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OCR for page 5978
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 5978-5981, May 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Plants and Popak,fion: Is There Time?"
held December 5-6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA.
Transgenic plants for tropical regions: Some considerations about
their development and their transfer to the small farmer
LUIS HERRERA-ESTRELLA~
Departamento de Ingenieria Genetica, Centro de Investigacion y Estudios Avanzados, Apartado Postal 629 C.P. 36500 Irapuato, Guanajuato, Mexico
ABSTRACT Biotechnological applications, especially
transgenic plants, probably hold the most promise in augment-
ing agricultural production in the first decades of the next
millennium. However, the application of these technologies to the
agriculture of tropical regions where the largest areas of low
productivity are located, and where they are most needed,
remains a major challenge. In this paper, some of the important
issues that need to be considered to ensure that plant biotech-
nology is effectively transferred to the developing world are
discussed.
The world's population is expected to double by the year 2050,
making food security the major challenge for the next millen-
nium. Food production will have to be doubled or preferably
tripled by the year 2050 to meet the needs of the expected 11
billion people, of whom ninety percent will reside in the devel-
oping world. The enormity of the challenge is significantly
increased by the declining availability of water and the fact that
this additional food will have to be produced on existing agricul-
tural land or in regions considered as marginal soils, if we want
to preserve the forested regions and the environment as a whole.
Agricultural research and technological improvements are, and
will continue to be, required for increasing agricultural produc-
tivity. There are numerous ways in which agricultural productivity
may be increased in a sustainable way, including the use of
biological fertilizers, improved pest control, soil and water con-
servation, and the use of improved plant varieties, produced by
either traditional or biotechnological means. Of these measures,
biotechnological applications, especially transgenic plant variet-
ies, probably hold the most promise for augmenting agricultural
production and productivity, when properly integrated into tra-
ditional systems.
The Case for Genetically Engineered Plants. The effectiveness
of tran~genic plant varieties in increasing production and lowering
production cost has been demonstrated in the cases of virus-,
insect-, and herbicide-resistant plants, in which an average in-
crease in production of SO to logo, and a saving in herbicides of
up to 40~o and in insecticides of between $60 to $120 per acre,
have been reported in 1996 and 1997 (1~. However, these
increases in productivity, impressive as they are in terms of their
economic and environmental value, will have a limited impact for
global food supply. In fact, most of the developments in transgenic
crops are aimed either at reducing production costs in agricultural
areas that already have high productivity levels or at increasing
the value added to the final product by improving, for instance,
oil quality. This trend has been stimulated by the current policies
of developed countries to limit production of key products such
as cereals, meat, and dairy products because of the reductions in
international prices of these products, and to reduce the intensive
use of fertilizers and pesticides because of their deleterious effects
on the environment.
PEAS jig available online at www.pnas.org.
In a global sense, a more effective strategy would be to increase
productivity in tropical areas, where an increase in food produc-
tion is needed and where crop yields are significantly lower than
those obtained in developed countries. In tropical areas, the
losses caused by pests, diseases, and soil problems are exacerbated
by climatic conditions that favor high levels of insect pests and
vectors and by the lack of the economic resources to apply
insecticides and fertilizers and to purchase high-quality seeds.
In addition to low productivity levels, postharvest losses in
tropical areas are very high, again because of climatic conditions
that favor fungal and insect infestation and because of the lack of
appropriate storage facilities. Despite efforts to prevent prehar-
vest and postharvest crop losses, pests destroy over half of all
world production. Preharvest losses caused by insects, the ma-
jority of which occur in the developing world, are calculated at
around 15% of the world's production.
