Agriculture Reference
In-Depth Information
Beneficial microorganisms can increase nutrient uptake and efficiency for crops. The
most remarkable process among plant nutrients is biological N fixation because there is an
inexhaustible supply from the atmosphere. Outside N fixation by the symbiotic relation-
ship of
Rhizobium
with leguminous plants, for nonsymbiotic microorganisms the most
notable example is on sugarcane, for which 40-70% of the required N can be supplied by
diazotrophs. However, for other nonlegume crops the rates of N fixation are much lower,
typically 5-10% at best. The major challenge for free-living N fixers likely is the large
amount of energy that is needed to support diazotrophs to produce N at levels that are
agronomically significant. Phosphorus availability can be enhanced by certain microor-
ganisms. This is accomplished by (1) solubilization of inorganic P by acidification (e.g.,
production of organic acids, carbonic acid, or H
+
) or chelation; (2) mineralization of organic
P by producing extracellular phosphatases or phytases; and (3) increased root growth or
root hair development by production of phytohormones to enable greater exploration of
soil for P uptake.
Interestingly, there are certain bacteria that can reduce crop water stress. Bacteria have
been isolated that can reduce water stress of crops by increasing seedling root elongation
(promotes stand establishment) and various crop physiological responses (reduction in
cell elasticity and osmotic potential and rise in apoplastic water fractions). Also, autoch-
thonous AM can increase plant resistance to water stress.
Of special note relative to beneficial microorganisms for crops is the symbiotic fungal
species mycorrhizae. For crop species, endomycorrhizae form arbuscular structures and
extensive hyphal networks that can transport P and other nutrients to the plant to help
relieve drought stress in crops.
This information indicates there is great potential for microorganisms to benefit crop
yields and quality and to develop truly sustainable systems that eliminate or greatly
reduce external, synthetic inputs. However, considerable research is needed to make these
practical and agronomically effective. This will require a multidisciplinary approach of
plant scientists and soil microbiologist to bioengineer crops and the associated manage-
ment systems to utilize beneficial microorganisms effectively. This necessitates fundamen-
tal microbial ecology research on microbial-plant interactions.
An emerging and potentially critical area of rhizosphere ecology is cell-to-cell commu-
nication
using signaling compounds. This communication
can be among microorganisms
and between microorganisms and plants. There are likely a large number of chemicals
that allow microorganisms to coordinate gene expression on a population-wide scale in
concert with crop growth responses. One such system is QS, which involves production,
secretion, and subsequent detection of small hormone-like signaling molecules known
as autoinducers. Fundamental research is needed on the types of communications, cata-
loguing of signaling compounds and their sources, microbial and plant gene expression
in response to signals, genetic controls on signaling, and signaling processes that elicit
beneficial crop growth and protection responses. Clearly, the convergence of new research
avenues of “-omics,” bioinformatics, and micro- or nanoscale biochemistry sensing tech-
nologies offers great potential to understand these complex cell-to-cell communications.
With such a foundation, one can imagine bioengineered crops with genes that can be
turned on when they are experiencing a nutrient, environmental, or pathogenic stress;
this results in an appropriate microbial response. If such a breakthrough could be made,
it offers the possibility to “program” plant roots to release compounds and activate benefi-
cial microbial responses at a critical growth stage.
To attain practical outcomes for microbial technology, it seems that the emphasis
should be placed on those approaches that depend on manipulating native populations.
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