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2-Plants: Poor record of 40 years of biological research to improve plant yields

  "Genetic engineering techniques are frequently proposed as ways to
   increase crop yields, especially in areas of the developing world where
   the people suffer from malnutrition and agricultural productivity is
   low. However, despite 40 years of biochemical and physiological
   research, there have been very few cases that led directly to
   improved cultivars with better yield. [...] Much biochemical and
   physiological research has focused on drought tolerance, to enable
   plants to survive long periods of drought. However, for most annual
   grain crops, a drought severe enough to threaten the plant's survival
   will inevitably result in such a low yield that survival is a moot
   point. Therefore, there is probably no point in trying to use inserted
   genes from so-called 'resurrection plants' to sustain food production
   during droughts. The original premise for the trait is irrelevant to
   the cropping situation."

-------------------------------- GENET-news -------------------------------

TITLE:  Integrating the GM approach to improve crop yield
SOURCE: SciDev.Net, UK, by Tom Sinclair
DATE:   18 Aug 2005

------------------- archive: -------------------

Integrating the GM approach to improve crop yield

Genetic engineering techniques are frequently proposed as ways to
increase crop yields, especially in areas of the developing world where
the people suffer from malnutrition and agricultural productivity is low.
However, despite 40 years of biochemical and physiological research,
there have been very few cases that led directly to improved cultivars
with better yield.

This research has greatly improved our understanding of molecular traits
and the factors associated with crop yields. However, the fact that there
are so few examples of this research leading directly to better crop-
yield suggests that we should be cautious how much molecular biology can
improve crop yields in the near term.

The challenge of boosting yield potential

There are two basic approaches to increasing yield potential:
- Increasing plants' overall physiological capacity to produce
harvestable yield under various environmental conditions (yield potential).
- Reducing the effects of biological stresses such as diseases, insects,
and weeds that prevent plants from reaching their theoretical yield potential.

The first approach depends on the straightforward logic of identifying a
crop plant's specific function or functions that could be improved to
increase yield potential. Scientists have used this approach to target
particular metabolic 'control points' so that plant genes thought to
limit a crop's basic yielding capacity could be targeted for improvement.

The concept of modifying a crucial biochemical or physiological step to
achieve yield increase is not new, however. The advent of 'scientific
agriculture' after World War II resulted in previously unimagined
increases in crop yield. Relatively cheap fertilisers became available,
and the addition of quantities of specific nutrients to the soil,
especially nitrogen, was an important factor in increasing yields. So
also was the development of plant varieties suitable for high-fertility
conditions. National averages in various countries reached six to eight
tons per hectare.

But progress was slow in research aimed at improving specific biochemical
or physiological traits. This was certainly not due to lack of effort.
Scientists identified superior cultivars and established genetic
heritability for some important traits that they thought were associated
with yield. However, few improved varieties with enhanced yield were
developed using this approach.

There are several explanations for these disappointments. First, the
anticipated benefits were sometimes simply not transferable to the field
situation. The activity of a metabolic pathway can be enhanced or
diminished, but it may be irrelevant in increasing crop yield.

Second, molecular modification often had a greatly diminished effect on
crop growth as its influence moved up through a crop's organisational
hierarchy, from the molecular level to the organelle, cellular, organ,
plant and crop levels. Biochemical compensation by other pathways may
also moderate the effect of the original modification

Finally, the challenge that appears to have often proved to be a critical
limitation was a research structure in which it was difficult to carry a
concept of genetic variation from the process level through to the
development of a viable commercial variety. The rare successes in the
past in developing crop varieties using this approach illustrate that
this research requires integrated, multidisciplinary teams with career
and financial commitments to sustain long-term research efforts lasting
12 to 15 years.

Possible target traits for genetic transformation


Photosynthesis is the process by which plants use sunlight to assimilate
carbon. Considerable early excitement was generated as genetic lines with
superior leaf photosynthetic activity were identified and the
photosynthetic capacity was successfully bred into progeny lines of
several crops, including maize, wheat, and soybean. Disappointment
followed when these 'improved' lines failed to produce yield increases.

The reason for this disappointment is that the benefits of improved
photosynthesis at the cellular and leaf level do not translate directly
into larger grain yield. In particular, improved photosynthesis -- which
boosts carbohydrate production -- may lead to larger vegetative growth,
which will increase the plant's demand for nitrogen. Unless the plant can
take up more nitrogen from the soil, grain yield may actually decrease.
This is because the limited supply of nitrogen may end up producing
bigger plants rather than more grain. The plant's needs for resources
such as carbohydrates, nitrogen and sulphur need to be addressed together.