Using biotechnology to produce transgenic plants that better
withstand diseases, insect attack, or unfavorable soil conditions,
is not a simple task. There are an estimated 67,000 species of
insects worldwide that damage crops and a similar or even higher
number of plant pathogens. For instance, in the case of Phaseolus
vulgar, over 200 diseases and 200-300 species of insects can
affect bean productivity (2~. These numbers give an idea of the
complexity of the task that scientists face in increasing produc-
tivity. There are of course a certain number of diseases and insect
pests that can be singled out as the most important constraints for
the production of each crop. However, it is also true that when a
particular disease or insect pest is controlled, others considered
as minor can then flourish, and themselves become major pro-
ductivity constraints.
One of the major advantages of plant biotechnology is that it
can generate strategies for crop improvement that can be applied
to many different crops. In this sense, genetically engineered virus
resistance, insect resistance, and delayed ripening are good
examples of strategies that can benefit many different crops.
Transgenic plants of over 20 plant species that are resistant to
more than 30 different viral diseases have been produced by using
different variations of the pathogen-derived resistance strategy.
Insect-resistant plant varieties, using the 6-endotoxin of Bacillus
thuringiensis, have been produced for several important plant
species including tobacco, tomato, potato, cotton, walnut, maize,
sugarcane, and rice. Of these, maize, potato, and cotton are
already under commercial production. It is envisaged that these
strategies can be used for many other crops important for
developing countries. Genetically engineered delayed ripening,
although tested only on a commercial scale for tomato, has an
enormous potential application for tropical fruit crops, which
suffer severe losses because they ripen rapidly, and in many
developing countries there are neither appropriate storage con-
ditions nor adequate transportation systems to allow their effi-
cient commercialization.
. . .
*To whom reprint requests should be addressed. e-mail: lherrera@
. . .
~rapuato.~ra.c~nvestav.mx.
5978
OCR for page 5979
Colloquium Paper: Herrera-Estrella
To date, most of the developments in plant gene transfer
technology and the different strategies for producing improved
transgenic plant varieties have been driven by the economic value
of the species or the trait. These economic values are in turn
mainly determined by their importance to agriculture in the
developed world, particularly the United States and Western
Europe. This economical emphasis is understandable, because
important investments are needed to develop, field test, and
commercialize new transgenic plant varieties. However, in terms
of global food production, it is necessary to ensure that this
technology is effectively transferred to the developing world and
adapted to the local crops and/or local varieties of crops for which
it was originally developed.
Developing improved transgenic versions of local varieties or
local crops is not a trivial issue; in most, if not all, cultures, the use
of specific crops has a deep social and/or religious meaning.
Cultural preservation is just as important as environmental
preservation. Cultural aspects of technology transfer need to be
considered because simply replacing crops to increase produc-
tivity could have an enormously negative effect for certain
cultures, and new introductions may not be accepted easily for
human consumption.
It is unfortunate that most developing countries do not have
sufficient resources to implement the biotechnological capacity
needed to solve the major problems that limit agricultural pro-
ductivity, at least not in the time frame that is required to cope
with the increasing demand for food. However, it is in the
developing world that biotechnology could have its major impact
in increasing crop production, especially in the areas of the world
where yields are low because of the lack of technology.
Plant genetic engineering could be considered a neutral tech-
nology that in principle does not require major changes in the
agricultural practices of farmers in developing countries. Perhaps
more importantly, it has the potential to bring about great
benefits to the small farmers who lack the economic resources to
purchase agrochemicals or prevent postharvest losses because of
the lack of storage facilities.
Whether there is time to increase agricultural productivity in
the developing world is a question with a complex answer,
because there are many factors that need to be taken into account
to make this happen. We need to identify and establish mecha-
nisms of technology transfer from developed countries, from both
academic institutions and the private sector, to the developing
world; there is a need to create a sufficient number of research
centers with the capability of acquiring this technology, adapting
it to local crops, and developing their own technologies. Seed
production facilities must be improved and an effective mecha-
nism implemented to reach subsistence farmers with this new
technology. To meet these requirements, several economic, po-
litical, and social issues must be dealt with to ensure the general
application of plant biotechnology to the agriculture of develop-
ing countries. The discussion of these issues goes beyond the
scope of this article. However, it is my personal opinion that it will
not be technological limitations but rather political and/or eco-
nomic constraints that will determine how successful we are in
supplying food to the hundreds of millions of people who will be
malnourished in the next millennium.
