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Introduction
Sentinels of the Prairie Speak (by Populus Species)
Harold S. McNabb, Jr.
This chapter is dedicated to the late Dr. Ma rie-France
Michel, Station d ' Ameliora tion d es Arbres Forestiers,
INRA, Centre d e Recherches d ' Orleans, Ardon - 45160
Olivet, France. Dr. Michel bridged the old and the new,
first researching the crown gall disease of poplar in nurseries, then helping to identify s trains of the causal agent,
Agrobacterium tumefaciens, w hich were ideal for foreign
gene transfer.
As we approach the 21 st century, our American Indian
friends should be proud o f us popla rs. As the research
writings in the following pages show, we are the model
system for new scientific ad vances in wood y plant biology. We continue to be the "guinea pig" of forest-tree breeding (Pauley 1949). This distinction means more than the
me re fac t tha t hum a n s ca n easily m a nipulate o u r
germp lasm to meet their needs. The following is our a ttempt to convey the mystique poplars ha ve possessed in
recent human history.
Although we resemble our ancestors in the Salicaceae
from approximately 58 million yea rs ago, we are comprised
of around 29 different species (Eckenwa lder 1996). Poplars were selected as the symbol of liberty during the
French Revolution of 1789, later becoming the banne r of
the resulting new republic (Tucker 1989). During the 18th
and 19th centuries, popla rs contributed to the overa ll
beauty, s tability, and economy of rural life. O ur wood was
importan t to the everyd ay Jives of people; shoes in the lowland a reas of western Europe, fuel and build ing ma terial
for rural a reas, and later, matches that fueled specialized
ind ustrial developmen t. Poplars were also part of an established economic system that designated b oundaries
between landowners and tenants. Mone t's famous popJar series from the 1890s illustrate the significance of popJars to p eople of that period. Our importance is conveyed
to subsequent socie ties as p eople continue to enjoy the
spirituality of these works of art (Tucker 1989).
Monet's tall, slender images of poplars suggests an early
selection of Populus x euramericana. A number of us were
first recognized in 1775 as "hybrids spontanes" between
the recently imported P. deltoides and the European na tive
P. nigra (Muhle La rsen 1960). Then, we we re called black
Italian poplars (P. serotina Ha rtig). The hybrid name P. x
USDA Forest Service Gen. Tech. Rep. RM-GTR-297. 1997.
canadensis was introduced in 1795 by Mi:inch (Pauley 1949).
Members of these early spontaneous hybrids are used by
our human friends throughout the world and constitute a
vast ne twork of our poplar clan tod ay.
Many of our natural hybrid s were identified and used
during the 19th century. However, in the beginning of the
20th century, artificial breeding was introduced at Kew
Gardens when Professor Henry fertilized flowers of P.
angulata with pollen from P. trichocarpa. The resulting progeny were subsequently named P. generosa (Muhle Larsen
1960). Later, many breeding p rog rams were initiated with
our species. For example, the Oxford Paper Co. in the
northeastern United States in 1924 with A. B. Stout and
E. J. Schreiner, C. Syrach Larsen in Denmark in 1926,
P. Guinier in France in 1926, and C. Heimburger and his
colleagues in Canad a in 1930. Probably the most in tensive
breeding efforts occu rred in Italy whe re the Institu te of
Poplar Culture at Casale Monferrato was founded in 1948
by Professor G. Piccarolo. A similar insti tute was established in 1949 by the l'Union Allumettiere, S.A. (U AL), a
match p roducing group, at Geraardsbe rgen (Grammont),
Belgium. C. Muhle Larsen came from Denmark to head
this new Belgium institute .
Vic Steenackers, recently retired head of the Belgian laboratory, remarked once w hile showing colleagues his famous fem ale poplar parents, "My supervisors always
wanted to know when I would begin biotechnology research?" My answer was, "What do you think I have been
doing d uring my career? Isn't breeding a form of biotechnology?" Although the modern biotechnology tools enable the gene ticis t to quickl y a nd comp reh ensive ly
understand germplasm questions, early poplar selection,
b reeding, and basic genetic studies p roduced valuable
practical and sc ie ntific benefits and were the basis of
today's work. Current use of the word biotechnology implies use of molecular tools, which was absent from early
poplar selection, breeding, and basic genetic studies.
We poplars believe that molecular tools w ill help alleviate many problems, such as disease susceptibility and insect injury, w ith establishmen t, maintenance, and use of
our productive systems. Our interpreter, H. S. McNabb,
Jr., and his colleagues believe that host resistance provides
the fundamental basis of management stra tegies for these
damaging agents. Breeding programs with our species
have recognized this principle and have used pest resistance as a major selection criteria. Consequently, people
look to genetic engineering and gene mapping as tools to
develop resistance in our poplar descendants. Molecular
tools should also reduce the time needed by breeders to
produce individuals with the desired traits.
