RA 00110.pdf - OAR@ICRISAT

nists, microbiologists, and agronomists in specialized
areas such as physiology, nutrition, and genetics,
must first describe the form and define the
function of the systems. They must determine the
range of conditions under which these systems do or
do not function. This information may then be used
by plant breeders to produce plant genotypes which
possess the characteristics needed to preferentially
favor rhizosphere colonization by potentially beneficial
indigenous soil microbes. The requisite microbes
are already there. A highly favorable environment in
the form of a "genetically engineered" rhizosphere
must be provided in order to increase their numbers
and activity.
Details of how this might be accomplished are the
domain of plant geneticists. One possible approach
may be for the breeder to avoid selecting plant genotypes
that show maximum fertilizer response. Many
root-microbe associations form more readily and
yield maximum benefits under low-fertility conditions.
Perhaps through utilization of more primitive
germplasm, plants could be bred or selected which
grow reasonably well under a range of adverse conditions.
Soil moisture, pH, temperature, etc., and
the microbiological component of the soil environment,
can be modified only to a limited extent on a
field scale, if at all. The most promising alternative
appears to be breeding plant genotypes which will
inherently encourage or support beneficial R - M
associations capable of enhancing plant growth.
Summary
The symbiotic nitrogen-fixing association of Rhizobium
with legumes is successfully exploited agronomically
via inoculation. The rhizobia infect and
fix nitrogen intracellularly in the roots via a mechanism
which permits direct and efficient exchange of
substrates and products in an environment largely
insulated from process-limiting environmental factors.
The rhizobia can be grown in the massive
amounts required for large-scale field application.
The symbiotic association of V A M fungi with
herbaceous plants, which enhances plant uptake of
phosphorus (and other beneficial effects), is also
biologically feasible for agronomic exploitation via
inoculation. Again, this is because the fungi establish
an intimate intracellular association in the roots
*
of the host plants; this permits efficient plantmicrobe
nutrient exchange in a system that is largely
insulated against potentially process-limiting factors.
Future agronomic exploitation on a practical
field scale may be largely dependent on development
of means to grow the fungi independently in mass
culture as an inoculant source.
Cryptic R - M associations, as exemplified by the
AzospiriJIum-grass association, are known to possess
characteristics which can enhance plant growth
under special conditions. These include nitrogenfixation
and production of PGPS. The microorganisms
can be grown in mass culture, so availability of
inoculant poses no limitation to agronomic exploitation.
However, the absence of a specialized morphological
and physiological intimacy in this type of
association makes successful, wide-scale exploitation
via inoculation impossible or highly improbable.
A highly promising alternative to inoculation as a
means of exploiting these cryptic associations is to
breed plant genotypes with genetically-based characteristics
which determine rhizosphere
properties conducive to establishment and function
of indigenous cryptic microbes. Whatever the mechanism,
these microbes would have the requisite ability
to enhance plant growth.
References
Alexander, M. (ed.) 1984. Biological nitrogen fixationecology,
technology, and physiology: proceedings of a
Training Course on Biological Nitrogen Fixation and its
Ecological Basis, 18-29 Jan 1982, Caracas, Venezuela. New
York, USA: Plenum Press.
Bagyaraj, D.J. 1984. Biological interactions with VA
mycorrhizal fungi. Pages 131-153/n V A mycorrhiza (Powell,
C.U., and Bagyaraj, D.J., eds.). Boca Raton, Florida,
USA: CRC Press.
Broughton, W.J. (ed.) 1982. Nitrogen fixation. Vol. 2.
Rhizobium. Oxford, UK: Clarendon Press. 353 pp.
Brown, M.E. 1974. Seed and root bacterization. Annual
Review of Phytopathology 127:181-197. 107 ref.
Dobereiner, J., and Day, J.M. 1976. Associative symbiosis
in tropical grasses: characterization of microorganisms and
dinitrogen fixing sites. Pages 518-538 in Proceedings of the
First International Symposium on Nitrogen Fixation, 3-7
Jun 1974, Pullman, Washington, USA (Newton, W.E., and
Nyman, C.J., eds.). Pullman, Washington, USA: Washington
State University Press.
Dommergues, Y., and Krupa, S.V. (eds.) 1977. Interactions
between non-pathogenic soil microorganisms and
plants. Amsterdam, Holland: Elsevier 475 pp.
Elmerich, C. 1984. Molecular biology and ecology of Diazotrophs
associated with non-leguminous plants. Biotechnology
November 1984: 967-984. 264 ref.
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