Agriculture Reference
In-Depth Information
certain rotations were better able to support the added beneficial organisms and amend-
ments to enable more effective biological disease control. Establishment and persistence
of microbial inoculants, soil amendments, or engineered rhizospheres likely depend on
many edaphic and climatic factors. This emphasizes the need for early testing of promis-
ing biological technologies at the farm or cropping systems level.
Besides biological interactions and adapting to local conditions, technology research
for developing countries must take into account regional cultural and socioeconomic con-
siderations. Since the 1960s, the developing world has become littered with agricultural
technologies that may have been agronomically superior under controlled research condi-
tions but were unacceptable for subsistence farmers because of local culture, lack of techni-
cal support, or scales and markets of rural economies.
To attain practical outcomes for microbial technology, it seems that the emphasis
should be placed on those approaches that depend on manipulating native populations.
For example, there is a body of research for the feasibility of using organic amendments
to create conditions and microbial responses to suppress soilborne diseases naturally.
Similarly, this appropriate technology consideration makes breeding or insertion of trans-
genic genes attractive. Such an approach is desirable because attributes like optimized
roots for efficient water and nutrient uptake or stimulation of beneficial microbial gene
expression would only require the introduction of new cultivars into existing systems—
with minimal technical support and reduction of the need for external inputs.
2.4 Conclusions
The rhizosphere (including the endophytic habitat) is a unique microbial environment that
has an impact on plant growth positively and negatively. Roots alter the physical environ-
ment of soils, release sloughed dead cellular material, and actively excrete organic com-
pounds that promote and sustain biological organisms.
It is now recognized, through the advent of nucleic acid analyses, that soils have a
high degree of diversity, with estimates of 1 million distinct bacterial genomes per gram
of soil. However, more than 99% of these are uncultured. Nonetheless, there have been
important discoveries of microorganisms that can deliver services to plants by stimulat-
ing plant growth and protecting plants from pests, diseases, and environmental stress.
Metagenomics can be used to construct BAC clone libraries to define these communities
based on particular functional genes that are present and potentially active in the com-
munity. This can be used to screen BAC libraries for novel and plant-beneficial functional
genes. A diverse array of bacteria and some fungi has been identified that can improve
crop productivity. One way is that microorganisms can produce PGP substances (e.g.,
plant hormones auxin, cytokinins, IAA, and gibberellin) that directly increase crop yields.
In the recent past, there have been surprising and exciting discoveries for microbial sup-
pression or attack of pathogens or plant protection.
There are two approaches to actively manage crop-associated microbial communities
for beneficial services: (1) adjusting the types and timing of organic inputs, such as cover
crops to stimulate favorable native populations; and (2) inoculation with known microbial
species. In addition, endophytic fungi or bacteria colonizing the host can stimulate mor-
phologically or physiologically systemic resistance in plants to pathogens that can control
or prevent plant diseases. Considerable progress has been made in identifying specific
organisms and managing organic inputs to suppress diseases or protect plants from infec-
tion in developed countries, but this is less so for other microbial-stimulated plant services.
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