Soil Acidity: A Problem for Agriculture in the Tropics. Esti-
mates of the world's potentially arable land resources indicate that
only 10.6% of the total land area of the world is cultivated, and
about 24.2~o is considered cultivable or is potentially arable land
(3-5~. Of these 2.5 billion hectares of potentially cultivable land,
68% is located in the humid tropics (6~. The acid soils of the
tropics, especially in the savannas, that historically have resisted
permanent settlement and agricultural use are considered to
represent the largest remaining potential for future agricultural
development (7~.
There are problems limiting food production that are specific
or more significant to the agriculture of tropical and subtropical
regions, but that unfortunately have not been given sufficient
Proc. Natl. Acad. Sci. USA 96 (1999) 5979
importance to deserve being considered priorities in the research
being done in developed countries. However, solutions to these
problems could significantly contribute to food production in
tropical areas. Because many of these problems are common to
many developing countries and affect the productivity of a wide
spectrum of crops, transgenic strategies to solve them that can be
applied to different plant species are urgently needed.
Among the problems common to tropical regions, probably the
most important is soil acidity. On a global scale, there are two
main geographical belts of acid soils: the humid northern tem-
perate zone that is covered by coniferous forest and the humid
tropics, which are (or in some cases were) covered mainly by
savanna and tropical rain forest. Soil acidification can develop
naturally in humid climates when basic cations are leached from
soils but can be considerably accelerated by certain farming
practices and by acid rain (84.
Acidic soils comprise about 3.95 billion hectares of the ice-free
land or approximately 40% of the world's arable land. Regions
with subsoil acidity occupy about 20% of the ice-free land surface.
Approximately 43% of the world's tropical land area is classified
as acidic, comprising about 68% of tropical America, 38% of
tropical Asia, and 27% of tropical Africa (9, 6~.
Tropical Acid Soils That Could Be Used for Agriculture.
Because a great proportion of forest land is located in acid soils,
it is important to remember that not all soils, although potentially
arable land, can be used for agriculture. Tropical forests are
invaluable with regard to their role in local, regional, and global
ecosystems and to the biodiversity found within them (over 90%
of plant and animal species live in forest ecosystems). Indiscrim-
inate conversion of tropical forest into agricultural land will have
far-reaching ecological consequences, whose effects will certainly
outweigh the potential gain in food production. In spite of these
consequences, 11 million or so hectares of forest are cleared every
year, of which only a small fraction is converted into productive
agricultural land, and most of it becomes unproductive grassland
(6~.
Policies to use acid soils for agriculture should be directed to
the acid savannas of the world such as the Cerrado in Brazil, Los
Llanos of Venezuela and Colombia, the savannas in Africa, and
the largely anthropic savannas of tropical Asia. These acid
savannas cover an area of over 700 million hectares (which is
approximately 50% of the global area that is currently under
cultivation), and their potential in food production for both
humans and animals could account for a large portion of that
required to satisfy the need of the growing population in the next
millennium. There are good examples in Brazil and Asia of
successful development of acid savanna into productive land for
the cultivation of sugarcane and soybean (64. The use of biotech-
nology could facilitate enormously the conversion of low-
productivity acid savannas into productive cropland.
Aluminum Toxicity. Poor crop productivity and soil fertility in
acid soils are mainly caused by a combination of aluminum and
manganese toxicity and nutrient deficiencies (mainly deficiencies
in P. Ca, Mg, and K). Among these problems, aluminum toxicity
has been identified as the most important constraint for crop
production in acid soils. Aluminum toxicity problems are of
enormous importance for the production of maize, sorghum, and
rice in developing countries located in tropical areas of Asia,
Africa, and Latin America. Most maize, sorghum, and rice
cultivars currently being used are susceptible to toxic aluminum
in the soil, and decreases in yield of up to 80% resulting from
aluminum toxicity have been extensively reported in the literature
(10-12~. In particular, maize and sorghum production is severely
limited in tropical Africa, where over 45% of the total land area
in countries such as Zaire, Zambia, and the Ivory Coast is covered
by acidic soils.
