Using Transgenic Plants as a Source of Genetic Diversity by rez13585


									        ISB News Report                                                                                        July 2005

      Using Transgenic Plants as a Source of Genetic Diversity for Breeding Greater Drought
                                      Tolerance into Wheat
                            Alessandro Pellegrineschi, M Pulleman, S Sullivan, R Trethowan, and M Reynolds

The emergence of the agricultural biotechnology industry at the end of the 20th century brought with it products that
fall into two basic categories: herbicide resistant and insect resistant crops (i.e., maize, soybean, and cotton). In the late
1980s and the 1990s, attempts were made to develop a range of other products whose qualities were to include improved
fungal disease resistance, starch composition, fruit quality, and so on.

As the agricultural biotech industry matured, standard product development processes were adopted. For agricultural
products, the commercialization of initial discoveries generally takes 8–12 years, from gene discovery to target validation.
As genomics applications emerged, the agricultural biotechnology industry recognized the potential to identify candidate
intervention points for commercial traits.

In reaction to stresses, plants adjust themselves at the levels of morphology, phenology, physiology, and biochemistry.
Because these responses are presumed to be regulated by genes, efforts in recent years have focused on the isolation
and characterization of genes induced by stresses. Among stress-induced genes isolated to date, several major groups
have been targeted for improving abiotic stress resistance in plants. These include genes encoding enzymes for the
biosynthesis of compatible compounds, enzymes for scavenging active oxygen species, heat shock proteins (HSPs),
late embryogenesis-abundant (LEA) proteins, enzymes modifying membrane lipid saturation, transcription factors, and
proteins required for ion homeostasis.

Among abiotic stresses, drought is the most important from an economic standpoint and likely the most intractable to
breeders’ efforts. Difficulties in breeding for drought tolerance are compounded by an incomplete knowledge of the
genetic and physiological bases of yield in water-limited conditions. To overcome the minimal response to direct selection
for yield under drought conditions, substantial efforts have been directed toward manipulation of morpho-physiological
traits that influence drought adaptation through escape, avoidance, and/or tolerance mechanisms. However, this indirect
selection strategy has been successful in only a limited number of cases. More positively, in recent years several genes
that are responsible for different traits associated with drought tolerance have been isolated and characterized.

In another branch of the biotech discipline, an increasing number of studies have strived to map quantitative trait loci (QTL)
affecting drought-related traits and yield in major crops exposed to water deficit. In a limited number of cases, marker-
assisted selection (MAS) has been used as an integral component of breeding strategies to incorporate target traits and to
increase tolerance to drought1. More recently, bioinformatics and the flood of information generated by genomics platforms
have added new dimensions for understanding the role and function of genes governing the response to drought. Despite
these impressive technological breakthroughs, the overall impact of MAS and genomics on the release of drought resilient
cultivars has thus far been negligible.

Possible impact of transgenic drought tolerant crops
As water shortages approach critical levels around the world, particularly in developing countries where more than 60
percent of the inhabitants already live under precarious conditions with no access to safe sources of fresh water, we clearly
need to provide quick solutions to the multi-faceted dilemma of water shortages. One promising approach is to integrate
useful drought tolerance traits through genetic engineering (transgenic intervention points) into lines or varieties at the
advanced breeding level for drought tolerance. Genes carrying these traits are now coming on line and, applied together
with conservation agriculture techniques, may offer developing world farmers a response to water-limited conditions that
is sustainable, economical, and readily adoptable.

Transgenic intervention points and comparative advantages
The use of transgenic intervention points is based on the following premises:
        ISB News Report                                                                                          July 2005

More information about inducible promoters
The most widely used promoters in development of transgenic plants are constitutively expressed, i.e., they are turned on all
the time and throughout the plant. In cases where gene expression needs to be tailored to a specific organ or a specific time,
such a promoter is not a good choice, especially for the stress-induced genes. This is because the constitutive expression of
a stress-induced gene may have serious penalties with respect to energy loss or other adverse side effects. Thus, a stress-
inducible promoter should be considered when the transgenic plants are targeted to deal with abiotic stresses. Study of the
cis-elements responsible for stress induction is necessary to temporally and spatially target the transgenes. Several studies
provide successful examples of the use of an inducible promoter.

Ability to test transgenic plants under field conditions
Overexpression of stress-related genes has afforded some stress protection in transgenic plants2. However, the results
are not always consistent and can even be conflicting. Due to the multigenic nature of stress tolerance, comprehensive
physiological and biochemical testing of transgenic plants under stress conditions must be conducted, which requires a
careful evaluation of the methods for assessing stress tolerance, especially osmotic stress. Desiccation and salt stresses
applied by most researchers are ‘shock’ treatments. For most crops, drought tends to develop slowly as the soil dries.
Plants that are subjected to drought conditions in this gradual manner accumulate solutes that maintain cell hydration and
undergo complex adjustments in their morphology and photosynthetic characteristics. Thus, to ensure that the responses of
the transformed plants to water stress treatments are agronomically relevant, plants must be subjected to the same drought
regime that crops experience in the field.

