiently associated with the glyoxysomes from days 3 to 6 after seedling
germination. I_n vivo, Cat3 does not interact with Catl or Cat2 to form hybrid
enzymes, but such hybrids can be produced by dissociation-association techni
ques (Scandalios et al., 1980b; Sorenson, 1982).
All of the catalase isozymes are inactivated by aminotriazole. Six alleles
have been reported for Catl and three for Cat2, the latter including a null
variant (Scandalios et al., 1980; Tsaftaris and Scandalios, 1981). We have re
ported four Cat3 variants, including a null, among standard inbred lines and
have studied four additional alleles isolated from other materials.
Recently, Scandalios et al. (1980a) have reported the detection of a locus
having additive gene action and governing the duration of observed Cat2 acti
vity. The locus, Carl, is on chromosome 1, 37 map units away from Cat2, which
was used to determine the position of Carl.
Sorenson (1982) has suggested that the transient association of catalase
with the glyoxysomes involves an "anchor protein" which binds catalase to the
glyoxysomal membrane. Sorenson has isolated Catl mRNA (with a length of 1850
nucleotides) that hybridizes with a cloned Catl cDNA probe. It is capable of
synthesizing a polypeptide that is indistinguishable from native catalase and
that has a subunit molecular weight of 56,000 daltons.