In early genetic engineering discussions about foreign,
pest-resistance genes, our interpreter expressed concern
·about using genes that could promote higher applications
of silvicultural pesticides (e.g., herbicide tolerance genes)
(Fillatti et al. 1987) or genes that coded for constitutive internal production of pesticides (e.g., the Bt endotoxin genes)
(Raffa 1989). Based on their interactions with insects and
pathogens, resistant (R) genes can be classified as either
subtle or radical (Klopfenstein et al. 1992; McNabb et al.
1990). For example, subtleR genes (e.g., proteinase inhibitor genes) are generally nonspecific and may reduce growth
and/ or reproduction of pests, making them more susceptible to parasites, predators, and adverse environments.
Alternatively, radical R genes (e.g., Bt endotoxin genes) are
typically pest-specific and pesticidal in their mode of action. Misuse of these radical I specific genes can place high
selection pressure on pest populations; whereas, subtle I
nonspecific genes are less likely to be rapidly overcome by
shifts in pest populations.
Attempts to reduce selection pressure has fostered interest in wound-inducible constructs such as the Proteinase Inhibitor II (PIN2) gene. Such constructs were
used in genetic engineering research with three of our
hybrid relatives (Chun et al. 1988; Heuchelin et al. 1997;
Kang et al. 1997; Klopfenstein et al. 1989, 1991, 1997)
and were tested in an early field planting of transgenic
poplar in July 1989 (Klopfenstein et al. 1991; McNabb
et al. 1991). However, questions remain: Do subtle reductions in pest populations significantly diminish the
selection pressure on pests? Would subtle reduction be
sufficient to manage these pest populations below economic loss levels? The age-old questions remain about
the proper planting design for pest resistant poplars in
woody crop systems.
Although we poplars believe that pest problems are
potentially the most serious, molecular techniques can also
help in other areas. An intriguing topic concerns competition among ourselves. The phytochrome system has been
shown to be associated with a "communication mechanism" among adjacent individuals (Smith 1995). Poplars
"talk" to each other. In competition with each other, we
compensate by favoring height growth instead of diameter growth. But, if this effect was mitigated, you humans
might achieve greater biomass production from us. Genetic
modification of the phytochrome recognition of infra-red
reflectance could change this competitive growth response.
The potential importance of this research area has
prompted the recent establishment of an issue team at Iowa
State University led by R. B. Hall. People continue to unravel our secrets.
This book addresses the multi-faceted biological studies that are inspired and challenged by molecular tools.
Use of these tools wil.l rapidly convey greater understanding about us poplars, our relatives, and our role in the
diverse natural ecosystems in which we reside. Concurrently, our natural ecosystems can be preserved when
humans develop special needs and artificial systems, such
as fiber farms, for us. However, appropriate precautions
are needed to prevent pollution of our native germplasm
via nonindigenous seed and pollen. As more molecular
techniques are applied to poplars, the resulting information will become a vital part of our genetic and breeding
programs around the world. This new biology does not
exist in a vacuum. If people wish to optimize poplar
potential for sustainable growth in these artificial systems, future research must be integrated into system
development. This publication attempts to promote this
integration.
This book is divided into 5 general sections each containing chapters on related topics. The initial section discusses what has been achieved with in vitro cultures of
our poplar relatives including micropropagation, somatic
embryogenesis, protoplast culture, somaclonal variation,
and germplasm preservation. The second section concerns
methods of genetic engineering and evaluation of
transgene expression. Poplar researchers have included
topics on genetic marker technologies and molecular characterization of Populus in the third section. Methods to
improve our resistance to insects, pathogens, and air pollution are addressed in the fourth section. We find the fifth
section most interesting. These chapters include the biotechnological applications of modification of our wood
properties, alteration of our flowering processes, integration of molecular techniques into breeding programs, the
commercialization of propagule production, bioethics, and
economic considerations. We speak for all members of our
Populus genus when we say that this new treatment of
poplar biology is a welcome event. Humans have honored
us as woody plant leaders in the basic biological knowledge explosion.
This introductory chapter closes by asking the following questions: With these new techniques, will humans be
able to determine our gender before our reproductive structures develop? At the turn of the 20th century, when our
ancestors along the Platte River were cut, the loggers always harvested the female trees because they believed that
the wood was stronger. Is this true? And, if so, why? As
humans unravel more and more of our secrets, will they
have the same reverence for the natural systems as the
American Indian people possessed? Will we remain the
"Sentinels of the Prairie?"
2
USDA Forest Service Gen. Tech. Rep. RM-GTR-297. 1997.
Literature Cited
Chun, Y.W.; Klopfenstein, N.B.; McNabb, H.S., Jr.; Hall, R.B.
1988. Transformation of Populus species by an
Agrobacterium binary vector systems. J. Korean For. Soc.
77: 199-207.
Eckenwalder, J.E. 1996. Systematics and evolution of
Populus. In: Stettler, R.F.; Bradshaw, H.D., Jr.; Heilman,
P.E.; Hinckley, T.M., eds. Biology of Populus and its implications for management and conservation. Ottawa,
Ontario, Canada: NRC Research Press: 7-32. Chapter 1.
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