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VI. Conclusions and Research Needs
J. C. O’TOOLE AND W. L. BLAND
systems for plant culture or rapid screening must be viewed with skepticism.
Adequate resources should be allocated to field verification of such
Novel experiments will be required to gain an adequate understanding of
the impact of soil environments on phenotype, e.g., the study of phenotype
across a continuum of root environments between the endpoints of
aeroponic and field soil culture. The significance of numerous interacting
factors (temperature, oxygen, strength, water content, etc.) must be
recognized and experimental facilities constructed in which as many important physical parameters as possible are controlled.
Realization of the full potential for crop root system improvement remains a challenge to the quantitative geneticist, soil physicist, and root
physiologist. The traditional bounds of each discipline must be extended
until common ground is established and truly interdisciplinary studies
result. By comparison with the state of scientific research on plant shoots,
root science is in its infancy and may be expected to disclose many new and
exciting possibilities to further adapt crop plants to their environment.
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ADVANCES IN AGRONOMY, VOL. 41
APPLICATION OF CELL AND
TISSUE CULTURE TECHNIQUES
FOR THE GENETIC IMPROVEMENT
OF SORGHUM, Sorghum
bicolor (L.) Moench
PROGRESS AND POTENTIAL’
S. K r e s ~ v i c hR.
~ ~McGee,* L. Panella,4
A. A. R e i l l e ~ ,and
~ . ~F. R. Miller4
*Texas Agricultural Experiment Station, The Texas A&M University System
Wesiaco, Texas 78596
Department of Soil and Crop Sciences, Texas A&M University
College Station, Texas 77843
Having evolved over a great period of time and a broad spectrum of
species, plant breeding is a science-based technology directed towards
economic objectives (Simmonds, 1983). It is one component of agricultural
production and, along with agronomic practices, agricultural chemistry,
plant pathology, entomology, and agricultural engineering, has been
responsible for past successes associated with increased yields of food, feed,
fiber, and fuel at low costs to the consumer. Plant breeding has long since
passed from being an “art,” as some of the older plant breeders would lead
one to believe, and is now heavily dependent on the disciplines of genetics
During the past two decades, an integration of plant biochemistry and
physiology has allowed for the development of basic techniques, i.e., cell
and tissue culture, which have the potential for improving our understanding of plant biology. Early proponents of the cell and tissue culture
methodology expounded its advantages and predicted that this array of
I This article is a contribution of the Texas Agricultural Experiment Station, College Station, Texas 77843. Approved as Technical Article No. 22187.
’ Present address: United States Department of Agriculture, Agricultural Research Service,
Germplasm Resources Unit, NYSAES, Geneva, New York 14456.
’ Present address: DNA Plant Technology Corporation, Watsonville, California 95076.
Copyright 0 1987 by Academic Press. Inc.
All rights of reproduction in any form reserved.
S. KRESOVICH ET A L .
of tools might supplant conventional plant breeding. It was suggested that
their implementation would cause an agricultural “revolution” which
greatly would exceed preceding revolutions initiated through the application of fertilizer and mechanization to the agricultural production system.
Fortunately, we all have mellowed a bit from the initial rhetoric
associated with cell and tissue culture techniques. Plant scientists now
realize that these techniques really cannot supplant plant breeding in all of
its many-faceted forms, but rather serve as adjuncts to it. Furthermore, the
uniqueness of individual crop species requires researchers to fit new techniques into current frameworks of crop improvement. With these considerations in mind, we attempt to highlight the “state of the discipline”
with regard to the status and potential of these techniques for the genetic
improvement of sorghum, Sorghum bicolor (L.) Moench. Sorghum is
recognized not only as a source of food and feed in marginal production
areas, but as a potential “biomass” energy crop because of its productivity,
efficiency, and adaptability. Within this article, a framework is developed
in which the potential applications of cell and tissue culture techniques to
the genetic improvement of sorghum may be viewed and, also, their current
limitations may be identified. Specific developments in molecular biology as
related to sorghum improvement are considered beyond the scope of this
The origin of cultivated sorghum is agreed generally to have occurred on
the African continent (Mann et al., 1983); however, disagreement exists
whether the origin has a monophyletic or a polyphyletic basis. Therefore,
the systematics of sorghum are quite complex (de Wet et al., 1970; Doggett,
1970). Sorghum bicolor (L.) Moench includes a diverse collection (Fig. 1)
including grass types and cultivated sorghum, all having a diploid
chromosome complement of 2n = 20. Harlan and de Wet (1972) have
devised a working classification of cultivated sorghum that encompasses
five basic races including bicolor, guinea, caudatum, kafir, and durra.
