RA 00110.pdf - OAR@ICRISAT

Introduction
Root-microbe associations which enhanced plant
growth were observed in ancient times. The biological
nature of such phenomena was not understood
until the advent of the microscope and the discovery
of microorganisms. The more easily observed causeand-effect
relationship of microbial pathogens on
plants, and their obvious devastating effects, prompted
early and intensive study of root-microbe (R-M)
interactions of a pathological nature. Recognition
and study of beneficial R - M interactions or associations
has lagged far behind. Nonetheless, the concept,
or the vague realization of the presence of these
associations, has been with us for many years. Inoculation
of legumes with nitrogen-fixing Rhizobium to
enhance soil fertility was practiced in a crude form as
early as 300 BC. The practice was not put on a firm
scientific foundation until the classic studies of the
phenomenon by Hellriegel and Willfarth, and Lawes
and Gilbert, followed by isolation of the microorganism
by Beijerinck (reviewed by Fred et al. 1932).
The suspicion that plants, or more specifically
plant roots, had to contend with more than just
chemical and physical factors of the soil environment
has been confirmed. The concept that microorganisms
are a constant and critical biological factor
in the soil environment of plant roots was given
clear definition by Hiltner's statement of the "rhizosphere
effect" (Katznelson et al. 1948). Since that
time, soil microbiologists have achieved a better
understanding of the topic. The nature and implications
of many kinds of nonpathological R - M associations
has been comprehensively discussed (Dommergues
and Krupa 1977).
In this paper, the term R - M association is used
arbitrarily in a restricted sense, to imply R - M associations
which are to some degree mutually beneficial
or symbiotic in their interaction. Two cases in
which the major benefit seems to favor the plant
component are discussed: Rhizobium-legume and
mycorrhizal associations. This is followed by a
general discussion of "cryptic" or "associative" R - M
systems of a decidedly less obvious beneficial nature.
With that background, the relative potential of these
associations as plant growth enhancing systems is
discussed. Certain ideas, concepts, and generalities
are described which can be confidently extrapolated
to pearl millet and are relevant to R - M associations.
Literature citations are limited to monographs,
reviews, and in some cases, individual papers which
have been pivotal in developing our current conception
of R - M associations. This is done reluctantly; a
great debt is owed to the many scientists whose
excellent work, although uncited here, has been
invaluable to the formulation of this paper.
Symbiotic R - M Associations
Numerous characteristics of the Rhizobium-legume
system contribute to its successful role of facilitating
legume growth:
• There is a unique, almost absolute, specificity of
rhizobia for infection, nodulation, and nitrogen
fixation in plant roots of the Leguminosae.
• The rhizobia provide the plant with an essential
nutrient, nitrogen, which is the element that most
often limits plant growth. This is done by utilizing
atmospheric nitrogen, "fixing" or reducing it
from that unavailable form to a form which is
available to support growth of both the plant and
the rhizobia.
• Specialized structures, nodules, are induced to
form on the roots of successfully infected legumes.
These nodules are a rare example of an instance
where, in most cases, a pure culture of an organism
occurs in nature. The unique mechanism of
root infection by rhizobia generally precludes the
entry of other microbes, thereby evading competitive
effects.
• Simple microbiological methods can be used to
isolate the homologous rhizobia from these nodules.
These isolates can be grown, purified, and
maintained in a virulent state on common laboratory
media. This permits large-scale culture of the
rhizobia to produce the large amounts of inoculum
required. Nitrogen fixation, whether by
chemical or biological means, is an energyintensive
process. Indeed, spiraling energy costs
to produce nitrogen fertilizer by chemical means
are the strongest justification for our efforts to
enhance biological fixation of nitrogen in legumes
and other plant families.
• The root nodule is a specialized structure; it has
morphologically and functionally differentiated
tissues. A critical feature is the vascular strands
which ramify from the root stele into the socalled
bacteroid zone, located more or less centrally
in the nodule, where nitrogen fixation
occurs. This feature is vital to the success of the
interaction since it provides a means for direct
exchange of required nutrients between plant and
microbe. Rhizobia receive the energy substrate
required for fixation, from photosynthate trans-
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