Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

182j 9 Rice–Rhizobia Association: Evolution of an Alternate Niche
Table 9.3 Colonization potential of R. leguminosarum bv. trifolii
strain E11 and isolate BSS 202 from S. spontaneum on four rice
varieties preferred by low-income Filipino farmers.
Inoculum strain Sample location Colonization potential (viable plate count/three rice
roots)
Rlt E11 External rooting medium 1.03 10 7
BSS 202 External rooting medium 9.17 10 7
Rlt E11 Root surface 6.34 10 7
BSS 202 Root surface 1.08 10 7
Rlt E11 Root interior 1.50 10 8
BSS 202 Root interior 7.50 10 7
9.7
Mechanisms of Plant Growth Promotion by Endophytic Rhizobia
The ability of some endophytic rhizobial strains to promote the growth of rice and
wheat prompted follow-up studies to identify possible mechanisms operative in this
beneficial plant–microbe interaction. These studies have focused primarily on rice,
addressing the following possible mechanism of rhizobial growth promotion: (1)
induction of an expansive root architecture having enhanced efficiency in plant
mineral nutrient uptake; (2) production of extracellular growth-regulating phytohormones;
(3) solubilization of precipitated inorganic and organic phosphate complexes,
thereby increasing the bioavailability of this important plant nutrient; (4)
endophytic nitrogen fixation; (5) production of Fe-chelating siderophores; and (6)
induction of systemic disease resistance.
9.7.1
Stimulation of Root Growth and Nutrient Uptake Efficiency
Sinandomeng PSBRC 74 PSBRC 58 PSBRC 18
1.50 10 7 7.16 10 7 8.17 10 7
7.00 10 6 8.00 10 7 1.27 10 7
1.05 10 8 6.50 10 7 6.50 10 7
6.00 10 7 5.60 10 7 5.66 10 7
2.19 10 9 1.32 10 8 1.03 10 9
6.30 10 6 7.34 10 7 1.23 10 8
Source: From Yanni et al. [7].
Reprinted with permission from CSIRO Publishing (http://www.publish.csiro.au/journals/
fpb).
Responsive rice varieties commonly develop expanded root architectures when
inoculated with candidate biofertilizer strains of rhizobia. This suggests that these
rhizobial endophytes alter root development in ways that could make them better
miners , more capable of exploiting a larger reservoir of plant nutrients from
existing resources in the soil. This possibility was suggested in early studies showing
significantly increased production of root biomass in plants that had been inoculated
[1,7,42,57] and by studies using greenhouse-potted soil indicating significant increases
in N, P, K and Fe uptake by rice plants inoculated with selected rhizobia,
including rice endophyte strains [58]. More recent studies have confirmed this result
using plants grown gnotobiotically with rhizobia in nutrient-poor medium (50%
Hoaglands), followed by measurements of root architecture and mineral nutrient