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

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Rice–Rhizobia Association: Evolution of an Alternate Niche
of Beneficial Plant–Bacteria Association
Ravi P.N. Mishra, Ramesh K. Singh, Hemant K. Jaiswal, Manoj K. Singh, Youssef G. Yanni,
and Frank B. Dazzo
9.1
Introduction
The vision of self-fertilizing crops contributed to the euphoria created by the emergence
of biotechnology and the Green Revolution. In his 1970 Nobel Peace Prize
lecture, Norman Borlaug highlighted the need to extend the range of symbioses to
include nitrogen-fixing bacteria, such as rhizobia, and cereals to sustain the Green
Revolution. He went on to acknowledge that even though high-yielding dwarf rice
and wheat varieties were the catalysts that had ignited the Green Revolution, chemical
fertilizers were the fuel that gave it thrust [11]. Since then, there have been
extensive discussions on the prospects of establishing such novel symbiotic systems
including research plans for their implementation. However, it is now clear that the
energy required for the reduction of nitrogen to ammonia in nitrogen fixation is not
greater than that required for the production of ammonia by reduction of nitrate, the
main form of nitrogen assimilated by plants. Consequently, cereals such as rice
would not likely suffer any significant energy penalty if they were supporting nitrogen
fixation [12].
A huge amount of natural gas is consumed in the synthesis of nitrogenous
fertilizer as anhydrous ammonia. In addition, this industrial process produces
carbon dioxide, the main cause of greenhouse effect and global warming. The
industrial production of nitrogenous fertilizer is also expensive, and in developing
countries, the additional costs often exceed the means of low-income farmers,
limiting the yield potential of their crops. Once chemical fertilizers are applied,
additional residual problems can arise. Roughly one third of the nitrogen fertilizer
applied is actually used by the crop. The nonassimilated nitrogen may result in
nitrate (NO3 ) contamination of groundwater [13], posing a serious health hazard.
In addition, excess nitrogen can also lead to soil acidification [14] and increased
denitrification resulting in higher emission of nitrous oxide (N2O), another potent
greenhouse gas that may exacerbate global warming [15]. Therefore, cropping
Plant-Bacteria Interactions. Strategies and Techniques to Promote Plant Growth
Edited by Iqbal Ahmad, John Pichtel, and Shamsul Hayat
Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31901-5
j165