In tropical South America, aluminum toxicity is a problem
shared by several countries, where about 850 million hectares, or
66% of the region, has acid soils. In Brazil alone, acid savannas
with low cation exchange capacity and high toxic aluminum
OCR for page 5980
5980 Colloquium Paper: Herrera-Estrella
saturation cover 205 million hectares, of which 112 million are
suitable for maize and sorghum production (9~.
Aluminum has a dear toxic effect on roots, disturbing plant
metabolism by decreasing mineral nutrition and water absorp-
tion. The most easily recognized symptom of Al toxicity is the
inhibition of root growth, and this has become a widely accepted
measure of Al stress in plants. Although Al toxicity primarily
restricts root growth, given sufficient exposure, myriad different
symptoms appear on both roots and shoots that are often
mistaken for soil nutrient deficiencies. Therefore, crop produc-
tion in acid soils is, to a great extent, limited by nutrient uptake
deficiency caused by the inhibition of root growth and function
that results from the toxic effects of Al (13~. Moreover, in some
acid soils, plant growth is affected not only by aluminum toxicity
but also by low availability of some essential elements such as P.
Ca, Mg, and Fe, some of which form complexes with Al and
consequently are not readily available for root uptake (14~.
It is well documented that many plant species exhibit significant
genetic variability in their ability to tolerate A1. Although it is
clear that certain plant genotypes have evolved mechanisms that
confer Al resistance, the cellular and molecular basis for Al
resistance is still poorly understood (13~.
Two basic strategies by which plants can tolerate Al have been
proposed: (i) the ability to exclude Al entry into the root apex and
root hairs, and (ii) the development of mechanisms that allow the
plant to tolerate toxic concentrations of Al within the cell.
Several conceptually attractive hypotheses have been proposed
to explain how plants could exclude A1 from entering into the
root. For example, mechanisms based on alteration in rhizo-
sphere pH, low cell-wall cation-exchange capacity, or Al +3
efflux across the plasma membrane (13~. However, experimental
evidence from several research groups supports a mechanism that
results in Al exclusion from the root apex via the release of
Al-binding ligands such as malic and citric acids. When these
ligands are released into the rhizosphere, they can effectively
chelate Al+3 and prevent its entry into the root.
The potential role of organic acid release in Al tolerance was
originally proposed by Miyasaka et al. (15~. Their work showed
that the root system of an al-tolerant snapbean cultivar grown in
Al-containing solutions released 10 times as much citrate as an
Al-sensitive cultivar grown In the presence of A1. the most
complete analysis of the possible role of organic acids as Al+3-
chelating molecules in naturally resistant plants comes from the
work, done by Delhaize and coworkers (16, 17), using near-
isogenic wheat lines differing at the Al tolerance locus (Altl).
These researchers found that, on treatment with Al, tolerant
wheat varieties release 5- to 10-fold more malate than do sus-
ceptible lines, and that this increased capacity to excrete malate
correlated with Al resistance and Al exclusion from the root apex.
Because malic acid excretion is located in the root apex, the
amount of malic acid excreted depended on the external Al
concentration, and the Al tolerance cosegregates with high rates
of malate excretion, Delhaize et al. (16, 17) proposed that the Altl
locus in wheat encodes a component of an Al-tolerance mecha-
nism based on the Al-stimulated excretion of malic acid.
The existence of Al-tolerance mechanisms based on the ex-
cretion of organic acids has also been reported for plant species
other than wheat: citrate in the case of maize, snap beans, and
Cassia tora (18-20), and oxalic acid for buckwheat (Fagopyrum
esculentum Moench) (20~.
