Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 152

The physiological benefits of the endophytic rhizobia-rice association are mani-
fested in many ways. These include increased phytohormone production (auxins and
gibberellins) accompanied by an enhanced root architecture that enables the plants
to be better miners of essential nutrients within the soil; increased nutrient uptake
efficiency and photosynthetic capacity; solubilization of both inorganic and organic
P reserves; and modulation of plant phenolic metabolism involved in innate plant
defenses (Biswas et al. 2000a; Chi et al. 2005; Mishra et al. 2006; Yanni et al. 2001).
More recently, extensive field inoculation tests (Yanni and Dazzo 2010) have
demonstrated the efficacy of using rhizobial endophytes as biofertilizer inoculants
for rice based on findings that certain selected strains boost fertilizer-N use effi-
ciency and grain production of rice. This enables farmers to significantly reduce
their requirements for chemical fertilizer inputs to achieve maximal grain yield
potential under real-world agronomic field conditions.
6.6 EFFECTS OF SOIL MICROBIAL ENDOPHYTES
IN PLANT CANOPIES
The research in Egypt was consistent with the effects seen in the research reported in
Section 6.2 from Madagascar, where populations of beneficial symbiotic endophytes,
Azospirillum rather than rhizobia, were apparently enhanced by the crop, soil, water,
and nutrient management practices introduced by SRI. That soil microbes would
enter into plant roots and live within them to mutual benefit was somewhat surpris-
ing, but could be easily understood given that roots were intimately associated with
the rhizosphere soil surrounding them, and this soil supported high concentrations
of beneficial soil organisms (Vessey 2003 and references therein).
These insights were taken a step further by research carried out in China by Chi
and associates, together with Dazzo and Yanni (Chi et al. 2005). Their data showed
that certain microorganisms migrate from the soil into the roots, as discussed below,
and then ascend upward through the roots and stems into the sheaths and lower
leaves of rice plants. As seen below, the presence of soil rhizobia in the canopy
of plants is associated with both morphological and physiological changes that are
advantageous for plant productivity.
Chi and his colleagues used five different strains of soil rhizobia that are known
to inhabit the root nodules of leguminous plants as endosymbionts. The roots of five
sets of rice plants (Japonica varieties Zhonghua 8 and Nipponbare) were inoculated,
each set with a different strain of rhizobia, with an additional set of uninoculated
plants grown as a control. The soil had been sterilized so that the trials were gnoto-
biotic. The experimenters had genetically tagged the rhizobia with a reporter gene
that constitutively expresses the green fluorescence protein so that movements of
these microbes could be definitively tracked and their local population densities in
the plant could be evaluated by computer-assisted fluorescence microscopy, so as to
assess their influence on the growth physiology of rice plants.
The migration of rhizobia within the plants after inoculation was documented
by laser scanning confocal microscope examination, as was their surface and endo-
phytic colonization of healthy rice plant tissues. The study definitely confirmed
earlier findings (Prayitno et al. 1999; Reddy et al. 1997; Yanni et al. 1997, 2001),