The release of biological nitrification inhibitors from sorghum root is regulated at the transcriptional level
Sorghum (Sorghum bicolor) roots release biological nitrification inhibitors (BNIs) to suppress soil-nitrifier activity and soil-nitrification. The presence of NH4+ in the rhizosphere stimulates the release of BNIs from roots and is hypothesized to be functionally linked to plasma membrane (PM) H+-ATPase activity. However, whether the H+-ATPase is regulated at the transcriptional level, and if so, which iso-forms of H+-ATPases are involved in BNIs release are not known. Also, the stimulation of BNI release, whether it is due to NH4+ uptake or its assimilation in roots, is unclear, and it would be subsequently addressed by this study.
NH4+ concentrations up to 1.0 mM positively stimulated both PM H+-ATPase activity and BNI release from sorghum roots; but at higher concentrations (>1.0 mM), NH4+ did not further increase BNI release and a decline in PM H+-ATPase activity was observed (Fig. 1a, b). Vanadate, an inhibitor of H+-ATPases, suppresses BNI release from intact sorghum plants (Fig. 1c). Twelve PM H+-ATPase genes (iso-forms, designated as SbA1 to SbA12) were identified in sorghum genome; however, only five H+-ATPase genes were stimulated by NH4+ in the rhizosphere. They have a similar expression pattern and is consistent with the observed variation in H+-ATPase activity (Fig. 2). Methyl-ammonium (MeA), a non-metabolizable analogue of NH4+, had no significant effect on BNI release, H+-ATPase activity, or the expression of H+-ATPase genes (Fig. 3). These results suggest that the functional link between PM H+-ATPase activity and BNI release is operational only at NH4+ concentrations of ≤1.0 mM in the rhizosphere. The variation in PM H+-ATPase activity by NH4+ is due to transcriptional regulation of five iso-forms of H+-ATPases. The stimulatory effect of NH4+ on BNI release is functionally associated with NH4+ assimilation and not from NH4+ uptake alone.
A mechanistic understanding of BNI release in sorghum helps in choosing suitable agro-ecological niche production systems where BNI function is expressed to its genetic potential for controlling soil nitrification. In addition, the use of slow-release N-fertilizers can allow soil ammonium levels ≤1.0 mM. This, coupled with the development of genetically modified crops with accelerated PM H+-ATPase activity, can further improve BNI release from sorghum root systems to make production systems low-nitrifying and low-N2O emitting with improved nitrogen-use efficiency, which in turn will be ultimately beneficial to human society and the environment.
Crop, Livestock and Environment Division, JIRCAS, Japan International Research Center for Agricultural Sciences
Zeng，H.et al. (2015) Plant and Soil (Online First) DOI: 10.1007/s11104-015-2675-2