Agriculture Reference
In-Depth Information
Considered collectively, these findings indicated that the natural endophytic
Rhizobium -rice association is more complex, inclusive, dynamic, and invasive than
previously thought, with soil microbes colonizing both above-ground and below-
ground tissues. Physiological and morphological parameters were demonstrably
enhanced in response to inoculation with several rhizobial species. In addition, sig-
nificant agronomic benefits to inoculation of rice with selected strains of endophytic
rhizobia have been repeatedly established under field conditions. But the specific
mechanisms for these effects, beyond some positive effect on phytohormone produc-
tion, remained to be identified and demonstrated. Results from further investigation
into mechanisms are reported in Section 6.7 after considering some results of con-
temporaneous research on fungal endophytes in rice seeds.
6.7 EFFECTS OF SEED ENDOPHYTES FOR ROOT
EMERGENCE AND GROWTH
That microbial endophytes by inhabiting rice plant tissues can enhance crop growth
and production outcomes attributed to soil system fertility are documented (Biswas
et al. 2000a,b). Less known is that microorganisms can also inhabit seeds to mutual
benefit; and while much is known about rhizobacteria as endophytes, fungi can also
have symbiotic relationships with plants (Rodriguez et al. 2009a,b). Note that we are
not considering here the well-known contributions of mycorrhizal fungi that also
live symbiotically in the roots of most terrestrial plants and confer many functional
advantages to their plant hosts, particularly water and nutrient uptake.
We focus here on other fungal endophytes that have been shown to protect plants
from both biotic and abiotic stresses and that can have profound impacts on plant
physiology. Plants colonized with symbiotic fungal endophytes can be more robust,
e.g., with greener leaves, more tillers in rice, and larger root systems, and can grow
faster and use less water than nonsymbiotic plants without endophytes (Rodriguez et
al. 2009b; Aly et al. 2011; Redman et al. 2011).
Fungal endophyte-induced changes in plant physiology occur very early in seed-
ling development (Rodriguez et al. 2009a). Upon germination, symbiotic seedlings
develop root systems more rapidly, allocate carbon differently, and produce more
biomass (shoots and roots) compared to nonsymbiotic seedlings. Trials showed that
rice seedlings emerging from seeds inoculated with the fungus Fusarium culmorum ,
generally considered a plant pathogen, had more rapid root growth after germina-
tion, as much as five times faster in the first 5 days, with root hairs emerging 2 days
sooner ( Figure 6.1 ). Such endophyte-induced plant growth responses occur under
greenhouse or field conditions, with symbiotic plants accumulating 25% to 50%
more biomass than nonsymbiotic plants (Redman et al. 2002, 2011) ( Figure 6.2 ).
Why more of the inoculated plant's growth resources should be allocated in early
growth to the root relative to the shoot is not clear; but the effect of inoculation with
fungal endophytes on seedlings' performance is very visible and statistically sig-
nificant. Plants with more vigorous root growth in their early stages of development
are better positioned for subsequent nutrient and water uptake and for supporting
the growth of the canopy. It has also been determined that plants inoculated with
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