Ability to transfer multiple genes
While introduction of the key abiotic stress resistance/tolerance genes into plants increased their stress resistance in some
recent experiments, simultaneous transfer of several genes will be the likely next step to achieve practical levels of plant
resistance/tolerance. As abiotic stress resistance is polygenic, plant engineering will require manipulation of complex
metabolic or regulatory pathways involving multiple genes. If, for example, osmoprotectant-producing, transcription factor-
expressing, ion homeostasis-maintaining, in the case of salt, and antioxidant enzymatic activities are all incorporated into
a single cultivar, there is a strong possibility that they could work in concert to overcome concurrent abiotic stresses. This
could be achieved either by transformation with multiple genes or by crossing plants containing different stress tolerance

Developing sustainable agriculture systems
Agronomic measures to reduce yield losses related to drought stress are a key aspect of an integrated set of technologies
called conservation agriculture (CA). The overall objective of CA is to increase yields and sustain crop production and
environmental quality through more efficient use of natural resources and chemical inputs. International and national research
has contributed to the development and testing of CA technologies suitable for local agroecological and socioeconomic
conditions in Africa, Asia, and Latin America, and a lot of expertise is available. To optimally exploit the combined benefits
from agronomic and GE technologies to reduce yield losses due to drought stress, collaborative efforts between genetic
engineers, breeders, physiologists, and agronomists are required. Moreover, integration between agronomic and GE
approaches is pivotal to ensure the sustainability of the technologies with respect to the conservation of soil fertility and
natural resources in the long term. Three basic principles of CA are: (i) zero or reduced tillage; (ii) retention of soil cover
with crop residues and/or cover crops that form a barrier to water loss by evaporation and runoff; and (iii) crop rotations
including, where possible, biological nitrogen-fixing crops. Such systems are increasingly adopted to stop or reverse soil
degradation resulting from unsustainable management practices. Today CA is used on more than one-third of the cropped
areas in Brazil and Argentina, and about 70 million hectares worldwide. CA systems demonstrate clear benefits in terms
of reduced labor and/or fuel inputs. Other advantages include reduced soil erosion, improved water infiltration and soil
structure, increased soil fertility, soil organic matter accumulation, carbon sequestration, soil water-holding capacity, water-
use efficiency, soil biodiversity, and resilience to climate change3. The combined effect of all these factors is claimed
to increase and sustain agricultural productivity with lower water, energy, and labor inputs. Increased adoption of CA
technologies, therefore, not only enhances food security for millions of smallholders in the developing world, but also
reduces detrimental environmental effects.
        ISB News Report                                                                                             July 2005

CA management practices for rain-fed conditions have been developed and their effects investigated in some well-
designed studies. For example, results from the long-term management trials of CIMMYT in the subtropical highlands
of Central Mexico indicate that small-scale maize and wheat farmers in this region may expect yield improvements of
67 and 84 percent for wheat and maize, respectively, after adoption of zero tillage, appropriate rotations, and retention of
sufficient residues as compared to current practices of heavy tillage, monocropping, and burning or removal of residue.

An additional CA technology generating considerable interest is raised permanent beds, where the benefits of zero-tillage
are combined with a bed and furrow system. The practice of planting crops on beds/ridges that are formed between furrows
used to supply irrigation water is widely applied in many (semi)arid regions under irrigated agriculture, although generally
beds are tilled and reshaped before each crop cycle. However, permanent bed systems have also been shown to be a
promising technology in water-limited, rain-fed environments, because they tend to conserve rainfall, prevent runoff, and
provide time for water to infiltrate, with the surplus advantage that more varied weeding and fertilizer application practices
are possible3,4.

Concluding Remarks
Use of transgenics is a large challenge for public breeding programs, but also one of the biggest opportunities. A more efficient
and sustainable agriculture depends on plant varieties and cropping systems with increased resistance to diseases, pests, and
other environmental stresses. Transgenics are a robust source for these traits, although target traits should be identified in
close collaboration with agronomists. CA technologies aimed to establish sustainable, water-conserving cropping systems
may require specific tolerances/resistances or traits, for example herbicide resistance or tolerance against plant pathogens
and drought that are favored in zero-till systems.

In addition, transgenic breeding can provide plant varieties with new or improved nutritional qualities and plants that will
produce renewable industrial products. Such novel varieties are relevant for environmental conservation both in industrial
and developing countries, and for food security and poverty reduction in the developing world. Moreover, the unique and
diverse expertise available within CIMMYT and sister CGIAR centers provides an environment that fosters collaboration
among genetic engineers, breeders, physiologists, and agronomists. Only through such a joint effort can a broad-based
systems approach be developed that incorporates the benefits offered by GE crops and CA systems, and indeed enhances
them, for the good of both farmers and the environment.

1. Nguyen TT, Klueva N, Chamareck V, Aarti A, Magpantay G, Millena AC, Pathan MS & Nguyen HT. (2004) Saturation mapping
of QTL regions and identification of putative candidate genes for drought tolerance in rice. Mol. Gen. Genet. 272, 35-46

2. Govaerts B, Sayre KD, & Deckers J. (2005) Stable high
yields with zero tillage and permanent bed planting? Field Crops Research (in press)

3. Pellegrineschi A, Reynolds M, Pacheco M, Brito RM, Almeraya R, Yamaguchi-Shinozaki K & Hoisington D. (2004) Stress-
induced expression in wheat of the Arabidopsis thaliana DREB1A gene delays water stress symptoms under greenhouse conditions.
Genome 47(3), 493-500

4. Sayre KD. (2004) Raised-bed cultivation. In: Lal, R. (Ed.), Encyclopedia of Soil Science. Marcel Dekker, Inc. eBook Site Online
publication 04/03/2004

                                                                                                     Alessandro Pellegrineschi
                                                                                               Genetic Engineering Laboratory
                                                                   CIMMYT – International Maize and Wheat Improvement Center
                                                                                                               Houston, Texas
                                                                       web: email:

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