Nitrogen assimilation

In the past, a plant's nitrogen accumulation has been a crucial feature
of yield increases. Usually, this has resulted from making more nitrogen
available to plants (for example, by applying nitrogen fertilisers to the
soil), and by breeding new plant varieties that can take up and store
more nitrogen. A key factor in these successes has been improving the
amount of accumulated nitrogen that gets stored in the grain, rather than
being 'locked up' in the plant's vegetative tissues.

A target for genetic engineers has been to increase 'nitrogen-use
efficiency.' However, efforts to improve a plant's nitrogen metabolism by
changing its genome are unlikely to succeed because plant biochemistry is
already extremely 'efficient' in nitrogen uptake and use. It may be
difficult to improve whole plant traits because it seems unlikely that
engineering a single or even a few genes can easily manipulate these traits.

Seed growth

A large amount of past research has focused on increasing seed growth
rates and overcoming the problem of seed-embryo abortion. However,
researchers have found that plants are well endowed with redundancies and
backups to optimise grain production in a range of environments. Plants
tend to compensate for an increase in one factor by decreasing others.
Research has shown that in a community of plants, if one seed is growing
more quickly, the plant compensates by changing the number of seeds or
the duration of seed growth, resulting in little or no increase in
overall yield.

In a few isolated cases, researchers have succeeded in improving seed
growth rates by genetically engineering plants to be less sensitive to
phosphorus feedback inhibition in the grain. However, they found that
seed growth was accompanied by increases in the growth of individual
plants. As a result, there was no change in the overall harvest index
(the ratio between the harvested grain and the total accumulated crop
mass) for rice and only a small increase in wheat.

Drought stress

Scarce water is a critical limitation on crop yield in many places. How
efficiently a crop uses water directly influences yield potential, but it
is not very flexible because of the physical and physiological
characteristics of gas exchange in leaves (transpiration). Although there
has been some success in improving water-use efficiency in wheat, any
major increases in yield still depend on more water being available. This
means that the crop must access more soil water in dry-land conditions.

Much biochemical and physiological research has focused on drought
tolerance, to enable plants to survive long periods of drought. However,
for most annual grain crops, a drought severe enough to threaten the
plant's survival will inevitably result in such a low yield that survival
is a moot point. Therefore, there is probably no point in trying to use
inserted genes from so-called 'resurrection plants' to sustain food
production during droughts. The original premise for the trait is
irrelevant to the cropping situation.

Successful research to achieve yield increase

Although this wealth of research has considerably improved our
understanding of plant growth and crop yield in grain crops, a targeted
approach of increasing specific physiological traits has resulted in very
few improved cultivars with increased yield potential. The lack of
success illustrates the difficulties involved in translating insights and
breakthroughs at the micro level into real improvements in crop quality
at the macro level.

These failures offer important lessons for molecular genetics research,
which is even further removed from grain yield. The lesson is that
genetic engineering research will probably confront many of the same
obstacles that have limited the impact of previous biochemical and
physiological research.

Nevertheless, there are a few successful examples in which physiological
research and genotypic selection has played an integral role in
developing useful new cultivars that produce better yields. Three such
cases are described in boxes 1, 2 and 3.

Box 1: Heat tolerance in cowpea

In the early 1980s researchers under the leadership of Dr Anthony E. Hall
at the University of California, Riverside, United States, observed that
a failure in seed set was a potential problem in cowpea. They found that
high night-time temperatures were damaging pollen viability. The research
team identified the sensitive step in pollen formation where the problem
was occurring. A field screen to identify lines that were heat tolerant
was developed and the tolerant lines were crossed with lines having
desirable agronomic traits. In 1999 the programme released a heat-
tolerant variety of cowpea for commercial use.

Ehlers J.D., Hall A.E., Patel P.N., Roberts P.A., Matthews W.C. 2000.
Registration of 'California Blackeye 27' Cowpea. Crop Science 40:854-855.

Box 2: Water-use efficiency in wheat

Water deficits can cause serious losses of yield in wheat production in
Australia. Researchers at the Australian National University and Plant
Industry, CSIRO in Canberra (Australia) initiated a research programme to
improve the water-use efficiency of wheat. Measuring this trait proved to
be very difficult and they put considerable effort into developing a
technique for characterising differences in water-use efficiency among
various lines. Eventually, they identified wheat lines with superior
water use efficiency and used them as parents in a breeding programme.
Ultimately, several new wheat varieties have been made available to
farmers, which increase yield by up to 10% under dry conditions.

Rebetzke, G.J., Condon, A.G., Richards R.A., Farquhar G.D. 2002.
Selection for reduced carbon isotope discrimination increases aerial
biomass and grain yield of rain-fed bread wheat. Crop Science 42:739-745.