These races are identified by mature spikelet and panicle type. In addition,
10 hybrid races are recognized, of which kafir-caudatum is the source of
most hybrid grain sorghum grown in the United States (Harlan, 1972).
Other important sorghum types grown include sorghum (sorgos) for syrup
and sugar, grass types for forages, and specialty types such as popping
sorghum and broomcorn. Although sorghum encompasses a great diversity
of types, the basic morphology and anatomy is consistent. This is particularly
important when considering source tissues for culture establishment.
CELL AND TISSUE CULTURE TECHNIQUES
FIG. 1. Phenotypic diversity exhibited in Sorghum hicolor (L.) Moench. (Courtesy of L.
W . Panella.)
S. KRESOVICH ET A L .
The seed germinates with the emergence of a coleoptile and coleorhiza.
The shoot apex is forced to the soil surface by the expansion of cells in the
mesocotyl region, while one to several primary roots may emerge from the
embryonic axis. A node forms at the juncture of the mesocotyl and the
shoot apex, from which secondary roots and adventitious buds develop.
The primary roots and mesocotyl deteriorate at this early stage, and it is the
secondary roots that continue to support the sorghum plant. Secondary
shoots may form from the adventitious buds at this basal node. The culm is
comprised of nodes and internodes. Internode tissue may be thick or pithy,
juicy or dry, and either insipid or sweet. Leaves arise from the nodal areas
and consist of a sheath region and blade. The sheath base is primarily
meristematic tissue which allows the sheath to elongate.
In cultivated sorghum, floral initiation may occur 30-75 days or more
following germination, with anthesis (flowering) occurring from 15-30 days
later. The sorghum inflorescence is a panicle that matures from the apex
downward. The rachis branches contain paired spikelets, one sessile and
one pedicellate. The sessile spikelet generally contains one fertile and one
sterile floret; sometimes both are fertile, resulting in twin seededness. The
pedicelled spikelet may contain a floret with functional anthers but is usually without a functional ovary. Anthesis generally occurs during the early
morning (around sunrise) when paired lodicules at the base of the spikelet
swell, forcing the glumes apart and exposing the stigmas and anthers. This
flowering process takes about 10-30 min.
The caryopsis reaches physiological maturity about 30-35 days following
pollination. The mature seed coat consists of a pericarp and fused testa. The
pericarp color may appear as red, yellow, or white. The testa may also contain pigmented compounds, composed mainly of phenolics and tannins
(Rooney and Miller, 1982; Oberthur et al., 1983; Doherty et al., 1987). (For
more details of growth and development, see Rangaswami Ayyangar and
Panduranga Rao, 1936; Artschwager, 1948; Artschwager and McGuire,
1949; Paulson, 1969; Doggett, 1970; Wall and Ross, 1970; Vanderlip and
GOALS IN BREEDING
Regardless of the crop involved, a breeding program must be defined
with set goals. In addition, the program must have the means to achieve
those goals. Naturally, the screening and selection criteria vary with the
crop, location, and goals. A plant breeder may try to address any one or any
combination of these goals. The earlier in the improvement program the
breeder can identify the trait@)desired, the more quickly progress can be