Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 160

Certain defense-related proteins were also up-regulated in the shoot tissues,
including S-adenosylmethionine synthetase, which is involved in methyl group
transfers and polymerization of lignin monomers to reinforce the plant cell walls,
and catalase, a well-known ROS [H 2 O 2 ] detoxification enzyme. These results indi-
cated that the rhizobial-mediated activation of the plant's defensive reactions occurs
not only in the roots but also in above-ground tissues, which might improve the stress
resistance of inoculated rice plants.
Taken together, these results predict that plant growth-promotion mechanisms
affected by endophytic rhizobia may operate in these modes: (1) activation of defense
mechanisms in different plant tissues below and above ground to minimize the nega-
tive effects of environmental and pathogenic factors; (2) enhancement of anabolism,
for example, photosynthesis, to increase the biomass of the plant; and (3) regulation
of auxin levels or status that promotes vegetative growth.
Exactly what signals the rhizobia send to trigger these genetic-expression
responses remain to be determined. But the molecular details involved in the inti-
mate and mutually beneficial association between these soil microorganisms and
crop plants have been experimentally validated. This relationship is one in which
soil scientists, crop scientists (especially physiologists), and microbiologists (and
microbial ecologists) should all have mutual interest.
6.9 IMPLICATIONS FOR ADVANCEMENT OF SOIL SCIENCE
It is noteworthy that many of the phenotypical advantages that have been reported
for rice plants grown under SRI management based on many observations and
validated by scientific evaluations match the physical characteristics that Chi,
Dazzo, Rodriguez, and their colleagues have reported as associated with symbi-
otic endophytes: more vigorous growth of roots and canopy; increased plant nutri-
ent uptake efficiency; more resistance to water and other abiotic stresses; more
resistance to insects and pathogens; slower or delayed senescence; higher levels of
chlorophyll and higher rates of photosynthesis; higher agronomic N-fertilizer use
efficiency; and higher grain productivity (see particularly the findings in Thakur
et al. 2010).
Correlations of course do not establish causation, so there is much scientific
research to be done in this promising area. But the recently reported findings of
Chi and associates identifying differences in gene expression—with up-regulation
or down-regulation of specific genes fitting patterns of observed phenotypical varia-
tion—are consistent with observed relationships between certain management prac-
tices for plants, soil, water, and nutrients and improved crop outcome. While the
latter could be taken as evidence of soil fertility, probably we should be construct-
ing a different language and analytical framework to encompass the more complex
biological relationships seen here in relation to the results now associated with soil
fertility.
We should get beyond the tacit assumptions that plants operate like organic,
C-based machines that extract inorganic nutrients and water from the soil, trans-
forming them into carbohydrates and other compounds by utilizing solar energy in
photosynthetic processes that lead to biomass outputs from a mechanistic process.