Principles of Plant Genetics and Breeding

elow a certain predetermined level, the accession is
regrown to obtain fresh seed.
Field growing
Accessions are regrown to obtain fresh seed or to
increase existing supplies (after filling orders by scientists
and other clients). To keep the genetic purity, the
accessions are grown in isolation, each plant covered
with a cotton bag to keep foreign sources of pollen out
and also to ensure self-pollination.
Cryopreservation
Cryopreservation or freeze-preservation is the storage
of materials at extremely low temperatures of between
−150 to −196°C in liquid nitrogen. Plant cells, tissue, or
other vegetative material may be stored this way for
a long time without loosing regenerative capacity.
Whereas seed may also be stored by this method, cryopreservation
is reserved especially for vegetatively propagated
species that need to be maintained as living plants.
Shoot tip cultures are obtained from the material to
be stored and protected by dipping in a cryoprotectant
(e.g., a mixture of sugar and polyethylene glycol plus
dimethylsulfoxide).
In vitro storage
Germplasm of vegetatively propagated crops is normally
stored and distributed to users in vegetative forms such
as tubers, corms, rhizomes, and cuttings. However, it
is laborious and expensive to maintain plants in these
forms. In vitro germplasm storage usually involves tissue
culture. There are several types of tissue culture systems
(suspension cells, callus, meristematic tissues). To use
suspension cells and callus materials, there must be an
established system of regeneration of full plants from
these systems, something that is not available for all
plant species yet. Consequently, meristem cultures are
favored for in vitro storage because they are more stable.
The tissue culture material may be stored using the
method of slow growth (chemicals are applied to retard
the culture temperature) or cryopreservation.
Molecular conservation
The advent of biotechnology has made it possible for
researchers to sequence DNA of organisms. These
sequences can be searched (see Bioinformatics in
Chapter 14, p. 238) for genes at the molecular level.
PLANT GENETIC RESOURCES FOR PLANT BREEDING 101
Specific genes can be isolated by cloning and used in
developing transgenic products.
Using genetic resources
Perceptions and challenges
Breeders, to varying degrees, acknowledge the need to
address the genetic vulnerability of their crops. Further,
they acknowledge the presence of large amounts of
genetic variation in wild crop relatives. However, much
of this variability is not useful to modern plant breeding.
In using wild germplasm, there is a challenge to sort
out and detect those germplasms that are useful to
breeders. Modern cultivars have resulted from years
of accumulation of favorable alleles that have been
gradually assembled into adapted interacting multilocus
combinations. Introgression of unadapted genes may
jeopardize these combinations through segregation and
recombination. Hence, some breeders are less inclined
to use unadapted germplasm. However, there are occasions
when the breeder has little choice but to take
the risk of using unadapted germplasm (e.g., specific
improvement of traits such as new races of disease, quality
issues), because alleles for addressing these problems
may be non-existent in the adapted materials. Plant
breeders engaged in the breeding of plant species that
have little or no history of improvement are among the
major users of active collections in germplasm banks.
For such breeders, they may have no alternative but
to evaluate primitive materials to identify those with
promise for use as parents in breeding.
Plant breeders may use germplasm collections in one
of two basic ways: (i) as sources of cultivars; or (ii) as
sources of specific genes. A breeding collection contains
alleles for specific traits that breeders can transfer into
adapted genotypes using appropriate breeding methods.
Accessions must be properly documented to facilitate the
search by users. This means, there should be accurate
passport and descriptor information for all accessions.
Unfortunately, this is not the case for many accessions.
The redundancy in germplasm banks is viewed by
some breeders as unacceptable. A study showed that of
the 250,000 accessions of barley at that time in repositories,
only about 50,000 were unique. Such discrepancy
leads to false estimation of the true extent of diversity in
the world collection. A large number of the accessions
are also obsolete and have little use to modern plant
breeding programs. Germplasm evaluation at the level
of germplasm banks is very limited, making it more