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Rhizobium-Initiated Rice Growth Inhibition Caused by Nitric Oxide Accumulation

¹ARC Centre of Excellence for Integrative Legume Research; ²Genomics Interaction Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia; ³Fakultät für Biologie, Universität Bielefeld, 33594 Bielefeld, Germany

Isolates of Rhizobium leguminosarum bv. trifolii (the clover root-nodule endosymbiont) from the Nile River delta have been found to infect rice roots and colonize the intercellular spaces of the rice roots. Some of these isolates inhibit rice seedling growth but one in particular, R4, has been found in rice roots which develop and grow normally. We present evidence that the induced growth inhibition is due to a toxic accumulation of nitric oxide (NO), from the reduction of nitrate, and suggest that the reason that R4 does not inhibit rice root growth is because it is capable of completing the reduction of NO through to nitrogen gas. Thus, strain R4 is a candidate for engineering into a future biological nitrogen fixation system within these roots.

Additional keywords: denitrification, nitrate reduction, nitrite.