Principles of Plant Genetics and Breeding

412 CHAPTER 22
have been developed using traditional approaches of
hybridization, recurrent selection, and chemical mutagenesis.
Studies have shown that reduced palmitic acid
is conditioned by at least two loci, without maternal
effects. Some of these genes have been designated fap1,
fap2, and fap3. Genes modifying the major palmitic
acid loci have also been found to influence the trait by
about 2–23 g/kg. Molecular markers, including RFLPs
(restriction fragment length polymorphisms), SSRs
(simple sequence repeats), and QTLs (quantitative trait
loci) associated with palmitic acid reduction have been
cloned and mapped.
Germplasm containing between about 75% and 90%
of oleate have been developed in soybean, sunflower,
and rape, among other species. Transgenic high oleate
soybean has been developed. Soybean is also a major
source of protein for animal feed. Reduced linolenate
soybean is conditioned by three independent mutants –
fan1(A5), fan 2, and fan 3.
Research has shown that seed protein and oil content
are negatively correlated. Consequently, developing
high protein and high oil seed has limited success.
Alternatively, breeders have devoted efforts to breeding
cultivars with high protein and low oil, and those with
high oil but low protein.
In sunflower, a breeding objective is to increase the
stearic acid (C18:0) content of the seed oil. Mutagenesis
was used to develop different lines with a higher C18:0
content of about 50 g/kg. The results of one genetic
analysis indicated the inheritance of high stearic content
to be under the control of one locus (Es 1 , es 1 ) and partial
dominance. Further studies indicated the presence of a
second locus (es 2 , es 2 ). Generally, fatty acid composition
is controlled by the genotype of the developing embryo.
Hence, selection for fatty acid may be conducted at the
single-seed level, by non-destructive techniques such as
gas-liquid chromatography.
Breeding low phytate cultivars
Human and monogastric animals produce small amounts
of the phytate enzyme needed to utilize phytate phosphorus.
Soybean, for example, contains about 4.3 g/kg
of phytate phosphorus and only 0.7 g/kg inorganic
phosphorus. In humans, diets high in phytic acid
decrease the absorption of essential minerals such as
calcium, iron, and zinc. It would be desirable to remove
phytate from cereals and oil seeds.
In soybean, low phytate mutants with about 1.9 g/kg
phytate phosphorus and 3.1 g/kg inorganic phospho-
rus have been discovered. The trait is conditioned by
recessive alleles designated pha 1 and pha 2 at two independent
loci that exhibit duplicate dominant epistasis.
Both of the alleles must be homozygous in order for low
phytate seed to be obtained. In wheat, low phytic acid
mutants lpa 1 and lpa 2 have been indentified.
Breeding end-use quality
Breeders set breeding objectives to meet the needs of
producers as well as consumers. The crop producer
needs to focus on traits that facilitate crop production
and increase crop yield (e.g., pest resistance, maturity,
high yield, lodging resistance, drought resistance).
Consumers are more concerned about nutritional quality
traits (taste, protein content, appearance). Another
group of consumer needs that is not nutritional but
concerned with how they are used or stored is end-use
quality. Certain cultivars are bred for specific industrial
quality traits (e.g., for processing, cooking).
Extended shelf-life
Plant products that are harvested and used fresh (e.g.,
fruits, vegetables) are perishable and highly susceptible
to spoilage soon after harvesting. In cases where production
is far from marketing centers, the fresh produce
has to be transported over long distances, and hence
requires protection from bruising and rotting in transit.
Fully vine- or plant-ripened fruits, even though desirable
for their superior taste, are more susceptible to
damage under such conditions than unripened fruits.
Grocery stores need to display their produce for a period
of time in good condition while waiting to make a sale.
Extended shelf-life is hence an important plant trait
from the point of view of producers, wholesalers, and
consumers.
Delayed ripening
Delayed ripening is desired in crops such as tomato and
banana. Biotechnology has been successfully used to
develop this quality in some crops. Certain fruits exhibit
elevated respiration during ripening with concomitant
evolution of high levels of ethylene. Called climacteric
fruits (e.g., apple, banana, tomato), the ripening process
of these fruits involves a series of biochemical
changes leading to fruit softening. Chlorophyll, starch,