An Example of How Transgenic Plants Could Improve Pro-
ductiv~ty in Acid Soils. The production of Al-tolerant transgenic
plant varieties should be considered an important part of crop
management strategies to increase agricultural production on
acid soils and to protect forests around strongly acidified indus-
trial regions.
Generation of metal-tolerant plants through genetic engineer-
ing has been demonstrated to be a valid approach. For instance,
expression of the alpha domain of human metallothionine IA in
transgenic tobacco plants confers cadmium resistance (21~.
Proc. Natl. Acad. Sci. USA 96 (1999)
The production of transgenic plants with an increased capacity
to produce and/or excrete organic acids that chelate and detoxify
Al in the rhizosphere is an appealing strategy to produce Al-
tolerant plants. The effectiveness of citric acid in alleviating Al
toxicity has also been demonstrated by adding citrate to solutions
containing toxic levels of Al, which reverses the inhibition of
wheat root growth caused by Al (22~. Citric acid forms a strong
chelate with Al, typified by a stability constant of 5 x 108 M-~,
which is about 700-fold greater than the corresponding value for
the malate-Al complex (234.
Citrate overproduction, therefore, appears to be an ideal
candidate to produce Al-tolerant transgenic plants. To test
whether citrate overproduction could be achieved in transgenic
plants and to assess the impact of elevated levels of citrate on
aluminum tolerance, our research team produced transgenic
tobacco lines that overexpress the citrate synthase from Pseudo-
monas aeruginosa in their cytoplasm (24~.
To produce these plants, a chimeric gene, in which the coding
sequence of the P. aerug~nosa citrate synthase gene (25) tran-
scriptionally fused to the 35S promoter from the cauliflower
mosaic virus, was introduced into the genome of tobacco plants.
Biochemical analysis of these transgenic tobacco lines showed
that most of them had elevated levels of citrate synthase and that
they contained in their roots 10-fold higher levels of citrate and
exuded five times more of this organic acid into the rhizosphere
than did their nontransformed siblings.
Because the evidence for the role of organic acid excretion in
aluminum tolerance is rather indirect, it was important to deter-
mine whether the lines with elevated levels of citrate synthesis and
excretion are less or equally susceptible than wild-type plants to
phytotoxic concentrations of A1. It was observed that the inhi-
bition of root growth by phytotoxic concentrations of Al is
significantly lower in the citrate synthase overproducing lines than
in the control (24~.
To test whether the same strategy could be used in other plant
species, the chimeric gene encoding the bacterial citrate synthase
was used to transform papaya plants, a crop that is grown in
tropical areas where aluminum toxicity limits its cultivation. It was
found that transgenic papaya plants expressing the bacterial
enzyme developed roots at concentrations of up to 150 mM Al,
whereas the controls failed to do so in concentrations above 50
mM (24~.
The finding that in two different plant species an increased
capacity to produce and excrete citrate led to Al tolerance
suggests that this strategy might be useful in many different plant
spec~es.
The production of Al-tolerant plants is just one example of
what plant biotechnology could do to improve productivity in
developing countries. Drought-tolerant plants or plants with an
enhanced capacity to take up nutrients that are present in tropical
soils, but that are not readily available for plant nutrition are
examples, among others, of technology that could be produced by
genetic engineering means, and that could significantly elevate
productivity.
Transfer of Technology to Developing Countries. Most of the
available technology for producing improved transgenic plant
varieties could effectively be used to improve productivity in
developing countries. Because most, if not all, of these technol-
ogies have been patented and belong to private corporations, a
major challenge is to identify and establish the mechanisms to
effectively transfer this technology to developing countries. Sev-
eral avenues could be followed: one would be the training of
scientists from developing countries in universities, research
institutes, and companies in developed countries; a second one is
to assist developing countries in establishing their own facilities
for biotechnological research; and the third one is to transfer
technology, by means of gene constructs or transgenic plants,
from universities or companies to the existing research centers in
the developing world.
