Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 157

of photosynthate to use for generating shoot and root biomass. More research on
these relationships is needed, however.
Considered together, these data suggest that there are active forms of communica-
tion between fungal endophytes and plant hosts affecting the expression of genetic
potentials and metabolism of the latter. Individual fungal endophytes have broad
host ranges and confer similar benefits to both monocot and eudicot plants, suggest-
ing that the symbiotic communication involved in these fitness benefits is highly
conserved, predating the divergence of these plant lineages approximately 200 mya
(Chaw et al. 2004; Wolfe et al. 1989; Yang et al. 1999). Since endophytes alter plant
gene expression and phenotype without causing changes in the underlying plant
DNA sequences, as discussed in the next section, and can be vertically transmitted
between generations via seed coats, these symbioses can be defined as intergenomic
epigenetic phenomena.
The ability of fungal endophytes to confer fitness benefits to plants makes them
ideal potential tools to improve agricultural sustainability. These tools represent a
symbiogenic technology (symbio = symbiosis and genic = gene influence), and meth-
ods for large-scale endophyte production, seed treatments, and field applications to
implement this technology are currently under development.
6.8 INFLUENCE OF SOIL MICROBES ON GENE
EXPRESSION IN THE PLANT CANOPY
Establishing the underlying molecular mechanisms that explain how these various
plant growth and health responses are achieved by microbial endophytes living in
plant roots, shoots, and seeds has been challenging. Accepting empirical results,
even if well documented, is often resisted until the mechanisms involved are dem-
onstrated. Since the growth and functioning of plant tissues is regulated ultimately
at the level of gene expression, a proper understanding of endophytic effects needs
to be linked to what is occurring in terms of the plants' DNA and in their protein
expression. Proteomic analysis, which investigates accumulative changes and modi-
fications of proteins, can provide a more precise and comprehensive understanding
of the physiological responses that occur in host plants when in association with soil
microbes.
These relationships have been explored by Chi and associates (2010), following
up their observations reported in Section 6.5. Proteomic analysis of the expression
of specific genes in rice plant cells, causing certain proteins to be produced in plant
tissues above or below ground, has shown that the presence of a certain rhizobia
Sinorhizobium meliloti 1021—versus its absence—induces the production of identi-
fiable proteins that contribute to better plant performance and health. These effects
of rhizobial inoculation are expressed in the leaves, sheaths, and roots of rice plants.
Two-dimensional gel-based comparative proteomic approaches coupled with a
mass spectrometric strategy were used to investigate endophyte-induced changes
in protein expression that could improve the growth of rice and resistance to dis-
ease. The protein profiles in tissues sampled from the roots, the leaf sheaths, and the
leaves of inoculated rice seedlings were compared to the profiles for similar tissues
taken from uninoculated plants, all grown under gnotobiotic conditions.