Agriculture Reference
In-Depth Information
Another beneficial mechanism is the optimization of growth conditions by
Pseudomona
s
spp. by removing ethylene (naturally produced by plants during metabolism; inhibits root
growth) from the root zone. They do this by producing the enzyme 1-aminocyclopropane-
1-carboxylate (ACC)-deaminase, which hydrolyzes the ethylene precursor ACC (Penrose
and Glick, 2001).
Some examples of organisms inoculated on crop roots that increase yields or growth
include
Azospirillum
on a variety of crops;
Rhizobium leguminosarium
on rice and wheat (but
interestingly only in the presence of N fertilizer) (Biswas et al., 2000a, 2000b);
Pseudomonas
on potatoes (Kloepper et al., 1980);
Bacillus
spp., producing various gibberellins, on alder
(Gutierrez Munero et al., 2001);
Pseudomonas fluorescens
strain on radish; and
Bacillus licheni-
formis
on
Pinus pinea L.
seedlings (Probanza et al., 2002). However, regionality and local
environments need to be investigated to determine the universality for using phytohor-
mone-producing microorganisms. This was shown on a strain of
B. japonicum
that grew
best under cooler soil conditions, with increased soybean protein (Zhang et al., 2002).
Earlier reviews have reported yield increases of inoculated over uninoculated controls
of 5-30% in about 70% of inoculation trials reported, mostly with
Azospirillum
(Okon and
Labandera-Gonzalez, 1994) and 12-30% on maize at various sites in the United States
(Riggs et al., 2001).
Why rhizobacteria have evolved to produce phytohormones is not well under-
stood. It may be that this stimulates greater production of root exudates to increase
root development, which provides enhanced nutritional benefits and habitable space
for microbial colonization, giving those microorganisms with phytostimulating prop-
erties an evolutionary advantage. The possible role of rhizosphere microorganisms
that produce plant growth stimulation presents an exciting potential to increase crop
yields naturally (Broughton et al., 2003). However, the mechanisms underlying this are
poorly understood. These organisms operate within complex communities and often
have multiple effects on plants. For example,
Azospirillum
produces phytohormones
but also can fix N
2
. Commercially available biological inoculation technologies are
emerging in Western countries, but most of these products are not based on solid sci-
ence. Larkin (2008) showed that several commercial biologicals did increase yields but
only under certain crop rotations.
The research required to fully develop phytostimu-
lating microbial systems will require work at all levels, from ecology to proteomics
and metabolomics.
2.2.2 Disease-suppressive soils and plant-protecting microorganisms
Since the mid-1990s, there have been surprising and exciting discoveries for natural meth-
ods to suppress or eliminate pathogens or protect plants. Intensive studies of disease-
suppressive soils have led to the development of new methods of analysis (Gross et al.,
2007; Borneman et al., 2007; Bolwerk et al., 2005; Benitez et al., 2007) and new insights into
the nature of soilborne disease suppression (Weller et al., 2002; Hoitink and Boehm, 1999).
Such advances indicate that active management of soil microbial communities can be an
effective approach to develop natural suppression of soilborne diseases and improve crop
productivity (Mazzola, 2004; Raaijmakers et al., 2009). Generally, there are two approaches
to actively manage crop-associated microbial communities.
The first approach is to develop disease-suppressive soils through manipulation of
carbon inputs. This involves adjusting the types and timing of organic inputs, such as
cover crops (Widmer et al., 2002), animal manures (Darby et al., 2006), composts (Abbasi
et al., 2002; Darby et al., 2006), organic amendments (Rotenberg et al., 2005; Stone et al.,
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