OCR for page 5981
Colloquium Paper: Herrera-Estrella
In terms of training and capacity building, several foundations
and government agencies have important programs. Good ex-
amples of successful programs of this kind are the rice and cassava
programs of the Rockefeller Foundation, the program of the
Biotechnology Action Council of the United Nations Educa-
tional, Scientific and Cultural Organization (UNESCO) and the
International Cooperation (INCO) program of the European
Commission, among others. In particular, the success of the
Rockefeller rice biotechnology program can be highlighted,
which in a few years stimulated biotechnological research for this
crop in the U.S. and Europe and facilitated the establishment of
rice molecular biology research in many laboratories in many
countries in the developing world.
In the short term, the direct transfer of technology may be the
most effective strategy to implement plant biotechnology to
increase productivity in the developing world. Technology trans-
fer can be done by using end products (transgenic seed) that have
been developed for the agriculture of advanced countries. Trans-
fer of end products in some cases will be done anyhow if the
market is of value to the companies; however, the use of such seed
will probably be limited to intensive agriculture of a nature similar
to that for which the transgenic seed was originally developed. It
would be more interesting if transgenic seeds were transferred to
national breeding programs, which could be used as the basis for
developing local varieties better suited to local environment and
soil conditions.
Another possibility is to transfer gene constructs to research
institutes that have the capacity to introduce this genetic material
into local crops or varieties. How to achieve this is still not
completely clear, but a number of mechanisms are beginning to
be explored. Transferring this technology has some problems; for
instance, when royalties can be waived and when not. Perhaps a
naive approach would be to reach agreements in which the
technology is donated on a royalty-free basis if it will be used only
for production aimed at internal markets of developing countries.
In cases where export is possible, royalties should of course be
paid; if the farmers can export their products, however, they
should at least have certain capacities to share their increased
income with the providers of the technology.
To be able to meet the needs of developing countries with
technology available in public and private institutions in devel-
oping countries, a source of easily accessible information will be
needed, which preferentially indicates which institutions or com-
panies agree in principle to donate technology. Initial attempts in
this direction have been carried out by the International Service
for the Acquisition of Agrobiotech Applications (ISAAA). This
nonprofit organization is attempting to play the role of an "honest
broker," identifying needs in their target countries and assisting
a national institution to reach an agreement with the companies
that have the technology that can potentially solve the problem.
An example of this is the agreement between the Centro de
Investigaci6n y Estudios Avanzados in Mexico and Monsanto to
develop virus-resistant potatoes for the Mexican market (26~.
Having a company as a partner makes it more likely that all the
steps, from basic research to field evaluation, are carried out
successfully. These still-limited initiatives should be significantly
enhanced to make sure that plant biotechnology is transferred to
developing countries at an adequate pace.
To ensure effective technology transfer, each recipient country
must have a research center with the capacity to assimilate the
technology and apply it to local crops or local varieties. Although
several developing countries, such as Brazil, Argentina, India, and
China, have at least some of this capacity, it is clear that not all
developing countries have research institutes with sufficient in-
frastructure and trained personnel to effectively participate in this
Proc. Natl. Acad. Sci. USA 96 (1999) 5981
process. It is therefore urgent that the countries that do not have
such capabilities give priority to the establishment of research
groups, as well as the regulatory bodies required to assess and
approve the use and commercialization of genetically modified
Organisms.
Even if technology is successfully transferred to developing
countries and transgenic varieties are developed for local crops,
the problem of getting this technology to the small farmer is still
an important challenge. The government of each country needs
to implement a system for producing and distributing transgenic
seeds and any other input, at low or no cost, to the small farmer.
Whether technology transfer to developing countries takes place
will, of course, depend on the political will of each national
government and the resources required.
The work on acid soils in my laboratory was carried out with support
of the Howard Hughes Medical Institute and the Rockefeller Foun-
dation. I thank June Simpson for critically reviewing this manuscript.
3.
4.
20.
21.
22.
23.
24.
25.
26.
