Principles of Plant Genetics and Breeding

222 CHAPTER 13
the chromosomes of one genome may share a function
in common with some chromosomes in a different
genome. Chromosomes from two genomes are said
to be homeologous when they are similar but not
homologous (identical).
Most alloploids have evolved certain genetic systems
that ensure that pairing occurs between chromosomes
of the same genome. A classic example occurs in wheat
(2n = 6x = 42) in which a gene on chromosome 5B,
designated Ph, enforces this diploid-like paring within
genomes of the alloploid. When this gene is absent,
pairing between homeologous chromosomes, as well as
corresponding chromosomes of the three genomes,
occurs, resulting in the formation of multivalents at
meiosis I.
Alloploids exhibit a variety of meiotic features.
Sometimes chromosomes pair as bivalents and thereby
produce disomic ratios. Where the component genomes
have genes in common, duplicate factor ratios will
emerge from meiosis, an event that sometimes is an
indication of alloploid origin of the species. Whereas
significant duplications of genetic material have been
found in wheat, the genomes of upland cotton have
little duplication. Tetrasomic ratios are expected for
some loci where multivalent associations are found in
allotetraploids.
Industry highlights
Application of tissue culture for tall wheatgrass improvement
Kanyand Matand and George Acquaah
Department of Agriculture and Applied Sciences, Langston University, Langston, OK 73050, USA
Introduction
Cultivar “Jose” of tall wheatgrass (Thinopyrum ponticum Podp.) is a perennial, cross-pollinating, bunchgrass, used as a cool-season
livestock forage. It is adapted to the Oklahoma environment primarily because of its relative ease-to-establish and tolerance of a
wide range of soil pH. This bunchgrass can produce high levels of dry matter per season (Coleman 1999) and exhibit superior
characteristics such as high biomass production, persistence through extended periods of grazing, and saline, flood, and drought
tolerance (Moser et al. 1996; Coleman 1999; Redmon 1999). Although used successfully to improve plants for many years, limitations
associated with traditional breeding approaches coupled with the large genome of tall wheatgrass (2n = 10x = 70) makes it
improbable and difficult to improve this crop by relying on traditional breeding methods alone (Kindiger 2002). Because tissue
culture is a prerequisite for the successful integration of classic plant breeding and genetic engineering for genetic crop improvement,
researchers at Langston University, Oklahoma, and the US Department of Agriculture–Agricultural Research Service
(USDA-ARS) Grazinglands Research Laboratory at El Reno, Oklahoma, collaborated to develop an efficient in vitro method for
propagating this crop as an additional tool for its improvement. The discussion in this box focuses primarily on a one-step, reliable
approach to microclone tall wheatgrass for germplasm development for its improvement; it also highlights cautions and pitfalls
associated with the proposed method in particular, and tissue culture in general. This one-step method may be adapted for tissue
culture research in other forage species.
Basic approaches to plant regeneration
There are a variety of ways to regenerate plants in vitro, but all of them can be grouped into two basic approaches, direct and
indirect.
1 Direct plant regeneration. Direct plant regeneration occurs without an intervening callus phase. Potential growth focal
structures consist of apical and axillary meristems, nodes, leaves, stem, roots, zygotic embryos, cotyledons, the whole seed,
and other structures. These organs can be excised from the mother plant to induce somatic embryos or adventitious shoots,
or both, without an intervening callus phase.
2 Indirect plant regeneration. This process is characteristic of the callus phase mediation for somatic plant formation from
explants. Explants similar to those listed for direct plant regeneration can also be used to induce somatic embryos or adventitious
shoots, or both, via callus formation.