Box 3: Nitrogen fixation in soybean

Soybean has the advantage of being able to accumulate atmospheric
nitrogen through symbiotic nitrogen fixation. In the 1980s it was shown
that this process was especially sensitive to soil drying. Differences in
sensitivity among soybean lines were documented. A research team
including me and colleagues at the Universities of Florida and Arkansas
(USA) found that the loss in nitrogen fixation activity was associated
with the accumulation of ureides, which are the transport products from
nitrogen fixation. We used this correlation to identify soybean lines
with a nitrogen fixation tolerance to soil drying. Using these as
parental lines, we established a breeding programme, which will soon
release higher-yielding soybean varieties for non-irrigated conditions.

Sinclair, T.R., Purcell L.C., Vadez V., Serraj R., King C.A., Nelson R.
2000. Identification of soybean genotypes with N2 fixation tolerance to
water deficits. Crop Science 40:1803-1809.

The rare successes of increasing crop yield potential by altering a
targeted physiological trait may offer important lessons for achieving
success using GM techniques. It is clear that the existence of genetic
diversity for a particular trait -- whether from natural diversity or
transgenic techniques -- is only a small, first step in achieving
measurable yield improvements for farmers. The challenges have been to
understand the altered trait's effect on crop performance in the field
and to exploit the trait in a crop-breeding programme. The examples
described in the boxes had characteristics in common, that were crucial
in achieving success using the physiological approach.

(1) Early assessment of the potential beneficial trait.

Early in each programme, the researchers gave considerable attention to
understanding and documenting the trait of interest under field
conditions, rather than relying on extrapolations from laboratory study.
Under what conditions would the trait be beneficial? What are the
consequences of trait expression on crop performance?

Integrating genomics, mapping and physiology might enable scientists to
develop molecular hypotheses that begin at the top of the trait hierarchy
rather than in the laboratory. Input from whole-plant physiologists and
agronomists will be needed to make an early assessment of how proposed
genetic modifications might improve the plant. It may also be advisable
to use systems analysis technology to make an early assessment of the
proposed GM trait or crop's economic or commercial viability.

(2) Effective phenotyping of genetic modifications

A crucial challenge in using a transgenic approach to improve yield is
forecasting what will happen when a transformed trait is expressed. Trait
expression depends on both the physical environment in which the plants
grow and the genetic environment into which the trait has been inserted.

The past successful studies gave considerable attention to characterising
trait expression under a range of field environments. It was not
sufficient simply to know that the genetic advantage existed in the
plant. Rather, it was necessary to document the trait's level of
expression under a range of conditions in which successful varieties will
be commercially grown.

Phenotypic expression requires extensive testing, including evaluating
plant performance and yield in a cropping situation. This is likely to
increase the demand for rapid and inexpensive methods for phenotyping
plants, especially for traits that do not have a readily visible expression.

(3) Multi-disciplinary effort

The successes documented in boxes 1 to 3 involved contributions from
different disciplines throughout the research effort, including crop
physiologists, agronomists and breeders. These three disciplines are
still important and necessary in developing GM plants offering new
genetic variability. The molecular genetic approach merely adds a further
layer to this team. Indeed, system and environmental analysts may need to
be involved as well, to assess where and when the expression of a GM
trait might be commercially beneficial.

The GM approach will likely need early involvement of all disciplines
simply to move the new plants forward for field assessment. In past
successes, researchers used early field screening to identify candidate
lines with both the desired trait and a reasonable capacity for growth
under field conditions. GM plants may require early attention from all
participants, to move the trait into more viable plant material before
starting trait assessments.

This interactive, team-based approach will require the team to be well
integrated and coordinated at all stages of the research programme. This
will probably present new challenges, especially for public research

(4) Long-term commitment

The examples show that it took 12 to 15 years to move from the initial
studies of a physiological trait to the release of a commercial variety
with improved yield. Although the molecular genetics approach can speed
up some of the steps involved, additional efforts to fully document the
consequences of introducing a new trait could well offset this gain. It
seems likely that a successful programme to generate improved cultivars
will require a team of scientists to work together for more than a
decade. This may be difficult because the traditional time horizons among
the various disciplines tend to differ widely. Further, commitment to a
team effort may be difficult when the probability of individual
recognition may not be high.

The greatest limitation, however, may be the financial commitment
required for long-term team research. Public funding based on a series of
two- to three-year grants does not encourage initiating team research to
undertake a high-risk, multi-year project. Private companies may be in no
better position to fund long-term research, because of the large
uncertainty about eventually generating a commercially viable product.

Reprinted from Trends in Plant Science, 9, Sinclair et al, Crop
transformation and the challenge to increase yield potential, pp 70-75,
Copyright (2004), with permission from Elsevier


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