Agriculture Reference
In-Depth Information
Mayang Segumpal when colonized by
Rhizobium
sp. (SB16) (4.47% tissue N), while the
MR219 rice cultivar had the highest N content when inoculated with
Corynebacterium
sp.
(4.30% tissue N). These results were similar to those of Rosenblueth and Martínez-Romero
(2006) and Gyaneshwar et al. (2002), which showed an interaction between rice cultivars
and microbial species relative to N fixation. Naher et al. (2009) showed that both strains
were infecting surface and endophytic locations in both rice cultivars and speculated that
the interaction of diazotroph by rice cultivar may be due to differences in root exudates
and the plant root tissue architecture. These results demonstrated the importance of mul-
tidisciplinary research among microbiologists, plant physiologists, and plant breeders to
optimize the N fixation by diazotrophs.
The most notable example for practical utilization of biological N
2
fixation is on
sugarcane (
Saccharum
spp.), for which 60-70% of the N comes from endophytic bacteria
(Boddey et al., 1995, 2003; Saravanan et al., 2008). An important diazotrophic species on
sugarcane (and other crops) is
Gluconacetobacter diazotrophicus
, which is a N-fixing, aerobic
α-proteobacterium (Alvarez et al., 1995) isolated from sugarcane that colonizes the plant's
apoplast and xylem (Fuentes-Ramírez et al., 1999; Dong et al., 1994) in an endophytic man-
ner (Baldani and Baldani, 2005; Reis et al., 2000).
Typically, the largest plant response to diazotrophic bacteria is on degraded soils
and in water-stressed regions when low or intermediate levels of fertilizer are applied
(Dobbelaere et al., 2001). This is encouraging for farmers in developing countries because
these are exactly the kind of soils and conditions they regularly encounter.
To develop this technology, fundamental research is needed as the detailed mecha-
nisms of this N-fixing symbiosis are not well understood. There needs to be a clear sepa-
ration of the role of phytohormones in both stimulating plant growth and its role on N
fixation. Information is needed on the role of physiological plant responses to endophytic
diazotrophs and which plant responses are needed to enhance N fixation. For example,
roots treated with natural, synthetic, or precursors of auxin will result in the formation of
nodular-like tumors (paranodules) that diazotrophs colonize and exhibit some degree of
nitrogenase activity. However, it is unclear whether this is advantageous over other sym-
biotic relationships between diazotrophs and normal roots (Vessey, 2003).
Another question that needs to be answered is the energy, nutritional, or signaling
requirements needed to stimulate N-fixing bacteria. It may well be that considerably more
C is needed than is currently produced in modern nonlegume crop rhizospheres. For
example, legumes can fix agronomically significant levels of N but do so by utilizing over
30% of the photosynthate produced by the host plant. Modern breeding programs that
have developed crops under high N levels that emphasize aboveground yield may have
selected against rhizospheres that promote N-fixing microorganisms.
Most soils are deficient in P readily available to plants. In contrast to N, which can be
obtained from the atmosphere through biological fixation, P must be supplied to plants by
native P or through the application of fertilizer. The majority of applied P is rapidly “fixed”
in plant-unavailable soil fractions (Sanyal and De Datta, 1991). The strong chemical reactiv-
ity of phosphate (P) results in very low plant recovery of only 10-20% of the P applied to
soils (McLaughlin et al.,
1988; Holford, 1997), and global annual consumption of P-based fer-
tilizers exceeds 30 million tons (International Fertilizer Industry Association [IFIA] 2001). It
is expected the world's known reserves of high-quality rock phosphate will be consumed
within the next 80 years (Isherwood, 2000). However, most farmers in developing coun-
tries have used very little P fertilizer because of a lack of infrastructure for delivery or low
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