1. James, C. (1997) Int. Ser. App. Agro. Apply Briefs 5, 1-20.
2. Van Schoonhoven, A. & Voysest, O. (1980) in Bean Production
Problems in the Tropics, eds. Schwartz, M. & Pastor-Corrales, J.
(Centro Internacional de Agricultura Tropical, Call, Colombia),
2nd Ed. pp. 33-58.
U.S. President's Advisory Committee Report (1967), pp. 20-45.
Food and Agriculture Organization (FAO) (1991) World Soil
Resources Report 66.
Buringh, P., van Haenst, H. D. & Staring, Y. (1975) J. Exp. Bot.
24, 1189-1195.
6. Von Uexkull, H. R. & Mutert, E. (1995) Plant Soil 171, 1-15.
7. Dunal, R. (1988) in Management and Fertilization of Upland Crops
in the Tropics, ed. Wang, Y. (Nanjing Institute of Soil Science,
Nanjing, China), pp. 1-5.
8. Kennedy, I. R. (1986) in The impact on the Environment of
Nitrogen and Sulfur Cycling, ed. Kennedy, I. R. (Cambridge Univ.
Press, Cambridge, U.K.), pp. 34-92.
9. Pandey, S., Ceballos, H., Granados, G. & Knapp, E. (1994) in
Stress Tolerance Breeding: Maize That Resist Insects, Drought, Low
Nitrogen and Acidic Soils. Maize Program, A Special Report, eds.
Edmeades, G. E & Deutsch, J. A. (Centro Internacional de
Mejoramiento de Ma~z y Trigo, Mexico D. A.
10. Brenes, E. & Pearson, R. W. (1973) Soil Sci. 116, 295-302.
11. Lopes, A. S. & Cox, F. R. (1977) Soil Sci. Am. J. 41, 743-747.
12. Saigusa, M., Shoji, S. & Takahashi, T. (1980) Soil Sci. 130,
241-250.
13. Kochian, L. V. (1995) Annul Rev. Plant Physiol. Plant Mol. Biol.
46, 237-260.
14. Haug, A. (1984) CRC Crit. Rev. Plant Sci. 1, 345-373.
15. Miyasaka, S. C., Buta, J. G., Howell, R. K. & Foy, C. D. (1991)
Plant Physiol. 96, 737-743.
16. Delhaize, E., Craig, S., Beaton, C. D., Bennet, R. J., Jagadish,
V. D. & Randall, P. J. (1993) Plant Physiol. 103, 685-693.
17. Delhaize, E. & Ryan, P. R. (1995) Plant Physiol. 107, 315-321.
18. Pellet, D. M., Grunes, D. L. & Kochian, L. V. (1995) Plan ta 196,
788-795.
19. Ma, J. F., Zheng, S. J. & Matsumoto, H. (1997) Plant Cell Physiol.
38, 1019-1025.
Ma, J. F., Zheng, S. J., Matsumoto, H. & Hiradate, S. (1997)
Nature (London) 390, 569-570.
Pan, A., Tie, F., Duau, Z., Yang, M., Wang, Z., Li, L., Chen Z.
& Ru, B. (1994) Mol. Gen. Genet. 242, 666-674.
Ownby, J. D. & Popham, H. R. (1989) J. Plant Physiol. 135, 588-
591.
Suhayda, C. G. & Haug, A. (1986) Physiol. Plant. 68, 189-195.
De la Fuente, J. M., Ram~rez-Rodr~guez, V., Cabrera-Ponce, J. L.
& Herrera-Estrella, L. (1997) Science 276,1566-1568.
Donald, L. J., G. F. Molgat & Duckworth, H. W. (1989) J.
Bacteriol. 171, 5542-5550.
Rivera-Bustamante, R. (1997) in Plant Biotechnology Transfer to
Developing Countries, eds. Altman, D. & Watanabe, S., R. G.
(Lances, Austin, TX).
Representative terms from entire chapter:
developing world
